EP0645022A1

EP0645022A1 - Processes of preparing tabular grain emulsions

Info

Publication number: EP0645022A1
Application number: EP93907632A
Authority: EP
Inventors: Gary Lawrence House; Thomas Brownell Brust; Debra Lynn Hartsell; Donald Lee Black; Michael George Antoniades; Allen Keh-Chang Tsaur; Yun Chea Chang
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1993-03-22
Filing date: 1993-03-22
Publication date: 1995-03-29
Anticipated expiration: 2013-03-22
Also published as: DE69318405T2; DE69318405D1; JPH07507646A; EP0645022B1; JP3193381B2; WO1994022051A1

Abstract

A process is disclosed of preparing silver halide emulsions containing tabular grains bounded by [100] major faces comprised of the steps of (1) introducing silver and halide salts into a dispersing medium and (2) maintaining conditions within the dispersing medium that promote the formation of tabular grains bounded by [100] major faces. The process is characterized in that of the tabular grains bounded by [100] major faces a portion accounting for 50 percent of total grain projected area selected on the criteria of adjacent major face edge ratios of less than 10 and thicknesses of less than 0.3 νm internally at their nucleation site contain iodide and at least 50 mole percent chloride, at least the selected portion of the tabular grains being formed ty nucleation in a continuous double jet reaction in the presence of iodide with chloride accounting for at least 50 mole percent of the halide present in the dispersing medium and the pC1 of the dispersing medium being maintained in the range of from 0.5 to 3.5. Following nucleation tabular grain growth is undertaken in a reaction vessel which receives emulsion from the continuous double jet reactor until the tabular grains having a thickness of less than 0.3 νm exhibit achieve a selected average aspect ratio.

Description

PROCESSES OF PREPARING TABULAR GRAIN EMULSIONS

Field of the Invention

The invention relates to processes for preparing radiation sensitive silver halide emulsions.

Background

During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developa- bility, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be achieved by employing high and intermediate aspect ratio tabular grain emulsions.

An emulsion is generally understood to be a "high aspect ratio tabular grain emulsion" when tabular grains having a thickness of less than 0.3 μm have an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. The aspect ratio of a tabular grain is the ratio of its equivalent circular diameter (ECD) to its thickness (t) . The equivalent circular diameter of a grain is the diameter of a circle having an area equal to the projected area of the grain. The term "intermediate aspect ratio tabular grain emulsion" is employed when, through tabular grain thickening above 0.3 μm and/or low grain mean ECD, an average aspect ratio in the range of from 5-8 is exhibited. Generally, tabular grain emulsions exhibit average tabular grain aspect ratios of at least 2. The term "thin tabular grain" is generally understood to be a tabular grain having a thickness of less than 0.2 μ . The term "ultrathin tabular grain" is generally understood to be a tabular grain having a thickness of 0.06 μm or less. The term "high chloride" refers to grains that contain at least 50 mole percent chloride based on silver. In referring to grains of mixed halide content, the halides are named in order of increasing molar concentrations-- e.g., silver iodochloride contains a higher molar concentration of chloride than iodide.

The overwhelming majority of tabular grain emulsions contain tabular grains that are irregular octahedral grains. Regular octahedral grains contain eight identical crystal faces, each lying in a different {111} crystallographic plane. Tabular irregular octahedra contain two or more parallel twin planes that separate two major grain faces lying in {111} crystallographic planes. The {111} major faces of the tabular grains exhibit a threefold symmetry, appearing triangular or hexagonal. It is generally accepted that the tabular shape of the grains is the result of the twin planes producing favored edge sites for silver halide deposition, with the result that the grains grow laterally while increasing little, if any, in thickness after parallel twin plane incorporation. While tabular grain emulsions have been advantageously employed in a wide variety of photographic and radiographic applications, the requirement of parallel twin plane formation and {111} crystal faces pose limitations both in emulsion preparation and use. These disadvantages are most in evidence in considering tabular grains containing high chloride concentrations. It is generally recognized that silver chloride grains prefer to form regular cubic grains--that is, grains bounded by six identical {100} crystal faces. Tabular grains bounded by {111} faces in silver chloride emulsions often revert to nontabular forms unless morphologically stabilized. While high and intermediate aspect ratio tabular grain silver bromide emulsions were known to the art long before the 1980's, Wey U.S. Patent 4,399,215 produced the first tabular grain silver chloride emulsion. The tabular grains were of the twinned type, exhibiting major faces of threefold symmetry lying in {111} crystallographic planes. An ammoniacal double-jet precipitation technique was employed. The thicknesses of the tabular grains were high compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions because the ammonia ripening agent thickened the tabular grains. To achieve ammonia ripening it was also necessary to precipitate the emulsions at a relatively high pH, which is known to produce elevated minimum densities (fog) in high chloride emulsions. Further, to avoid degrading the tabular grain geometries sought both bromide and iodide ions were excluded from the tabular grains early in their formation.

Wey et al U.S. Patent 4,414,306 developed a twinning process for preparing silver chlorobromide emulsions containing up to 40 mole percent chloride based on total silver. This process of preparation has not been successfully extended to high chloride emulsions. The highest average aspect ratio reported in the Examples was 11. Maskasky U.S. Patent 4,400,463 (hereinafter designated Maskasky I) developed a strategy for preparing a high chloride emulsion containing tabular grains with parallel twin planes and {111} major crystal faces with the significant advantage of tolerating significant internal inclusions of the other halideε. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with a grain growth modifier having as its function to promote the formation of {111} crystal faces. Adsorbed aminoazaindenes, preferably adenine, and iodide ions were disclosed to be useful grain growth modifiers.

Maskasky U.S. Patent 4,713,323 (hereinafter designated Maskasky II), significantly advanced the state of the art by preparing high chloride emulsions containing tabular grains with parallel twin planes and {111} major crystal faces using an aminoazaindene growth modifier and a gelatino-peptizer containing up to 30 micromoles per gram of methionine. Since the methionine content of a gelatino-peptizer, if objectionably high, can be readily reduced by treatment with a strong oxidizing agent (or alkylating agent, King et al U.S. Patent 4,942,120), Maskasky II placed within reach of the art high chloride tabular grain emulsions with significant bromide and iodide ion inclusions prepared starting with conventional and universally available peptizers.

Maskasky I and II have stimulated further investigations of grain growth modifiers capable of preparing high chloride emulsions of similar tabular grain content. Tufano et al U.S. Patent 4,804,621 employed di (hydroamino)azines as grain growth modifiers; Takada et al U.S. Patent 4,783,398 employed heterocycles containing a divalent sulfur ring atom; Nishikawa et al U.S. Patent 4,952,491 employed spectral sensitizing dyes and divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al U.S. Patent 4,983,508 employed organic bis-quaternary amine salts.

Bogg U.S. Patent 4,063,951 reported the first tabular grain emulsions in which the tabular grains had parallel {100} major crystal faces. The tabular grains of Bogg exhibited square or rectangular major faces, thus lacking the threefold symmetry of conventional tabular grain {111} major crystal faces. In the sole example Bogg employed an ammoniacal ripening process for preparing silver bromoiodide tabular grains having aspect ratios ranging from 4:1 to 1:1. The average aspect ratio of the emulsion was reported to be 2, with the highest aspect ratio grain (grain A in Figure 3) being only 4. Bogg states that the emulsions can contain no more than 1 percent iodide and demonstrates only a 99.5% bromide 0.5% iodide emulsion. Attempts to prepare tabular grain emulsions by the procedures of Bogg have been unsuccessful. Mignot U.S. Patent 4,386,156 represents an improvement over Bogg in that the disadvantages of ammoniacal ripening were avoided in preparing a silver bromide emulsion containing tabular grains with square and rectangular major faces. Mignot specifically requires ripening in the absence of silver halide ripening agents other than bromide ion (e.g., thiocyanate, thioether or ammonia) .

Endo and Okaji, "An Empirical Rule to Modify the Habit of Silver Chloride to form Tabular Grains in an Emulsion", The Journal of Photographi c Sci ence, Vol. 36, pp. 182-188, 1988, discloses silver chloride emulsions prepared in the presence of a thiocyanate ripening agent. Emulsion preparations by the procedures disclosed has produced emulsions containing a few tabular grains within a general grain population exhibiting mixed {111} and {100} faces.

Mumaw and Haugh, "Silver Halide Precipitation Coalescence Processes", Journal of Imaging Science, Vol. 30, No. 5, Sept./Oct. 1986, pp. 198-299, is essentially cumulative with Endo and Okaji, with section IV-B being particularly pertinent.

Symposium: Torino 1963, Photographic Science, Edited by C. Semerano and U. Mazzucato, Focal Press, pp. 52-55, discloses the ripening of a cubic grain silver chloride emulsion for several hours at 77°C. During ripening tabular grains emerged and the original cubic grains were depleted by Ostwald ripening. As demonstrated by the comparative Example below, after 3 hours of ripening tabular grains account for only a small fraction of the total grain projected area, and only a small fraction of the tabular grains were less than 0.3 μm in thickness. In further investigations going beyond the actual teachings provided, extended ripening eliminated many of the smaller cubic grains, but also degraded many of the tabular grains to thicker forms.

Japanese published patent application (Kokai) 02/024,643, laid open January 26, 1990, was cited in a Patent Cooperation Treaty search report as being pertinent to the subject matter claimed, but is in Applicants' view unrelated. The claim is directed to a negative working emulsion containing a hydrazide derivative and tabular grains with an equivalent circular diameter of 0.6 to 0.2 μm. Only conventional tabular grain preparations are disclosed and only silver bromide and bromoiodide emulsions are exemplified.

In the precipitation of silver halide emulsions it is the most common practice to perform the entire precipitation reaction in a single reaction vessel. Nevertheless, so-called "dual-zone" precipitations have also been reported. In dual-zone arrangements silver and halide ions are brought together to form grain nuclei in a first area and then transported to a second area for grain growth. For many years emulsion was recirculated from the second (growth) area to the first (nucleation) area, but more recently arrangements have been reported that do not recirculate any portion of the emulsion from the second (growth) area to the first (nucleation) area, thereby completely isolating grain nucleation from grain growth. Specific illustrations of dual-zone precipitation are provided by Mignot U.S. Patent 4,334,012, Urabe U.S. Patent 4,879,208, and European published patent applications 326,852, 326,853, 355,535, 370,116, 368,275 and 374,954.

Summary of the Invention

In one aspect the invention is directed to a process of preparing silver halide emulsions in which tabular grains of less than 0.3 μm in thickness exhibiting {100} major faces with adjacent edge ratios of less than 10 account for at least 50 percent of total grain projected area and internally at their nucleation site contain iodide and at least 50 mole percent chloride, comprised of the steps of (1) introducing silver and halide salts and a dispersing medium into a continuous double jet reactor so that nucleation of the tabular grains occurs in the presence of iodide with chloride accounting for at least 50 mole percent of the halide present in the dispersing medium and the pCl of the dispersing medium being maintained in the range of from 0.5 to 3.5 and (2) following nucleation completing grain growth in a reaction vessel which receives emulsion from the continuous double jet reactor under conditions that maintain the {100} major faces of the tabular grains.

The present invention is based on the discovery of a novel approach to forming tabular grains. Instead of introducing parallel twin planes in grains as they are being formed to induce tabularity and thereby produce tabular grains with {111} major faces, it has been discovered that the presence of iodide in the dispersing medium during a high chloride nucleation step in a continuous double-jet reactor coupled with maintaining the chloride ion in solution within a selected pCl range results in the formation of grain nuclei that, when transported to a second reactor, can be grown to form a tabular grain emulsion in which the tabular grains are bounded by {100} crystal faces.

Not only does the invention represent the discovery of a novel process for preparing tabular grain emulsions, the emulsions that are produced by the process are novel. The invention places within the reach of the art tabular grains bounded by {100} crystal faces with halide contents, halide distributions and grain thicknesses that have not been heretofore realized. The present invention provides the first ultrathin tabular grain emulsion in which the grains are bounded by {100} crystal faces. The invention in a preferred form provides intermediate and high aspect ratio tabular grain high chloride emulsions exhibiting high levels of grain stability. Unlike high chloride tabular grain emulsions in which the tabular grains have {111} major faces, the emulsions of the invention do not require a morphological stabilizer adsorbed to the major faces of the grains to maintain their tabular form. Finally, while clearly applicable to high chloride emulsions, the present invention extends beyond high chloride emulsions to those containing a wide range of bromide, iodide and chloride concentrations. Brief Description of the Drawings

Figure 1 is a shadowed photomicrograph of carbon grain replicas of an emulsion of the invention;

Figure 2 is a shadowed photomicrograph of carbon grain replicas of a control emulsion and;

Figure 3 is a schematic diagram of a dual- zone reactor.

Description of Preferred Embodiments

The photographically useful, radiation sensitive emulsions that can be prepared by the preparation processes of the invention include those comprised of a dispersing medium and silver halide grains which are at least in part tabular grains bounded by {100} major faces. Of the tabular grains bounded by {100} major faces those accounting for 50 percent of the total grain projected area, selected on the criteria of (1) adjacent major face edge ratios of less than 10, (2) thicknesses of less than 0.3 μm and (3) higher aspect ratios than any remaining tabular grains satisfying criteria (1) and (2), have an average aspect ratio of at least 2, preferably at least 5 and optimally greater than 8.

The identification of emulsions satisfying the requirements of the invention and the significance of the selection parameters can be better appreciated by considering a typical emulsion. Figure 1 is a shadowed photomicrograph of carbon grain replicas of a representative emulsion of the invention, described in detail in Example 1 below. It is immediately apparent that most of the grains have orthogonal tetragonal (square or rectangular) faces. The orthogonal tetragonal shape of the grain faces indicates that they are {100} crystal faces.

The projected areas of the few grains in the sample that do not have square or rectangular faces are noted for inclusion in the calculation of the total grain projected area, but these grains clearly are not part of the tabular grain population having {100} major faces. A few grains may be observed that are acicular or rod-like grains (hereinafter referred as rods) . These grains are more than 10 times longer in one dimension than in any other dimension and can be excluded from the desired tabular grain population based on their high ratio of edge lengths. The projected area accounted for by the rods is low, but, when rods are present, their projected area is noted for determining total grain projected area.

The grains remaining all have square or rectangular major faces, indicative of {100} crystal faces. Some of these grains are regular cubic grains. That is, they are grains that have three mutually perpendicular edges of equal length. To distinguish cubic grains from tabular grains it is necessary to measure the grain shadow lengths. From a knowledge of the the shadow angle it is possible to calculate the thickness of a grain from a measurement of its shadow length. The projected areas of the cubic grains are included in determining total grain projected area. To quantify the characteristics of the tabular grains, a grain-by-grain examination of each of the remaining grains presenting square or rectangular faces is required. The projected area of each grain is noted for determination of total grain projected area. Each of the grains having a square or rectangular face and a thickness of less than 0.3 μm is examined. The projected area (the product of edge lengths) of the upper surface of each grain is noted. From the grain projected area the ECD of the grain is calculated. The thickness (t) of the grain and its aspect ratio (ECD/t) of the grain are next calculated.

After all of the grains having a square or rectangular face and a thickness of less than 0.3 μm have been measured, these grains are rank ordered according to aspect ratio. The grain with the highest aspect ratio is rank ordered first and the grain with the lowest aspect ratio is rank ordered last.

Proceeding from the top of the aspect ratio rank ordering, sufficient tabular grains are selected to account for 50 percent of total grain projected area. The aspect ratios of the selected tabular grain population are then averaged. In the emulsion of Figure 1 the average aspect ratio of the selected tabular grain population is greater than 8. In specifically preferred emulsions according to the invention average aspect ratios of the selected tabular grain population are greater than 12 and optimally at least 20. Typically the average aspect ratio of the selected tabular grain population ranges up to 50, but higher aspect ratios of 100, 200 or more can be realized.

The selected tabular grain population accounting for 50 percent of total grain projected area preferably exhibits major face edge length ratios of less than 5 and optimally less than 2. The nearer the major face edge length ratios approach 1 (i.e., equal edge lengths) the lower is the probability of a significant rod population being present in the emulsion. Further, it is believed that tabular grains with lower edge ratios are less susceptible to pressure desensitization.

Instead of rank ordering tabular grains accounting for 50 percent of total grain projected area as described above to arrive at an average aspect ratio a simpler approach can be employed in characterizing many of the emulsions satisfying the requirements of the invention in which tabular grains are the primary grain population present. Following this approach an average grain ECD and an average grain thickness (t) are obtained, excluding only rods and grains lacking {100} major faces. When average grain thickness is less than 0.3 μm and average grain aspect ratio (ECD/t) satisfies the selected criterion of 2, 5 or greater than 8, the emulsion in every instance is one which satisfies the parameter requirements noted above by the more laborious rank ordering procedure.

In one specifically preferred form of the invention the tabular grain population is selected on the basis of tabular grain thicknesses of less than 0.2 μm instead of 0.3 μm. In other words, the emulsions are in this instance thin tabular grain emulsions.

Surprisingly, ultrathin tabular grain emulsions have been prepared by the preparation process of the invention. Ultrathin tabular grain emulsions are those in which the selected tabular grain population is made up of tabular grains having thicknesses of less than 0.06 μ . Prior to the present invention the only ultrathin tabular grain emulsions of a halide content exhibiting a cubic crystal lattice structure known in the art contained tabular grains bounded by {111} major faces. In other words, it was thought essential to form tabular grains by the mechanism of parallel twin plane incorporation to achieve ultrathin dimensions. Emulsions can be prepared according to the invention in which the selected tabular grain population has a mean thickness down to 0.02 μm and even 0.01 μm. Ultrathin tabular grains have extremely high surface to volume ratios. This permits ultrathin grains to be photographically processed at accelerated rates. Further, when spectrally sensitized, ultrathin tabular grains exhibit very high ratios of speed in the spectral region of sensitization as compared to the spectral region of native sensitivity. For example, ultrathin tabular grain emulsions prepared according to the invention can have entirely negligible levels of blue sensitivity, and are therefore capable of providing a green or red record in a photographic product that exhibits minimal blue contamination even when located to receive blue light.

The characteristic of tabular grain emulsions that sets them apart from other emulsions is the ratio of grain ECD to thickness (t) . This relationship has been expressed quantitatively in terms of aspect ratio. Another quantification that is believed to assess more accurately the importance of tabular grain thickness is tabularity:

T = ECD/t² = AR/t where

T is tabularity; AR is aspect ratio;

ECD is equivalent circular diameter in micrometers (μ ) ; and t is grain thickness in micrometers.

The selected tabular grain population accounting for 50 percent of total grain projected area preferably exhibits a tabularity of greater than 25 and most preferably greater than 100. Since the selected tabular grain population can be ultrathin, it is apparent that extremely high tabularities, ranging to 1000 and above are within the contemplation of the invention.

The selected tabular grain population can exhibit an average ECD of any photographically useful magnitude. For photographic utility average ECD's of less than 10 μm are contemplated, although average

ECD's in most photographic applications rarely exceed 6 μm. Within ultrathin tabular grain emulsions satisfying the requirements of the invention it is possible to satisfy intermediate aspect ratio requirements with ECD's of the selected tabular grain population of 0.10 μm and less. As is generally understood by those skilled in the art, emulsions with selected tabular grain populations having higher ECD's are advantageous for achieving relatively high levels of photographic sensitivity while selected tabular grain populations with lower ECD's are advantageous in achieving low levels of granularity. So long as the selected population of tabular grains satisfying the parameters noted above accounts for at least 50 percent of total grain projected area a photographically desirable grain population is available. It is recognized that the advantageous properties of the emulsions of the invention are increased as the proportion of tabular grains having thicknesses of less than 0.3 μm and {100} major faces is increased. The preferred emulsions according to the invention are those in which at least 70 percent and optimally at least 90 percent of total grain projected area is accounted for by tabular grains having {100} major faces. It is specifically contemplated to provide emulsions satisfying the grain descriptions above in which the selection of the rank ordered tabular grains extends to sufficient tabular grains to account for 70 percent or even 90 percent of total grain projected area.

So long as tabular grains having the desired characteristics described above account for the requisite proportion of the total grain projected area, the remainder of the total grain projected area can be accounted for by any combination of coprecipitated grains. It is, of course, common practice in the art to blend emulsions to achieve specific photographic objectives. Blended emulsions that satisfy the selected tabular grain descriptions above are specifically contemplated.

If tabular grains having a thickness of less than 0.3 μm do not account for 50 percent of the total grain projected area, the emulsion does not satisfy the requirements of the invention and is, in general, a photographically inferior emulsion. For most applications (particularly applications that require spectral sensitization, require rapid processing and/or seek to minimize silver coverages) emulsions are photographically inferior in which many or all of the tabular grains are relatively thick--e.g., emulsions containing high proportions of tabular grains with thicknesses in excess of 0.3 μm. Emulsions containing thicker (up to 0.5 μ ) tabular grains with {111} major faces, though generally inferior, have been suggested for use in the art to maximize capture of light in the spectral region to which silver halide exhibits native sensitivity (e.g., blue light). Emulsions containing thicker tabular grains having {100} major faces can be applied, if desired, to similar applications.

More commonly, inferior emulsions prepared by processes failing to satisfy the requirements of the invention have an excessive proportion of total grain projected area accounted for by cubes, twinned nontabular grains, and rods. Such an emulsion is shown in Figure 2. Most of the grain projected area is accounted for by cubic grains. Also the rod population is much more pronounced than in Figure 1. A few tabular grains are present, but they account for only a minor portion of total grain projected area.

The tabular grain emulsion of Figure 1 prepared by a process satisfying the requirements of the invention and the predominantly cubic grain emulsion of Figure 2 were prepared under conditions that were identical, except for iodide management during nucleation. The Figure 2 emulsion is a silver chloride emulsion while the emulsion of Figure 1 additionally includes a small amount of iodide introduced during grain nucleation.

A preferred procedure for obtaining high chloride {100} tabular grain emulsions of the type described above has been realized by the discovery of a novel dual-zone precipitation process. A preferred dual-zone precipitation apparatus is shown in Figure 3, wherein a continuous double-jet nucleation reactor 1 is provided to receive a dispersing medium through jet 2, a silver salt solution through jet 3 and a halide salt solution through jet 4. Within the reactor the silver and halide salts react to form grain nuclei. The reaction mixture containing the grain nuclei is then transported, as indicated by arrow 5, to a growth reaction vessel 6 containing a liquid medium 7 comprised of an initially present dispersing medium and/or an earlier transported portion of the emulsion formed in the nucleation reactor. The growth reaction vessel is shown equipped with a stirring device 8. If desired additional silver and halide ions can be supplied to the growth reaction vessel.

In the dual-zone precipitation process of the invention grain nucleation occurs in a high chloride environment in the presence of iodide ion under conditions that favor the emergence of {100} crystal faces. As grain formation occurs the inclusion of iodide into the cubic crystal lattice being formed by silver ions and the remaining halide ions is disruptive because of the much larger diameter of iodide ion as compared to chloride ion. The incorporated iodide ions introduce crystal irregularities that in the course of further grain growth result in tabular grains rather than regular (cubic) grains.

It is believed that at the outset of nucleation the incorporation of iodide ion into the crystal structure results in cubic grain nuclei being formed having one or more irregularities in one or more of the cubic crystal faces. The cubic crystal faces that contain at least one irregularity thereafter accept silver halide at an accelerated rate as compared to the regular cubic crystal faces (i.e., those lacking an irregularity) . When only one of the cubic crystal faces contains an irregularity, grain growth on only one face is accelerated, and the resulting grain structure on continued growth is a rod. The same result occurs when only two opposite parallel faces of the cubic crystal structure contain the growth accelerating irregularities. However, when any two contiguous cubic crystal faces contain the irregularity, continued growth accelerates growth on both faces and produces a tabular grain structure. It is believed that the tabular grains of the emulsions of this invention are produced by those grain nuclei having two, three or four faces containing the growth accelerating irregularities. Following conventional practice, both the nucleation reactor 1 and the growth reaction vessel 6 are fitted with conventional silver and reference electrodes for monitoring halide ion concentrations within the dispersing medium. Halide ion is introduced into the nucleation reactor that is at least 50 mole percent chloride--i.e. , at least half by number of the halide ions in the dispersing medium are chloride ions. The pCl of the dispersing medium is adjusted to favor the formation of {100} grain faces on nucleation--that is, within the range of from 0.5 to 3.5, preferably within the range of from 1.0 to 3.0 and, optimally, within the range of from 1.5 to 2.5.

Since precipitation is conducted in the presence of a stoichiometric excess of halide ion, the dispering medium and halide jets are first opened. The grain nucleation step is initiated when the silver jet is opened to introduce silver ion into the dispersing medium. Iodide ion is preferably introduced into the dispersing medium concurrently with or, optimally, before opening the silver jet. Effective tabular grain formation can occur over a wide range of iodide ion concentrations ranging up to the saturation limit of iodide in silver chloride. The saturation limit of iodide in silver chloride is reported by H. Hirsch, "Photographic Emulsion Grains with Cores: Part I. Evidence for the Presence of Cores", J. of Photog. Science, Vol. 10 (1962), pp. 129-134, to be 13 mole percent. In silver halide grains in which equal molar proportions of chloride and bromide ion are present up to 27 mole percent iodide, based on silver, can be incorporated in the grains. It is preferred to undertake grain nucleation and growth below the iodide saturation limit to avoid the precipitation of a separate silver iodide phase and thereby avoid creating an additional category of unwanted grains. It is generally preferred to maintain the iodide ion concentration in the dispersing medium at the outset of nucleation at less than 10 mole percent. In fact, only minute amounts of iodide at nucleation are required to achieve the desired tabular grain population. Initial iodide ion concentrations of down to 0.001 mole percent are contemplated. However, for convenience in replication of results, it is preferred to maintain initial iodide concentrations of at least 0.01 mole percent and, optimally, at least 0.05 mole percent.

In the preferred form of the invention silver iodochloride grain nuclei are formed during the nucleation step. Minor amounts of bromide ion can be present in the dispersing medium during nucleation. Any amount of bromide ion can be present in the dispersing medium during nucleation that is compatible with at least 50 mole percent of the halide in the grain nuclei being chloride ions. The grain nuclei preferably contain at least 70 mole percent and optimally at least 90 mole percent chloride ion, based on silver.

Grain nuclei formation occurs instantaneously upon introducing silver ion into the dispersing medium. The nucleation reactor can be operated in a steady state, continuous manner over any desired time period. It is possible to switch the reaction stream 5 from one growth reaction vessel to another as precipitation continues.

Any convenient conventional source of silver and halide ions can be employed during the nucleation step. Silver ion is preferably introduced as an aqueous silver salt solution, such as a silver nitrate solution. Halide ion is preferably introduced as alkali or alkaline earth halide, such as lithium, sodium and/or potassium chloride, bromide and/or iodide.

It is possible, but not preferred, to introduce silver chloride or silver iodochloride Lippmann grains into the dispersing medium during the nucleation step. In this instance grain nucleation has already occurred and what is referred to above as the nucleation step is in reality a step for introduction of grain facet irregularities. The disadvantage of delaying the introduction of grain facet irregularities is that this produces thicker tabular grains than would otherwise be obtained.

The dispersing medium introduced into the nucleation reactor is comprised of water and a peptizer. The dispersing medium can exhibit a pH within any convenient conventional range for silver halide precipitation, typically from 2 to 8. It is preferred, but not required, to maintain the pH of the dispersing medium on the acid side of neutrality (i.e., < 7.0) . To minimize fog a preferred pH range for precipitation is from 2.0 to 6.0. Mineral acids, such as nitric acid or hydrochloride acid, and bases, such as alkali hydroxides, can be used to adjust the pH of the dispersing medium. It is also possible to incorporate pH buffers. The peptizer can take any convenient conventional form known to be useful in the precipitation of photographic silver halide emulsions and. particularly tabular grain silver halide emulsions. A summary of conventional peptizers is provided in Research Disclosure, Vol. 308, December 1989, Item

308119, Section IX. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England. While synthetic polymeric peptizers of the type disclosed by Maskasky I, cited above and here incorporated by reference, can be employed, it is preferred to employ gelatino peptizers (e.g., gelatin and gelatin derivatives). As manufactured and employed in photography gelatino peptizers typically contain significant concentrations of calcium ion, although the use of deionized gelatino peptizers is a known practice. In the latter instance it is preferred to compensate for calcium ion removal by adding divalent or trivalent metal ions, such alkaline earth or earth metal ions, preferably magnesium, calcium, barium or aluminum ions.

Specifically preferred peptizers are low methionine gelatino peptizers (i.e., those containing less than 30 micromoles of methionine per gram of peptizer) , optimally less than 12 micromoles of methionine per gram of peptizer, these peptizers and their preparation are described by Maskasky II and King et al, cited above, the disclosures of which are here incorporated by reference. However, it should be noted that the grain growth modifiers of the type taught for inclusion in the emulsions of Maskasky I and II (e.g., adenine) are not appropriate for inclusion in the dispersing media of this invention, since these grain growth modifiers promote twinning and the formation of tabular grains having {111} major faces. Generally at least about 10 percent and typically from 20 to 80 percent of the dispersing medium forming the completed emulsion is present in the reaction vessel at the outset of the nucleation step. It is conventional practice to maintain relatively low levels of peptizer, typically from 10 to 20 percent of the peptizer present in the completed emulsion, in the reaction vessel at the start of precipitation. To increase the proportion of thin tabular grains having {100} faces formed during nucleation it is preferred that the concentration of the peptizer in the dispersing medium be in the range of from 0.5 to 6 percent by weight of the total weight of the dispersing medium at the outset of the nucleation step. It is conventional practice to add gelatin, gelatin derivatives and other vehicles and vehicle extenders to prepare emulsions for coating after precipitation. Any naturally occurring level of methionine can be present in gelatin and gelatin derivatives added after precipitation is complete.

The nucleation step can be performed at any convenient conventional temperature for the precipitation of silver halide emulsions. Temperatures ranging from near ambient--e.g. , 30°C up to about 90°C are contemplated, with nucleation temperatures in the range of from 35 to 70°C being preferred. Since grain nuclei formation occurs almost instantaneously, only a very small proportion of the total silver used to form the emulsion need be introduced during the nucleation step. Typically from about 0.1 to 10 mole percent of total silver is introduced through the nucleation reactor during the nucleation step.

In the growth reaction vessel the grain nuclei are grown until tabular grains having {100} major faces of a desired average ECD are obtained. Whereas the objective of the nucleation step is to form a grain population having the desired incorporated crystal structure irregularities, the objective of the growth step is to deposit additional silver halide onto (grow) the existing grain population while avoiding or minimizing the formation of additional grains. If additional grains are formed during the growth step, the polydispersity of the emulsion is increased and, unless conditions in the reaction vessel are maintained as described above for the nucleation step, the additional grain population formed in the growth step will not have the desired tabular grain properties described above.

It is usually preferred to prepare photographic emulsions with the most geometrically uniform grain populations attainable, since this allows a higher percentage of the total grain population to be optimally sensitized and otherwise optimally prepared for photographic use. Further, it is usually more convenient to blend relatively monodisperse emulsions to obtain aim sensitometric profiles than to precipitate a single polydisperse emulsion that conforms to an aim profile.

In the preparation of emulsions according to the invention it is preferred to complete the nucleation step before adding additional silver and/or halide ions to the growth reaction vessel. In other words, there is preferably a holding period in the growth reaction vessel before proceeding with the growth step of the preparation process. The emulsions are held within the temperature ranges described above for nucleation for a period sufficient to allow reduction in grain dispersity. A holding period can range from a minute to several hours, with typical holding periods ranging from 5 minutes to an hour. During the holding period relatively smaller grain nuclei are Ostwald ripened onto surviving, relatively larger grain nuclei, and the overall result is a reduction in grain dispersity.

If desired, the rate of ripening can be increased by the presence of a ripening agent in the emulsion during the holding period. A conventional simple approach to accelerating ripening is to increase the halide ion concentration in the dispersing medium. This creates complexes of silver ions with plural halide ions that accelerate ripening. When this approach is employed, it is preferred to increase the chloride ion concentration in the dispersing medium. That is, it is preferred to lower the pCl of the dispersing medium into a range in which increased silver chloride solubility is observed. Alternatively, ripening can be accelerated and the percentage of total grain projected area accounted for by {100} tabular grains can be increased by employing conventional ripening agents. Preferred ripening agents are sulfur containing ripening agents, such as thioethers and thiocyanates. Typical thiocyanate ripening agents are disclosed by Nietz et al U.S. Patent 2,222,264, Lowe et al U.S. Patent 2,448,534 and Illingsworth U.S. Patent 3,320,069, the disclosures of which are here incorporated by reference. Typical thioether ripening agents are disclosed by McBride U.S. Patent 3,271,157, Jones U.S. Patent 3,574,628 and Rosencrantz et al U.S. Patent 3,737,313, the disclosures of which are here incorporated by reference. More recently crown thioethers have been suggested for use as ripening agents. Ripening agents containing a primary or secondary amino moiety, such as imidazole, glycine or a substituted derivative, are also effective. Sodium sulfite has also been demonstrated to be effective in increasing the percentage of total grain projected accounted by the {100} tabular grains.

Once the desired population of grain nuclei have been formed, grain growth can proceed according to any convenient conventional precipitation technique for the precipitation of silver halide grains bounded by

{100} grain faces. Grain growth is continued until the tabular grains having a thickness of less than 0.3 μm exhibit an average aspect ratio of greater than 8. To be considered tabular a grain must have an aspect ratio of at least 2.

Whereas iodide and chloride ions are required to be incorporated into the grains during nucleation and are therefore present in the completed grains at the internal nucleation site, any halide or combination of halides known to form a cubic crystal lattice structure can be employed during the growth step. Neither iodide nor chloride ions need be incorporated in the grains during the growth step, since the irregular grain nuclei faces that result in tabular grain growth, once introduced, persist during subsequent grain growth independently of the halide being precipitated, provided the halide or halide combination is one that forms a cubic crystal lattice. This excludes only iodide levels above 13 mole percent (preferably 6 mole percent) in precipitating silver iodochloride, levels of iodide above 40 mole percent (preferably 30 mole percent) in precipitating silver iodobromide, and proportionally intermediate levels of iodide in precipitating silver iodohalides containing bromide and chloride. When silver bromide or silver iodobromide is being deposited during the growth step, it is preferred to maintain a pBr within the dispersing medium in the range of from 1.0 to 4.2, preferably 1.6 to 3.4. When silver chloride, silver iodochloride, silver bromochloride or silver iodobro ochloride is being deposited during the growth step, it is preferred to maintain the pCl within the dispersing medium within the ranges noted above in describing the nucleation step. It has been discovered quite unexpectedly that up to 20 percent reductions in tabular grain thicknesses can be realized by specific halide introductions during grain growth. Surprisingly, it has been observed that bromide additions during the growth step in the range of from 0.05 to 15 mole percent, preferably from 1 to 10 mole percent, based on silver, produce relatively thinner {100} tabular grains than can be realized under the same conditions of precipitation in the absence of bromide ion. Similarly, it has been observed that iodide additions during the growth step in the range of from 0.001 to <1 mole percent, based on silver, produce relatively thinner {100} tabular grains than can be realized under the same conditions of precipitation in the absence of iodide ion.

During the growth step both silver and halide salts are preferably introduced into the dispersing medium. In other words, double jet precipitation is contemplated, with added iodide salt, if any, being introduced with the remaining halide salt or through an independent jet. The rate at which silver and halide salts are introduced is controlled to avoid renucleation--that is, the formation of a new grain population. Addition rate control to avoid renucleation is generally well known in the art, as illustrated by Wilgus German OLS No. 2,107,118, Irie U.S. Patent 3,650,757, Kurz U.S. Patent 3,672,900, Saito U.S. Patent 4,242,445, Teitschied et al European Patent Application 80102242, and Wey "Growth Mechanism of AgBr Crystals in Gelatin Solution", Photographic Science and Engineering, Vol. 21, No. 1, Jan. /Feb. 1977, p. 14, et seg.

In one form of the invention dual zone precipitation can be undertaken by recirculating the contents of the growth reaction vessel to the nucleation reactor. Referrrng to Figure 3, in this form the liquid medium 7 in the growth reaction vessel is fed to the nucleation reactor through jet 2. Precipitation arrangements of this type are illustrated by Posse et al U.S. Patent 3,790,386, Forster et al U.S. Patent 3,897,935, Finnicum et al U.S. Patent 4,147,551, and Verhille et al U.S. Patent 4,171,224, here incorporated by reference.

It is herein contemplated that various parameters important to the control of grain formation and growth, such as pH, pAg, ripening, temperature, and residence time, can be independently controlled in the separate nucleation and growth reaction vessels. To allow grain nucleation to be entirely independent of grain growth occurring in the growth reaction vessel down stream of the nucleation reaction vessel, no portion of the contents of the growth reaction vessel should be recirculated to the nucleation reaction vessel. Preferred arrangements that separate grain nucleation from the contents of the growth reaction vessel are disclosed by Mignot U.S. Patent 4,334,012 (which also discloses the useful feature of ultrafiltration during grain growth), Urabe U.S. Patent 4,879,208 and published European Patent Applications 0 326 852, 0 326 853, 0 355 535 and 0 370 116, Ichizo published European Patent ApplicationO 368 275, Urabe et al published European Patent Application 0 374 954, and Onishi et al published Japanese Patent Application (Kokai) 172,817-A (1990). The emulsions that can be produced by the process of the invention include silver iodochloride emulsions, silver iodobromochloride emulsions and silver iodochlorobromide emulsions. Dopants, in concentrations of up to 10^"2 mole per silver mole and typically less than 10^"^ mole per silver mole, can be present in the grains. Compounds of metals such as copper, thallium, lead, mercury, bismuth, zinc, cadmium , rhenium, and Group VIII metals (e.g., iron, ruthenium, rhodium, palladium, osmium, iridium, and platinum) can be present during grain precipitation, preferably during the growth stage of precipitation. The modification of photographic properties is related to the level and location of the dopant within the grains. When the metal forms a part of a coordination complex, such as a hexacoordination complex or a tetracoordination complex, the ligands can also be included within the grains and the ligands can further influence photographic properties. Coordination ligands, such as halo, aquo, cyano cyanate, thiocyanate, nitrosyl, thionitrosyl, oxo and carbonyl ligands are contemplated and can be relied upon to modify photographic properties.

Dopants and their addition are illustrated by Arnold et al U.S. Patent 1,195,432; Hochstetter U.S. Patent 1,951,933; Trivelli et al U.S. Patent 2,448,060; Overman U.S. Patent 2,628,167; Mueller et al U.S. Patent 2,950,972; McBride U.S. Patent 3,287,136; Sidebotham U.S. Patent 3,488,709; Rosecrants et al U.S. Patent 3,737,313; Spence et al U.S. Patent 3,687,676; Gilman et al U.S. Patent 3,761,267; Shiba et al U.S. Patent 3,790,390; Ohkubo et al U.S. Patent 3,890,154; Iwaosa et al U.S. Patent 3,901,711; Habu et al U.S. Patent 4,173,483; Atwell U.S. Patent 4,269,927; Janusonis et al U.S. Patent 4,835,093; McDugle et al U.S. Patents 4,933,272, 4,981,781, and 5,037,732;

Keevert et al U.S. Patent 4,945,035; and Evans et al U.S. Patent 5,024,931, the disclosures of which are here incorporated by reference. For background as to alternatives known to the art attention is directed to B. H. Carroll, "Iridium Sensitization: A Literature Review", Photographi c Science and Engineering,Vol. 24, NO. 6, Nov./Dec. 1980, pp. 265-257, and Grzeskowiak et al published European Patent Application 0 264 288.

The invention is particularly advantageous in providing high chloride (greater than 50 mole percent chloride) tabular grain emulsions, since conventional high chloride tabular grain emulsions having tabular grains bounded by {111} are inherently unstable and require the presence of a morphological stabilizer to prevent the grains from regressing to nontabular forms. Particularly preferred high chloride emulsions are according to the invention that are those that contain more than 70 mole percent (optimally more than 90 mole percent) chloride. Although not essential to the practice of the invention, a further procedure that can be employed to maximize the population of tabular grains having {100} major faces is to incorporate an agent capable of restraining the emergence of non- {100} grain crystal faces in the emulsion during its preparation. The restraining agent, when employed, can be active during grain nucleation, during grain growth or throughout precipitation.

Useful restraining agents under the contemplated conditions of precipitation are organic compounds containing a nitrogen atom with a resonance stabilized π electron pair. Resonance stabilization prevents protonation of the nitrogen atom under the relatively acid conditions of precipitation. Aromatic resonance can be relied upon for stabilization of the π electron pair of the nitrogen atom. The nitrogen atom can either be incorporated in an aromatic ring, such as an azole or azine ring, or the nitrogen atom can be a ring substituent of an aromatic ring.

In one preferred form the restraining agent can satisfy the following formula: (I)

where

Z represents the atoms necessary to complete a five or six membered aromatic ring structure, preferably formed by carbon and nitrogen ring atoms. Preferred aromatic rings are those that contain one, two or three nitrogen atoms. Specifically contemplated ring structures include 2H-pyrrole, pyrrole, imidazole, pyrazole, 1,2,3-triazole, 1,2,4-triazole,

1,3,5-triazole, pyridine, pyrazine, pyrimidine, and pyridazine.

When the stabilized nitrogen atom is a ring substituent, preferred compounds satisfy the following formula: (ID

Ar ¹-N—R²

where

Ar is an aromatic ring structure containing from 5 to 14 carbon atoms and R! and R² are independently hydrogen, Ar, or any convenient aliphatic group or together complete a five or six membered ring.

Ar is preferably a carbocyclic aromatic ring, such as phenyl or naphthyl. Alternatively any of the nitrogen and carbon containing aromatic rings noted above can be attached to the nitrogen atom of formula II through a ring carbon atom. In this instance, the resulting compound satisfies both formulae I and II. Any of a wide variety of aliphatic groups can be selected. The simplest contemplated aliphatic groups are alkyl groups, preferably those containing from 1 to 10 carbon atoms and most preferably from 1 to 6 carbon atoms. Any functional substituent of the alkyl group known to be compatible with silver halide precipitation can be present. It is also contemplated to employ cyclic aliphatic substituents exhibiting 5 or 6 membered rings, such as cycloalkane, cycloalkene and aliphatic heterocyclic rings, such as those containing oxygen and/or nitrogen hetero atoms. Cyclopentyl, cyclohexyl, pyrrolidinyl, piperidinyl, furanyl and similar heterocyclic rings are specifically contemplated.

The following are representative of compounds contemplated satisfying formulae I and/or II:

R-l

aniline

RA-2

α-naphthylamine

RA-3

β-naphthylamine

RA-4

benzidine RA-5

carbazole

RA-6

norharman

RA-7

pyrrole

RA-8

indole

RA-9

pyridine RA-10

quinoline

RA-11

isoquinoline

RA-12

acridine

RA-13

1, 8-naphthyridine

RA-14

1, 10-phenanthroline RA-15

nicotine

RA-16

benzoxazole

RA-17

pyrazole

RA-18

antipyrine

RA-19

H imidazole RA-20

H indazole

RA-21

pyrimidine

RA-22

pyrazme

RA-23

2,2' -bipyrazine

RA-24

pteridine RA-25

H

1,2, 3-triazole

RA-26

V H

1,2, 4-triazole

RA-27

3 -amino- 1, 2, 4-triazole

RA-28

H

3, 5-diamino-l, 2, 4-triazole

RA-29

benzotriazole RA-30

1 , 2 , 4-triazine RA-31

1 , 3 , 5-triazine

Selection of preferred restraining agents and their useful concentrations can be accomplished by the following selection procedure: The compound being considered for use as a restraining agent is added to a silver chloride emulsion consisting essentially of cubic grains with a mean grain edge length of 0.3 μm. The emulsion is 0.2 M in sodium acetate, has a pCl of 2.1, and has a pH that is at least one unit greater than the pKa of the compound being considered. The emulsion is held at 75°C with the restraining agent present for 24 hours. If, upon microscopic examination after 24 hours, the cubic grains have sharper edges of the {100} crystal faces than a control differing only in lacking the compound being considered, the compound introduced is performing the function of a restraining agent. The significance of sharper edges of intersection of the {100} crystal faces lies in the fact that grain edges are the most active sites on the grains in terms of ions reentering the dispersing medium. By maintaining sharp edges the restraining agent is acting to restrain the emergence of non-{100} crystal faces, such as are present, for example, at rounded edges and corners. In some instances instead of dissolved silver chloride depositing exclusively onto the edges of the cubic grains a new population of grains bounded by {100} crystal faces is formed. Optimum restraining agent activity occurs when the new grain population is a tabular grain population in which the tabular grains are bounded by {100} major crystal faces.

It is specifically contemplated to deposit epitaxially silver salt onto the tabular grains acting as hosts. Conventional epitaxial depositions onto high chloride silver halide grains are illustrated by Maskasky U.S. Patent 4,435,501 (particularly Example 24B) ; Ogawa et al U.S. Patents 4,786,588 and 4,791,053; Hasebe et al U.S. Patents 4,820,624 and 4,865,962; Sugimoto and Miyake, "Mechanism of Halide Conversion Process of Colloidal AgCl Microcrystals by Br" Ions", Parts I and II, Journal of Colloid and Interface Science, Vol. 140, No. 2, Dec. 1990, pp. 335-361; Houle et al U.S. Patent 5,035,992; and Japanese published applications (Kokai) 252649-A (priority 02.03.90-JP

051165 Japan) and 288143-A (priority 04.04.90-JP 089380 Japan). The disclosures of the above U.S. patents are here incorporated by reference.

The emulsions prepared by the process of the invention can be chemically sensitized with active gelatin as illustrated by T. H. James, The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, pp. 67-76, or with sulfur, selenium, tellurium, gold, platinum, palladium, iridium, osmium, rhenium or phosphorus sensitizers or combinations of these sensitizers, such as at pAg levels of from 5 to 10, pH levels of from 5 to 8 and temperatures of from 30 to 80°C, as illustrated by Research Disclosure, Vol. 120, April, 1974, Item 12008, Research Disclosure, Vol. 134, June, 1975, Item 13452, Sheppard et al U.S. Patent 1,623,499, Matthies et al U.S. Patent 1,673,522, Waller et al U.S. Patent 2,399,083, Damschroder et al U.S. Patent 2,642,361, McVeigh U.S. Patent 3,297,447, Dunn U.S. Patent 3,297,446, McBride U.K. Patent 1,315,755, Berry et al U.S. Patent 3,772,031, Gilman et al U.S. Patent 3,761,267, Ohi et al U.S. Patent 3,857,711, Klinger et al U.S. Patent 3,565,633, Oftedahl U.S. Patents 3,901,714 and 3,904,415 and Simons U.K. Patent 1,396,696; chemical sensitization being optionally conducted in the presence of thiocyanate derivatives as described in Damschroder U.S. Patent 2,642,361; thioether compounds as disclosed in Lowe et al U.S. Patent 2,521,926, Williams et al U.S. Patent 3,021,215 and Bigelow U.S. Patent 4,054,457; and azaindenes, azapyridazines and azapyrimidines as described in Dostes U.S. Patent 3,411,914, Kuwabara et al U.S. Patent 3,554,757, Oguchi et al U.S. Patent 3,565,631 and Oftedahl U.S. Patent 3,901,714; elemental sulfur as described by Miyoshi et al European Patent Application EP 294,149 and Tanaka et al European Patent Application EP 297,804; and thiosulfonates as described by Nishikawa et al European Patent Application EP 293,917. Additionally or alternatively, the emulsions can be reduction-sensitized--e.g. , with hydrogen, as illustrated by Janusonis U.S. Patent 3,891,446 and

Babcock et al U.S. Patent 3,984,249, by low pAg (e.g., less than 5), high pH (e.g., greater than 8) treatment, or through the use of reducing agents such as stannous chloride, thiourea dioxide, polyamines and amineboranes as illustrated by Allen et al U.S. Patent 2,983,609, Oftedahl et al Research Disclosure, Vol. 136, August, 1975, Item 13654, Lowe et al U.S. Patents 2,518,698 and 2,739,060, Roberts et al U.S. Patents 2,743,182 and '183, Chambers et al U.S. Patent 3,026,203 and Bigelow et al U.S. Patent 3,361,564. Chemical sensitization can take place in the presence of spectral sensitizing dyes as described by Philippaerts et al U.S. Patent 3,628,960, Kofron et al U.S. Patent 4,439,520, Dickerson U.S. Patent 4,520,098, Maskasky U.S. Patent 4,435,501, Ihama et al U.S. Patent 4,693,965 and Ogawa U.S. Patent 4,791,053. Chemical sensitization can be directed to specific sites or crystallographic faces on the silver halide grain as described by Haugh et al U.K. Patent Application 2,038,792A and Mifune et al published European Patent Application EP 302,528. The sensitivity centers resulting from chemical sensitization can be partially or totally occluded by the precipitation of additional layers of silver halide using such means as twin-jet additions or pAg cycling with alternate additions of silver and halide salts as described by Morgan U.S. Patent 3,917,485, Becker U.S. Patent 3,966,476 and Research Disclosure, Vol. 181, May, 1979, Item 18155. Also as described by Morgan, cited above, the chemical sensitizers can be added prior to or concurrently with the additional silver halide formation. Chemical sensitization can take place during or after halide conversion as described by Hasebe et al European Patent Application EP 273,404. In many instances epitaxial deposition onto selected tabular grain sites (e.g., edges or corners) can either be used to direct chemical sensitization or to itself perform the functions normally performed by chemical sensitization.

The emulsions of the invention can be spectrally sensitized with dyes from a variety of classes, including the polymethine dye class, which includes the cyanines, merocyanines, complex cyanines and merocyanines (i.e., tri-, tetra- and polynuclear cyanines and merocyanines) , styryls, merostyryls, streptocyanines, hemicyanines, arylidenes, allopolar cyanines and enamine cyanines.

The cyanine spectral sensitizing dyes include, joined by a methine linkage, two basic heterocyclic nuclei, such as those derived from quinolinium, pyridinium, isoquinolinium, 3H-indolium, benzindolium, oxazolium, thiazolium, selenazolinium, imidazolium, benzoxazolium, benzothiazolium, benzoselenazolium, benzotellurazolium, benzimidazolium, naphthoxazolium, naphthothiazolium, naphthoselen- azolium, naphtotellurazolium, thiazolinium, dihydronaphthothiazolium, pyrylium and imidazo- pyrazinium quaternary salts.

The merocyanine spectral sensitizing dyes include, joined by a methine linkage, a basic heterocyclic nucleus of the cyanine-dye type and an acidic nucleus such as can be derived from barbituric acid, 2-thiobarbituric acid, rhodanine, hydantoin, 2-thiohydantoin, 4-thiohydantoin, 2-pyrazolin-5-one, 2-isoxazolin-5-one, indan-1, 3-dione, cyclohexan-1, 3- dione, 1, 3-dioxane-4, 6-dione, pyrazolin-3, 5-dione, pentan-2, 4-dione, alkylsulfonyl acetonitrile, benzoylacetonitrile, malononitrile, malonamide, isoquinolin-4-one, chroman-2, 4-dione, 5H-furan-2-one, 5H-3-pyrrolin-2-one, 1, 1, 3-tricyanopropene and telluracyclohexanedione.

One or more spectral sensitizing dyes may be employed. Dyes with sensitizing maxima at wavelengths throughout the visible and infrared spectrum and with a great variety of spectral sensitivity curve shapes are known. The choice and relative proportions of dyes depends upon the region of the spectrum to which sensitivity is desired and upon the shape of the spectral sensitivity curve desired. Dyes with overlapping spectral sensitivity curves will often yield in combination a curve in which the sensitivity at each wavelength in the area of overlap is approximately equal to the sum of the sensitivities of the individual dyes. Thus, it is possible to use combinations of dyes with different maxima to achieve a spectral sensitivity curve with a maximum intermediate to the sensitizing maxima of the individual dyes.

Combinations of spectral sensitizing dyes can be used which result in supersensitization--that is, spectral sensitization greater in some spectral region than that from any concentration of one of the dyes alone or that which would result from the additive effect of the dyes. Supersensitization can be achieved with selected combinations of spectral sensitizing dyes and other addenda such as stabilizers and antifoggants, development accelerators or inhibitors, coating aids, brighteners and antistatic agents. Any one of several mechanisms, as well as compounds which can be responsible for supersensitization, are discussed by Gilman, Photographic Science and Engineering, Vol. 18, 1974, pp. 418-430.

Spectral sensitizing dyes can also affect the emulsions in other ways. For example, spectrally sensitizing dyes can increase photographic speed within the spectral region of inherent sensitivity. Spectral sensitizing dyes can also function as antifoggants or stabilizers, development accelerators or inhibitors, reducing or nucleating agents, and halogen acceptors or electron acceptors, as disclosed in Brooker et al U.S. Patent 2,131,038, Illingsworth et al U.S. Patent

3,501,310, Webster et al U.S. Patent 3,630,749, Spence et al U.S. Patent 3,718,470 and Shiba et al U.S. Patent 3,930,860.

Among useful spectral sensitizing dyes for sensitizing the emulsions of the invention are those found in U.K. Patent 742,112, Brooker U.S. Patents 1,846,300, '301, '302, '303, '304, 2,078,233 and 2,089,729, Brooker et al U.S. Patents 2,165,338, 2,213,238, 2,493,747, '748, 2,526,632, 2,739,964 (Reissue 24,292) , 2,778,823, 2,917,516, 3,352,857,

3,411,916 and 3,431,111, Sprague U.S. Patent 2,503,776, Nys et al U.S. Patent 3,282,933, Riester U.S. Patent 3,660,102, Kampfer et al U.S. Patent 3,660,103, Taber et al U.S. Patents 3,335,010, 3,352,680 and 3,384,486, Lincoln et al U.S. Patent 3,397,981, Fumia et al U.S. Patents 3,482,978 and 3,623,881, Spence et al U.S. Patent 3,718,470 and Mee U.S. Patent 4,025,349, the disclosures of which are here incorporated by reference. Examples of useful supersensitizing-dye combinations, of non-light-absorbing addenda which function as supersensitizers or of useful dye combinations are found in McFall et al U.S. Patent 2,933,390, Jones et al U.S. Patent 2,937,089, Motter U.S. Patent 3,506,443 and Schwan et al U.S. Patent 3,672,898, the disclosures of which are here incorporated by reference.

Spectral sensitizing dyes can be added at any stage during the emulsion preparation. They may be added at the beginning of or during precipitation as described by Wall, Photographi c Emulsions, American

Photographic Publishing Co., Boston, 1929, p. 65, Hill U.S. Patent 2,735,766, Philippaerts et al U.S. Patent 3,628,960, Locker U.S. Patent 4,183,756, Locker et al U.S. Patent 4,225,666 and Research Disclosure, Vol. 181, May, 1979, Item 18155, and Tani et al published European Patent Application EP 301,508. They can be added prior to or during chemical sensitization as described by Kofron et al U.S. Patent 4,439,520, Dickerson U.S. Patent 4,520,098, Maskasky U.S. Patent 4,435,501 and Philippaerts et al cited above. They can be added before or during emulsion washing as described by Asami et al published European Patent Application EP 287,100 and Metoki et al published European Patent Application EP 291,399. The dyes can be mixed in directly before coating as described by Collins et al U.S. Patent 2,912,343. Small amounts of iodide can be adsorbed to the emulsion grains to promote aggregation and adsorption of the spectral sensitizing dyes as described by Dickerson cited above. Postprocessing dye stain can be reduced by the proximity to the dyed emulsion layer of fine high-iodide grains as described by Dickerson. Depending on their solubility, the spectral-sensitizing dyes can be added to the emulsion as solutions in water or such solvents as methanol, ethanol, acetone or pyridine; dissolved in surfactant solutions as described by Sakai et al U.S. Patent 3,822,135; or as dispersions as described by Owens et al U.S. Patent 3,469,987 and Japanese published Patent Publication 24185/71. The dyes can be selectively adsorbed to particular crystallographic faces of the emulsion grain as a means of restricting chemical sensitization centers to other faces, as described by Mifune et al published European Patent Application EP 302,528. The spectral sensitizing dyes may be used in conjunction with poorly adsorbed luminescent dyes, as described by Miyasaka et al published European Patent Applications 270,079, 270,082 and 278,510.

The following illustrate specific spectral sensitizing dye selections: SS-1

Anhydro-5 ' -chloro-3 ' -di- (3-sulfopropyl)naphtho[1, 2-d] - thiazolothiacyanine hydroxide, sodium salt

SS-2 Anhydro-5 ' -chloro-3 ' -di- (3-sulfopropyl)naphtho[1, 2-d] - oxazolothiacyanine hydroxide, sodium salt SS-3 Anhydro-4,5-benzo-3 '-methyl-4' -phenyl-1- (3-sulfo- propyl)naphtho[1,2-d]thiazolothiazolocyanine hydroxide SS-4

1,1'-Diethylnaphtho[1,2-d]thiazolo-2'-cyanine bromide

SS-5 Anhydro-1, 1'-dimethyl-5,5'-di- (trifluoromethyl) -3- (4- sulfobuyl)-3 '- (2,2,2-trifluoroethyl)benzimidazolo- carbocyanine hydroxide

SS-6 Anhydro-3,3 '- (2-methoxyethyl) -5,5'-diphenyl-9-ethyloxa- carbocyanine, sodium salt

SS-7 Anhydro-ll-ethyl-l,l'-di- (3-sulfopropyl)naphtho[1,2-d]- oxazolocarbocyanine hydroxide, sodium salt

SS-8 Anhydro-5, 5' -dichloro-9-ethyl-3,3'-di- (3-sulfopropyl)- oxaselenacarbocyanine hydroxide, sodium salt SS-9

5, 6-Dichloro-3 ' ,3 *-dimethyl-1, 1' ,3-triethylbenzimid- azolo-3H-indolocarbocyanine bromide

SS-10 Anhydro-5, 6-dichloro-l, l-diethyl-3- (3-sulfopropyl- benzimidazolooxacarbocyanine hydroxide

SS-11 Anhydro-5, 5'-dichloro-9-ethyl-3,3 '-di- (2-sulfoethyl- carbamoylmethyl)thiacarbocyanine hydroxide, sodium salt SS-12

Anhydro-5' , 6' -dimethoxy-9-ethyl-5-phenyl-3- (3-sulfo- butyl) -3 '- (3-sulfopropyl)oxathiacarbocyanine hydroxide, sodium salt

SS-13 Anhydro-5, 5 ' -dichloro-9-ethyl-3- (3-phosphonopropyl) -3 ' - (3-sulfopropyl) thiacarbocyanine hydroxide

SS-14 Anhydro-3,3 ' -di- (2-carboxyethyl)-5,5' -dichloro-9-ethyl- thiacarbocyanine bromide

SS-15 Anhydro-5, 5 ' -dichloro-3- (2-carboxyethyl) -3 ' - (3-sulfo- propyl) thiacyanine sodium salt

SS-16 9- (5-Barbituric acid) -3, 5-dimethyl-3 ' -ethyltellurathia- carbocyanine bromide

SS-17 Anhydro-5, 6-methylenedioxy-9-ethyl-3-methyl-3 ' - (3- sulfopropyl) tellurathiacarbocyanine hydroxide SS-18

3-Ethyl-6, 6 ' -dimethyl-3 ' -pentyl-9.11-neopentylenethia- dicarbocyanine bromide

SS-19 Anhydro-3-ethyl-9, ll-neopentylene-3 ' - (3-sulfopropyl) - thiadicarbocyanine hydroxide

SS-20 Anhydro-3-ethyl-11, 13-neopentylene-3 ' - (3-sulfopropyl) - oxathiatricarbocyanine hydroxide, sodium salt

SS-21 Anhydro-5-chloro-9-ethyl-5 ' -phenyl-3 ' - (3-sulfobutyl) -3- (3-sulfopropyl)oxacarbocyanine hydroxide, sodium salt

SS-22 Anhydro-5, 5 ' -diphenyl-3, 3 ' -di- (3-sulfobutyl) -9-ethyl- oxacarbocyanine hydroxide, sodium salt

SS-23 Anhydro-5, 5 ' -dichloro-3, 3 ' -di- (3-sulfopropyl) -9-ethyl- thiacarbocyanine hydroxide, triethylammonium salt

SS-24 Anhydro-5, 5 ' -dimethyl-3, 3 ' -di- (3-sulfopropyl) -9-ethyl- thiacarbocyanine hydroxide, sodium salt

SS-25 Anhydro-5, 6-dichloro-l-ethyl-3- (3-sulfobutyl) -1 ' - (3- sulfopropyl)benzimidazolonaphtho[l,2-d] - thiazolocarbocyanine hydroxide, triethylammonium salt

SS-26 Anhydro-ll-ethyl-1, 1 '-di- (3-sulfopropyl)naphthfl, 2-d] - oxazolocarbocyanine hydroxide, sodium salt

SS-27 Anhydro-3, -diethyl-3 ' -methylsulfonylcarbamoylmethyl-5- phenyloxathiacarbocyanine p-toluenesulfonate

SS-28 Anhydro-6, 6 ' -dichloro-1, 1 ' -diethyl-3,3 ' -di- (3-sulfopro- pyl) -5,5' -bis (trifluoromethyDbenzimidazolo- carbocyanine hydroxide, sodium salt

SS-29 Anhydro-5' -chloro-5-phenyl-3, 3 ' -di- (3-sulfopropyl)- oxathiacyanine hydroxide, sodium salt

SS-30 Anhydro-5, 5 ' -dichloro-3, 3 ' -di- (3-sulfopropyl)thia- cyanine hydroxide, sodium salt

SS-31 3-Ethyl-5- [1, 4-dihydro-l- (4-sulfobutyl)pyridin-4- ylidene] rhodanine, triethylammonium salt

SS-32 l-Carboxyethyl-5- [2- (3-ethylbenzoxazolin-2-ylidene) - ethylidene] -3-phenylthiohydantoin SS-33

4- [2- ( (1, 4-Dihydro-l-dodecylpyridin-ylidene) ethyl¬ idene] -3-phenyl-2-isoxazolin-5-one

SS-34 5- (3-Ethylbenzoxazolin-2-ylidene) -3-phenylrhodanine SS-35 1, 3-Diethyl-5-{ [l-ethyl-3- (3-sulfopropyl)benzimid- azolin-2-ylidene] ethylidene}-2-thiobarbituric acid

SS-36 5- [2- (3-Ethylbenzoxazolin-2-ylidene) ethylidene]-1- methyl-2-dimethylamino-4-oxo-3-phenylimidazol- inium p-toluenesulfonate

SS-37 5- [2- (5-Carboxy-3-methylbenzoxazolin-2-ylidene) ethyl¬ idene] -3-cyano-4-phenyl-l- (4-methylsulfonamido-3- pyrrolin-5-one

SS-38 2- [4- (Hexylsulfonamido)benzoylcyanomethine] -2- {2-{3- (2- methoxyethyl) -5- [ (2-methoxyethyl) sulfonamido] - benzoxazolin-2-ylidene}ethylidenejacetonitrile SS-39

3-Methyl-4- [2- (3-ethyl-5, 6-dimethylbenzotellurazolin-2- ylidene) ethylidene] -l-phenyl-2-pyrazolin-5-one

SS-40 3-Heptyl-l-phenyl-5-{4- [3- (3-sulfobutyl)-naphtho[1, 2- d] thiazolin] -2-butenylidene}-2-thiohydantoin

SS-41 1, 4-Phenylene-bis (2-aminovinyl-3-methyl-2-thiazolinium] dichloride

SS-42 Anhydro-4-{2- [3- (3-sulfopropyl) thiazolin-2-ylidene] - ethylidene}-2-{3- [3- (3-sulfopropyl)thiazolin-2- ylidene]propenyl-5-oxazolium, hydroxide, sodium salt

SS-43 3-Carboxymethyl-5-{3-carboxymethyl-4-oxo-5-methyll, 3, 4- thiadiazolin-2-ylidene) ethylidene] thiazolin-2- ylidene}rhodanine, dipotassium salt

SS-44 1, 3-Diethyl-5- [l-methyl-2- (3, 5-dimethylbenzotellur- azolin-2-ylidene) ethylidene] -2-thiobarbituric acid SS-45 3-Methyl-4- [ 2- (3-ethyl-5, 6-dimethylbenzotellurazolin-2- ylidene) -1-methylethylidene] -l-phenyl-2-pyrazolin- 5-one SS-46

1,3-Diethyl-5- [l-ethyl-2- (3-ethyl-5, 6-dimethoxybenzo- tellurazolin-2-ylidene) ethylidene] -2-thiobar- bituric acid

SS-47 3-Ethyl-5-{ [ (ethylbenzothiazolin-2-ylidene) -methyl]- [ (1, 5-dimethylnaphtho[1, 2-d] selenazolin-2-yli- dene)methyl]methylene}rhodanine

SS-48 5-{Bis [ (3-ethyl-5, 6-dimethylbenzothiazolin-2-ylidene) - methyl]methylene}-1,3-diethyl-barbituric acid

SS-49 3-Ethyl-5-{ [ (3-ethyl-5-methylbenzotellurazolin-2- ylidene)methyl] [1-ethylnaphtho[1, 2-d] -tellur- azolin-2-ylidene)methyl]methylene}rhodanine SS-50

Anhydro-5, 5 ' -diphenyl-3, 3 ' -di- (3-sulfopropyl) thia- cyanine hydroxide, triethylammonium salt

SS-51 Anhydro-5-chloro-5 ' -phenyl-3, 3 ' -di- (3-sulfopropyl)thia- cyanine hydroxide, triethylammonium salt

Instability which increases minimum density in negative-type emulsion coatings (i.e., fog) can be protected against by incorporation of stabilizers, antifoggants, antikinking agents, latent-image stabilizers and similar addenda in the emulsion and contiguous layers prior to coating. Most of the antifoggants effective in the emulsions of this invention can also be used in developers and can be classified under a few general headings, as illustrated by C.E.K. Mees, The Theory of the Photographic Process , 2Nd Ed., Macmillan, 1954, pp. 677-680.

To avoid such instability in emulsion coatings, stabilizers and antifoggants can be employed, such as halide ions (e.g., bromide salts); chloropalladates and chloropalladites as illustrated by Trivelli et al U.S. Patent 2,566,263; water-soluble inorganic salts of magnesium, calcium, cadmium, cobalt, manganese and zinc as illustrated by Jones U.S. Patent 2,839,405 and Sidebotham U.S. Patent 3,488,709; mercury salts as illustrated by Allen et al U.S. Patent 2,728,663; selenols and diselenides as illustrated by Brown et al U.K. Patent 1,336,570 and Pollet et al U.K. Patent 1,282,303; quaternary ammonium salts of the type illustrated by Allen et al U.S. Patent 2,694,716,

Brooker et al U.S. Patent 2,131,038, Graham U.S. Patent 3,342,596 and Arai et al U.S. Patent 3,954,478; azomethine desensitizing dyes as illustrated by Thiers et al U.S. Patent 3,630,744; isothiourea derivatives as illustrated by Herz et al U.S. Patent 3,220,839 and Knott et al U.S. Patent 2,514,650; thiazolidines as illustrated by Scavron U.S. Patent 3,565,625; peptide derivatives as illustrated by Maffet U.S. Patent 3,274,002; pyrimidines and 3-pyrazolidones as illustrated by Welsh U.S. Patent 3,161,515 and Hood et al U.S. Patent 2,751,297; azotriazoles and azotetrazoles as illustrated by Baldassarri et al U.S. Patent 3,925,086; azaindenes, particularly tetraazaindenes, as illustrated by Heimbach U.S. Patent 2,444,605, Knott U.S. Patent 2,933,388, Williams U.S. Patent 3,202,512, Research Disclosure, Vol. 134, June, 1975, Item 13452, and Vol. 148, August, 1976, Item 14851, and Nepker et al U.K. Patent 1,338,567; mercaptotetrazoles, -triazoles and -diazoles as illustrated by Kendall et al U.S. Patent 2,403,927, Kennard et al U.S. Patent 3,266,897, Research Disclosure, Vol. 116, December, 1973, Item 11684, Luckey et al U.S. Patent 3,397,987 and Salesin U.S. Patent 3,708,303; azoles as illustrated by Peterson et al U.S. Patent 2,271,229 and Research Disclosure, Item 11684, cited above; purines as illustrated by Sheppard et al U.S. Patent 2,319,090, Birr et al U.S. Patent 2,152,460, Research Disclosure, Item 13452, cited above, and Dostes et al French Patent 2,296,204, polymers of 1,3-dihydroxy(and/or 1,3-carbamoxy)-2- methylenepropane as illustrated by Saleck et al U.S. Patent 3,926,635 and tellurazoles, tellurazolines, tellurazolinium salts and tellurazolium salts as illustrated by Gunther et al U.S. Patent 4,661,438, aromatic oxatellurazinium salts as illustrated by

Gunther, U.S. Patent 4,581,330 and Przyklek-Elling et al U.S. Patents 4,661,438 and 4,677,202. High-chloride emulsions can be stabilized by the presence, especially during chemical sensitization, of elemental sulfur as described by Miyoshi et al European published Patent Application EP 294,149 and Tanaka et al European published Patent Application EP 297,804 and thiosulfonates as described by Nishikawa et al European published Patent Application EP 293,917. Among useful stabilizers for gold sensitized emulsions are water-insoluble gold compounds of benzothiazole, benzoxazole, naphthothiazole and certain merocyanine and cyanine dyes, as illustrated by Yutzy et al U.S. Patent 2,597,915, and sulfinamides, as illustrated by Nishio et al U.S. Patent 3,498,792.

Among useful stabilizers in layers containing poly(alkylene oxides) are tetraazaindenes, particularly in combination with Group VIII noble metals or resorcinol derivatives, as illustrated by Carroll et al U.S. Patent 2,716,062, U.K. Patent 1,466,024 and Habu et al U.S. Patent 3,929,486; quaternary ammonium salts of the type illustrated by Piper U.S. Patent 2,886,437; water-insoluble hydroxides as illustrated by Maffet U.S. Patent 2,953,455; phenols as illustrated by Smith U.S. Patents 2,955,037 and '038; ethylene diurea as illustrated by Dersch U.S. Patent 3,582,346; barbituric acid derivatives as illustrated by Wood U.S. Patent 3,617,290; boranes as illustrated by Bigelow U.S. Patent 3,725,078; 3-pyrazolidinones as illustrated by Wood U.K. Patent 1,158,059 and aldoxi ines, amides, anilides and esters as illustrated by Butler et al U.K. Patent 988,052.

The emulsions can be protected from fog and desensitization caused by trace amounts of metals such as copper, lead, tin, iron and the like by incorporating addenda such as sulfocatechol-type compounds, as illustrated by Kennard et al U.S. Patent 3,236,652; aldoximines as illustrated by Carroll et al U.K. Patent 623,448 and meta- and polyphosphates as illustrated by Draisbach U.S. Patent 2,239,284, and carboxylic acids such as ethylenediamine tetraacetic acid as illustrated by U.K. Patent 691,715.

Among stabilizers useful in layers containing synthetic polymers of the type employed as vehicles and to improve covering power are monohydric and polyhydric phenols as illustrated by Forsgard U.S. Patent 3,043,697; saccharides as illustrated by U.K. Patent 897,497 and Stevens et al U.K. Patent 1,039,471, and quinoline derivatives as illustrated by Dersch et al U.S. Patent 3,446,618.

Among stabilizers useful in protecting the emulsion layers against dichroic fog are addenda such as salts of nitron as illustrated by Barbier et al U.S. Patents 3,679,424 and 3,820,998; mercaptocarboxylic acids as illustrated by Willems et al U.S. Patent 3,600,178; and addenda listed by E. J. Birr, Stabilization of Photographic Silver Halide Emulsions, Focal Press, London, 1974, pp. 126-218.

Among stabilizers useful in protecting emulsion layers against development fog are addenda such as azabenzimidazoles as illustrated by Bloom et al U.K. Patent 1,356,142 and U.S. Patent 3,575,699, Rogers U.S. Patent 3,473,924 and Carlson et al U.S. Patent 3,649,267; substituted benzimidazoles, benzothiazoles, benzotriazoles and the like as illustrated by Brooker et al U.S. Patent 2,131,038, Land U.S. Patent 2,704,721, Rogers et al U.S. Patent 3,265,498; mercapto-substituted compounds, e.g., mercaptotetrazoles, as illustrated by Dimsdale et al U.S. Patent 2,432,864, Rauch et al U.S. Patent 3,081,170, Weyerts et al U.S. Patent 3,260,597, Grasshoff et al U.S. Patent 3,674,478 and Arond U.S. Patent 3,706,557; isothiourea derivatives as illustrated by Herz et al U.S. Patent 3,220,839, and thiodiazole derivatives as illustrated by von Konig U.S. Patent 3,364,028 and von Konig et al U.K. Patent 1,186,441.

Where hardeners of the aldehyde type are employed, the emulsion layers can be protected with antifoggants such as monohydric and polyhydric phenols of the type illustrated by Sheppard et al U.S. Patent 2,165,421; nitro-substituted compounds of the type disclosed by Rees et al U.K. Patent 1,269,268; poly(alkylene oxides) as illustrated by Valbusa U.K. Patent 1,151,914, and mucohalogenic acids in combination with urazoles as illustrated by Allen et al U.S. Patents 3,232,761 and 3,232,764, or further in combination with maleic acid hydrazide as illustrated by Rees et al U.S. Patent 3,295,980. To protect emulsion layers coated on linear polyester supports, addenda can be employed such as parabanic acid, hydantoin acid hydrazides and urazoles as illustrated by Anderson et al U.S. Patent 3,287,135, and piazines containing two symmetrically fused

6-member carbocyclic rings, especially in combination with an aldehyde-type hardening agent, as illustrated in Rees et al U.S. Patent 3,396,023.

Kink desensitization of the emulsions can be reduced by the incorporation of thallous nitrate as illustrated by Overman U.S. Patent 2,628,167; compounds, polymeric latices and dispersions of the type disclosed by Jones et al U.S. Patents 2,759,821 and '822; azole and mercaptotetrazole hydrophilic colloid dispersions of the type disclosed by Research Disclosure, Vol. 116, December, 1973, Item 11684; plasticized gelatin compositions of the type disclosed by Milton et al U.S. Patent 3,033,680; water-soluble interpolymers of the type disclosed by Rees et al U.S. Patent 3,536,491; polymeric latices prepared by emulsion polymerization in the presence of poly (alkylene oxide) as disclosed by Pearson et al U.S. Patent 3,772,032, and gelatin graft copolymers of the type disclosed by Rakoczy U.S. Patent 3,837,861. Where the photographic element is to be processed at elevated bath or drying temperatures, as in rapid access processors, pressure desensitization and/or increased fog can be controlled by selected combinations of addenda, vehicles, hardeners and/or processing conditions as illustrated by Abbott et al U.S. Patent 3,295,976, Barnes et al U.S. Patent 3,545,971, Salesin U.S. Patent 3,708,303, Yamamoto et al U.S. Patent 3,615,619, Brown et al U.S. Patent 3,623,873, Taber U.S. Patent 3,671,258, Abele U.S. Patent 3,791,830, Research Disclosure, Vol. 99, July, 1972, Item 9930, Florens et al U.S. Patent 3,843,364, Priem et al U.S. Patent 3,867,152, Adachi et al U.S. Patent 3,967,965 and Mikawa et al U.S. Patents 3,947,274 and 3,954,474. In addition to increasing the pH or decreasing the pAg of an emulsion and adding gelatin, which are known to retard latent-image fading, latent- image stabilizers can be incorporated, such as amino acids, as illustrated by Ezekiel U.K. Patents 1,335,923, 1,378,354, 1,387,654 and 1,391,672, Ezekiel et al U.K. Patent 1,394,371, Jefferson U.S. Patent 3,843,372, Jefferson et al U.K. Patent 1,412,294 and Thurston U.K. Patent 1,343,904; carbonyl-bisulfite addition products in combination with hydroxybenzene or aromatic amine developing agents as illustrated by Seiter et al U.S. Patent 3,424,583; cycloalkyl-1, 3- diones as illustrated by Beckett et al U.S. Patent 3,447,926; enzymes of the catalase type as illustrated by Matejec et al U.S. Patent 3,600,182; halogen- substituted hardeners in combination with certain cyanine dyes as illustrated by Kumai et al U.S. Patent 3,881,933; hydrazides as illustrated by Honig et al U.S. Patent 3,386,831; alkenyl benzothiazolium salts as illustrated by Arai et al U.S. Patent 3,954,478; hydroxy-substituted benzylidene derivatives as illustrated by Thurston U.K. Patent 1,308,777 and Ezekiel et al U.K. Patents 1,347,544 and 1,353,527; mercapto-substituted compounds of the type disclosed by Sutherns U.S. Patent 3,519,427; metal-organic complexes of the type disclosed by Matejec et al U.S. Patent 3,639,128; penicillin derivatives as illustrated by Ezekiel U.K. Patent 1,389,089; propynylthio derivatives of benzimidazoles, pyrimidines, etc., as illustrated by von Konig et al U.S. Patent 3,910,791; combinations of iridium and rhodium compounds as disclosed by Yamasue et al U.S. Patent 3,901,713; sydnones or sydnone imines as illustrated by Noda et al U.S. Patent 3,881,939; thiazolidine derivatives as illustrated by Ezekiel U.K. Patent 1,458,197 and thioether-substituted imidazoles as illustrated by Research Disclosure, Vol. 136, August, 1975, Item 13651.

Apart from the features that have been specifically discussed the tabular grain emulsion preparation procedures, the tabular grains that they produce, and their further use in photography can take any convenient conventional form. Substitution for conventional emulsions of the same or similar silver halide composition is generally contemplated, with substitution for silver halide emulsions of differing halide composition, particularly tabular grain emulsions, being also feasible in many types of photographic applications. The low levels of native blue sensitivity of the high chloride {100} tabular grain emulsions of the invention allows the emulsions to be employed in any desired layer order arrangement in multicolor photographic elements, including any of the layer order arrangements disclosed by Kofron et al U.S. Patent 4,439,520, the disclosure of which is here incorporated by reference, both for layer order arrangements and for other conventional features of photographic elements containing tabular grain emulsions. Conventional features are further illustrated by the following incorporated by reference disclosures: ICBR-1 Research Disclosure, Vol. 308,

December 1989, Item 308,119; ICBR-2 .Research Disclosure, Vol. 225, January

1983, Item 22,534; ICBR-3 Wey et al U.S. Patent 4,414,306, issued Nov. 8, 1983; ICBR-4 Solbei-g et al U.S. Patent 4,433,048, issued Feb. 21, 1984; ICBR-5 Wilgus et al U.S. Patent 4,434,226, issued Feb. 28, 1984; ICBR-6 Maskasky U.S. Patent 4,435,501, issued

Mar. 6, 1984; ICBR-7 Maskasky U.S. Patent 4,643,966, issued

Feb. 17, 1987; ICBR-8 Daubendiek et al U.S. Patent

4,672,027, issued Jan. 9, 1987; ICBR-9 Daubendiek et al U.S. Patent

4,693,964, issued Sept. 15, 1987; ICBR-10 Maskasky U.S. Patent 4,713,320, issued

Dec. 15, 1987; ICBR-11 Saitou et al U.S. Patent 4,797,354, issued Jan. 10, 1989; ICBR-12 Ikeda et al U.S. Patent 4,806,461, issued Feb. 21, 1989; ICBR-13 Makino et al U.S. Patent 4,853,322, issued Aug. 1, 1989; and ICBR-14 Daubendiek et al U.S. Patent

4,914,014, issued Apr. 3, 1990. Photographic elements containing high chloride {100} tabular grain emulsions prepared according to this invention can be imagewise-exposed with various forms of energy which encompass the ultraviolet and visible (e.g., actinic) and infrared regions of the electromagnetic spectrum, as well as electron-beam and beta radiation, gamma ray, X-ray, alpha particle, neutron radiation and other forms of corpuscular and wave-like radiant energy in either noncoherent (random phase) forms or coherent (in phase) forms as produced by lasers. Exposures can be monochromatic, orthochromatic or panchromatic.

Imagewise exposures at ambient, elevated or reduced temperatures and/or pressures, including high- or low- intensity exposures, continuous or intermittent exposures, exposure times ranging from minutes to relatively short durations in the millisecond to microsecond range and solarizing exposures, can be employed within the useful response ranges determined by conventional sensitometric techniques, as illustrated by T. H. James, The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, Chapters 4, 6, 17, 18 and 23.

Examples

The invention can be better appreciated by reference to the following examples. Throughout the examples the acronym APMT is employed to designate 1- (3-acetamidophenyl)-5-mercaptotetrazole. The term "low methionine gelatin" is employed, except as otherwise indicated, to designate gelatin that has been treated with an oxidizing agent to reduce its methionine content to less than 30 micromoles per gram. The acronym DW is employed to indicate distilled water. The acronym mppm is employed to indicate molar parts per million, whereas ppm is employed to parts per million on a weight basis. The term "Rsens" is in some instances employed to indicate relative sensitivity.

Examples JL and 2

These examples demonstrate the preparation of emulsions satisfying the requirements of the invention employing a dual-zone growth process in which the growth reactants are premixed in a continuous reactor prior to being added to the growth reactor, to yield tabular grains with an ECD greater than 2 μm.

Example JL

To a stirred reaction vessel containing a 2945 mL solution that is 1.77 percent by weight bone gelatin, 0.0056 M sodium chloride, 1.86 x 10"⁴ M potassium iodide and at 55°C and pH 6.5, 15 mL of a 4.0 M silver nitrate solution and 15 mL of a 4.0 M sodium chloride solution were each added concurrently at a rate of 30 mL/min.

The mixture was then held for 5 minutes during which 7000 mL of distilled water were added and the temperature was raised to 65°C, while the pCl was adjusted to 2.15 and the pH to 6.5. Following the hold, the size of the resulting grains was increased through growth using a dual-zone process. In this process, a solution of 0.67 M silver nitrate was premixed with a 0.67 M solution of sodium chloride and a solution of 0.5 percent by weight bone gelatin at a pH of 6.5, in a continuous reactor with a total volume of 30 mL, which was well-mixed. The effluent from this premixing reactor was then added to the original reaction vessel, which during this step acted as a growth reactor. During the growth step the fine crystals from the continuous reactor were ripened onto the original crystals through Ostwald ripening. The total suspension volume of the growth reactor during this growth step was maintained constant at 13.5 L using ultrafiltration. The flow rates of the 0.67 M silver nitrate solution and the 0.67 M sodium chloride solution were linearly increased from 20 to 80 mL/min, 150 mL/min and 240 mL/min in 25 minute intervals. The flow rate of the 0.5 percent gelatin reactant was maintained constant at 500 mL/min. The continuous reactor in which these reactants were premixed was kept at 30°C and a pCl of 2.45, while the growth reactor was maintained at a temperature of 65°C, a pCl of 2.15, and a pH of 6.5.

This procedure resulted in 6 moles of a high aspect ratio tabular grain iodochloride emulsion containing 0.01 mole % iodide. More than 90% of the total projected grain area was provided by tabular grains having {100} major faces, an average ECD of 2.55 μ , and an average thickness of 0.165 μm. Therefore, the tabular grain population had an average aspect ratio of 15.5 and an average tabularity of 93.7.

Example 2. Silver iodochloride nuclei were formed in a

30 mL well-mixed, continuous reactor by mixing a 0.447 M silver nitrate solution (at 100 mL/min) with a 0.487 M sodium chloride and 0.00377 M potassium iodide solution (at 100 mL/min) and a 2.0 percent by weight bone gelatin solution (at 1 L/min) at a pCl of 2.3 and a temperature of 40°C. The resulting mixture containing the nuclei was transferred to a stirred semi-batch reactor for 1.5 min. The semi-batch reactor was maintained at 65°C and a constant volume of 13.5 L (using ultrafiltration) and was initially at a pCl of 2.15, a pH of 6.5 and a bone gelatin concentration of 0.37 percent by weight. During the nuclei transfer from the continuous reactor to the semi-batch reactor the pCl of the latter was maintained at 2.15 by the addition of a 1 M sodium chloride solution.

After holding for 5 min, growth of the initial nuclei was achieved by the dual-zone process as follows. A solution of 0.67 M silver nitrate, a solution of 0.67 M sodium chloride and a solution of 0.5 percent by weight bone gelatin at a pH of 6.5 were premixed in the 30 mL continuous reactor, and then transferred to the semi-batch reactor. Growth occurred by Ostwald ripening whereby the crystals from the continuous reactor were dissolved in the semi-batch reactor and the original nuclei increased in size. The total suspension volume of the semi-batch reactor was maintained constant at 13.5 L during this step, as during the nucleation step.

During the growth step the flow rates of the 0.67 M silver nitrate solution and the 0.67 M sodium chloride solution were linearly increased from 20 to 80 mL/min, 150 mL/min and 240 mL/min in 25 minute intervals. The flow rate of the 0.5 percent gelatin reactant was maintained constant at 500 mL/min. The continuous reactor in which these reactants were premixed was kept at 30°C and a pCl of 2.45, while the growth reactor was maintained at a temperature of 65°C, a pCl of 2.15, and a pH of 6.5.

This procedure resulted in 6 moles of a large, high aspect ratio tabular grain iodochloride emulsion containing 0.01 mole % iodide. More than 80% of the total projected grain area was provided by tabular grains having {100} major faces, an average ECD of 2.28 μm, and an average thickness of 0.195 μm. Therefore, the tabular grain population had an average aspect ratio of 11.7 and an average tabularity of 60.0.

Example 3

This example demonstrates the preparation of an ultrathin tabular grain silver iodochloride emulsion satisfying the requirements of this invention.

A 2030 mL solution containing 1.75% by weight low methionine gelatin, 0.011 M sodium chloride and 1.48 x 10^~4 M potassium iodide was provided in a stirred reaction vessel. The contents of the reaction vessel were maintained at 40°C and the pCl was 1.95. While this solution was vigorously stirred, 30 mL of 1.0 M silver nitrate solution and 30 mL of a 0.99 M sodium chloride and 0.01 M potassium iodide solution were added simultaneously at a rate of 30 mL/min each. This achieved grain nucleation to form crystals with an initial iodide concentration of 2 mole percent, based on total silver.

The mixture was then held 10 minutes with the temperature remaining at 40°C. Following the hold, a 1.0 M silver nitrate solution and a 1.0 M NaCl solution were then added simultaneously at 2 mL/min for 40 minutes with the pCl being maintained at 1.95.

The resulting emulsion was a tabular grain silver iodochloride emulsion containing 0.5 mole percent iodide, based on silver. Fifty percent of total grain projected area was provided by tabular grains having {100} major faces having an average ECD of 0.84 μm and an average thickness of 0.037 μm, selected on the basis of an aspect ratio rank ordering of all {100} tabular grains having a thickness of less than 0.3 μm and a major face edge length ratio of less than 10. The selected tabular grain population had an average aspect ratio (ECD/t) of 23 and an average tabularity (ECD/t²) of 657. The ratio of major face edge lengths of the selected tabular grains was 1.4. Seventy two percent of total grain projected area was made up of tabular grains having {100} major faces and aspect ratios of at least 7.5. These tabular grains had a mean ECD of 0.75 μm, a mean thickness of 0.045 μm, a mean aspect ratio of 18.6 and a mean tabularity of 488.

A representative sample of the grains of the emulsion is shown in Figure 1. Examole 4 (Comparative)

This emulsion demonstrates the importance of iodide in the precipitation of the initial grain population (nucleation) . This emulsion was precipitated identically to that of Example 1, except no iodide was intentionally added.

The resulting emulsion consisted primarily of cubes and very low aspect ratio rectangular grains ranging in size from about 0.1 to 0.5 μm in edge length. A small number of large rods and high aspect ratio {100} tabular grains were present, but did not constitute a useful quantity of the grain population.

A representative sample of the grains of this emulsion is shown in Figure 2. Example 5

This example demonstrates an emulsion according to the invention in which 90% of the total grain projected area is comprised of tabular grains with {100} major faces and aspect ratios of greater than 7.5.

A 2030 mL solution containing 3.52% by weight low methionine gelatin, 0.0056 M sodium chloride and 1.48 x 10^~ M potassium iodide was provided in a stirred reaction vessel. The contents of the reaction vessel were maintained at 40°C and the pCl was 2.25.

While this solution was vigorously stirred, 30 mL of 2.0 M silver nitrate solution and 30 mL of a 1.99 M sodium chloride and 0.01 M potassium iodide solution were added simultaneously at a rate of 60 mL/min each. This achieved grain nucleation to form crystals with an initial iodide concentration of 1 mole percent, based on total silver.

The mixture was then held 10 minutes with the temperature remaining at 40°C. Following the hold, a 0.5 M silver nitrate solution and a 0.5 M NaCl solution were then added simultaneously at 8 mL/min for 40 minutes with the pCl being maintained at 2.35. The 0.5 M AgNθ3 solution and the 0.5 M NaCl solution were then added simultaneously with a ramped linearly increasing flow from 8 mL per minute to 16 mL per minute over 130 minutes with the pCl maintained at 2.35.

The resulting emulsion was a tabular grain silver iodochloride emulsion containing 0.06 mole percent iodide, based on silver. Fifty percent of total grain projected area was provided by tabular grains having {100} major faces having an average ECD of 1.86 μm and an average thickness of 0.082 μm, selected on the basis of an aspect ratio rank ordering of all {100} tabular grains having a thickness of less than 0.3 μm and a major face edge length ratio of less than 10. The selected tabular grain population had an average aspect ratio (ECD/t) of 24 and an average tabularity (ECD/t²) of 314. The ratio of major face edge lengths of the selected tabular grains was 1.2.

Ninety three percent of total grain projected area was made up of tabular grains having {100} major faces and aspect ratios of at least 7.5. These tabular grains had a mean ECD of 1.47 μm, a mean thickness of 0.086 μm, a mean aspect ratio of 17.5 and a mean tabularity of 222.

Example 6

This example demonstrates an emulsion prepared similarly as the emulsion of Example 3, but an initial 0.08 mole percent iodide and a final 0.04% iodide.

A 2030 mL solution containing 3.52% by weight low methionine gelatin, 0.0056 M sodium chloride and 3.00 x 10^~5 M potassium iodide was provided in a stirred reaction vessel. The contents of the reaction vessel were maintained at 40°C and the pCl was 2.25.

While this solution was vigorously stirred, 30 mL of 5.0 M silver nitrate solution and 30 mL of a 4.998 M sodium chloride and 0.002 M potassium iodide solution were added simultaneously at a rate of 60 mL/min each. This achieved grain nucleation to form crystals with an initial iodide concentration of 0.08 mole percent, based on total silver. The mixture was then held 10 minutes with the temperature remaining at 40°C. Following the hold, a 0.5 M silver nitrate solution and a 0.5 M sodium chloride solution were then added simultaneously at 8 mL/min for 40 minutes with the pCl being maintained at 2.95.

The resulting emulsion was a tabular grain silver iodochloride emulsion containing 0.04 mole percent iodide, based on silver. Fifty percent of the total grain projected area was provided by tabular grains having {100} major faces having an average ECD of 0.67 μm and an average thickness of 0.035 μm, selected on the basis of an aspect ratio rank ordering of all {100} tabular grains having a thickness of less than 0.3 μm and a major face edge length ratio of less than 10. The selected tabular grain population had an average aspect ratio (ECD/t) of 20 and an average tabularity (ECD/t²) of 651. The ratio of major face edge lengths of the selected tabular grains was 1.9. Fifty two percent of total grain projected area was made up of tabular grains having {100} major faces and aspect ratios of at least 7.5. These tabular grains had a mean ECD of 0.63 μm, a mean thickness of 0.036 μm, a mean aspect ratio of 18.5 and a mean tabularity of 595. Example 7

This example demonstrates an emulsion in which the initial grain population contained 6.0 mole percent iodide and the final emulsion contained 1.6% iodide.

A 2030 mL solution containing 3.52% by weight low methionine gelatin, 0.0056 M sodium chloride and 3.00 x 10^~5 M potassium iodide was provided in a stirred reaction vessel. The contents of the reaction vessel were maintained at 40°C and the pCl was 2.25. While this solution was vigorously stirred, 30 mL of 1.0 M silver nitrate solution and 30 mL of a 0.97 M sodium chloride and 0.03 M potassium iodide solution were added simultaneously at a rate of 60 mL/min each. This achieved grain nucleation to form crystals with an initial iodide concentration of 6.0 mole percent, based on total silver.

The mixture was then held 10 minutes with the temperature remaining at 40°C. Following the hold, a 1.00 M silver nitrate solution and a 1.00 M sodium chloride solution were then added simultaneously at 2 mL/min for 40 minutes with the pCl being maintained at 2.35.

The resulting emulsion was a tabular grain silver iodochloride emulsion containing 1.6 mole percent iodide, based on silver. Fifty percent of total grain projected area was provided by tabular grains having {100} major faces having an average ECD of 0.57 μm and an average thickness of 0.036 μm, selected on the basis of an aspect ratio rank ordering of all {100} tabular grains having a thickness of less than 0.3 μm and a major face edge length ratio of less than 10. The selected tabular grain population had an average aspect ratio (ECD/t) of 16.2 and an average tabularity (ECD/t²) of 494. The ratio of major face edge lengths of the selected tabular grains was 1.9. Sixty two percent of total grain projected area was made up of tabular grains having {100} major faces and aspect ratios of at least 7.5. These tabular grains had a mean ECD of 0.55 μm, a mean thickness of 0.041 μm, a mean aspect ratio of 14.5 and a mean tabularity of 421.

Example 8

This example demonstrates an ultrathin high aspect ratio {100} tabular grain emulsion in which 2 mole percent iodide is present in the initial population and additional iodide is added during growth to make the final iodide level 5 mole percent.

A 2030 mL solution containing 1.75% by weight low methionine gelatin, 0.0056 M sodium chloride and 1.48 x 10^~ M potassium iodide was provided in a stirred reaction vessel. The contents of the reaction vessel were maintained at 40°C and the pCl was 2.2.

While this solution was vigorously stirred, 30 mL of 1.0 M silver nitrate solution and 30 mL of a 0.99 M sodium chloride and 0.01 M potassium iodide solution were added simultaneously at a rate of 90 mL/min each. This achieved grain nucleation to form crystals with an initial iodide concentration of 2 mole percent, based on total silver.

The mixture was then held 10 minutes with the temperature remaining at 40°C. Following the hold, a 1.00 M silver nitrate solution and a 1.00 M sodium chloride solution were then added simultaneously at 8 mL/min while a 3.375 X 10^~2 M potassium iodide was simultaneously added at 14.6 mL/min for 10 minutes with the pCl being maintained at 2.35.

The resulting emulsion was a tabular grain silver iodochloride emulsion containing 5 mole percent iodide, based on silver. Fifty percent of total grain projected area was provided by tabular grains having {100} major faces having an average ECD of 0.58 μm and an average thickness of 0.030 μm, selected on the basis of an aspect ratio rank ordering of all {100} tabular grains having a thickness of less than 0.3 μm and a major face edge length ratio less than 10. The selected tabular grain population had an average aspect ratio (ECD/t) of 20.6 and an average tabularity (ECD/t²) of 803. The ratio of major face edge lengths of the selected tabular grains was 2. Eighty seven percent of total grain projected area was made up of tabular grains having {100} major faces and aspect ratios of at least 7.5. These tabular grains had a mean ECD of 0.54 μm, a mean thickness of 0.033 μm, a mean aspect ratio of 17.9 and a mean tabularity of 803.

Example 9

This example demonstrates a high aspect ratio

{100} tabular emulsion where 1 mole percent iodide is present in the initial grain population and 50 mole percent bromide is added during growth to make the final emulsion 0.3 mole percent iodide, 36 mole percent bromide and 63.7 mole percent chloride.

A 2030 mL solution containing 3.52% by weight low methionine gelatin, 0.0056 M sodium chloride and 1.48 x 10^~ M potassium iodide was provided in a stirred reaction vessel. The contents of the reaction vessel were maintained at 40°C and the pCl was 2.25. While this solution was vigorously stirred,

30 mL of 1.0 M silver nitrate solution and 30 mL of a 0.99 M sodium chloride and 0.01 M potassium iodide solution were added simultaneously at a rate of 60 mL/min each. This achieved grain nucleation.

The mixture was then held 10 minutes with the temperature remaining at 40°C. Following the hold, a 0.5 M silver nitrate solution and a 0.25 M sodium ^• chloride and 0.25 M sodium bromide solution were then added simultaneously at 8 mL/min for 40 minutes with the pCl being maintained at 2.60 to form crystals with an initial iodide concentration of 2 mole percent, based on total silver.

The resulting emulsion was a tabular grain silver iodobromochloride emulsion containing 0.27 mole percent iodide and 36 mole percent bromide, based on silver, the remaining halide being chloride. Fifty percent of total grain projected area was provided by tabular grains having {100} major faces having an average ECD of 0.4 μm and an average thickness of 0.032 μm, selected on the basis of an aspect ratio rank ordering of all {100} tabular grains having a thickness of less than 0.3 μm and a major face edge length ratio of less than 10. The selected tabular grain population had an average aspect ratio (ECD/t) of 12.8 and an average tabularity (ECD/t²) of 432. The ratio of major face edge lengths of the selected tabular grains was 1.9. Seventy one percent of total grain projected area was made up of tabular grains having {100} major faces and aspect ratios of at least 7.5. These tabular grains had a mean ECD of 0.38 mm, a mean thickness of 0.034 μm, a mean aspect ratio of 11.3 and a mean tabularity of 363.

Example 10

This example demonstrates the preparation of an emulsion satisfying the requirements of the invention employing phthalated gelatin as a peptizer. To a stirred reaction vessel containing a 310 mL solution that is 1.0 percent by weight phthalated gelatin, 0.0063 M sodium chloride and 3.1 X 10^~4 M KI at 40°C, 6.0 mL of a 0.1 M silver nitrate aqueous solution and 6.0 mL of a 0.11 M sodium chloride solution were each added concurrently at a rate of 6 mL/min.

The mixture was then held 10 minutes with the temperature remaining at 40°C. Following the hold, the silver and salt solutions were added simultaneously with a linearly accelerated flow from 3.0 mL/min to 9.0 mL/min over 15 minutes with the pCl of the mixture being maintained at 2.7.

The resulting emulsion was a high aspect ratio tabular grain silver iodochloride emulsion. Fifty percent of total grain projected area was provided by tabular grains having {100} major faces having an average ECD of 0.37 μ and an average thickness of 0.037 μ , selected on the basis of an aspect ratio rank ordering of all {100} tabular grains having a thickness of less than 0.3 μm and a major face edge length ratio of less than 10. The selected tabular grain population had an average aspect ratio (ECD/t) of 10 and an average tabularity (ECD/t²) of 330. Seventy percent of total grain projected area was made up of tabular grains having {100} major faces and aspect ratios of at least 7.5. These tabular grains had a mean ECD of 0.3 μm, a mean thickness of C.04 μm, and a mean tabularity of 210. Electron diffraction examination of the square and rectangular surfaces of the tabular grains confirmed major face {100} crystallographic orientation.

Example 11 This example demonstrates the preparation of an emulsion satisfying the requirements of the invention employing an unmodified bone gelatin as a peptizer.

To a stirred reaction vessel containing a 2910 mL solution that is 0.69 percent by weight bone -71- gelatin, 0.0056 M sodium chloride, 1.86 x 10^-4 M KI and at 55°C and pH 6.5, 60 mL of a 4.0 M silver nitrate solution and 60.0 mL of a 4.0 M sodium chloride solution were each added concurrently at a rate of 120 mL/min.

The mixture was then held for 5 minutes during which a 5000 mL solution that is 16.6 g/L of low methionine gelatin was added and the pH was adjusted to 6.5 and the pCl to 2.25. Following the hold, the silver and salt solutions were added simultaneously with a linearly accelerated flow from 10 mL/min to 25.8 mL/min over 63 minutes with the pCl of the mixture being maintained at 2.25.

The resulting emulsion was a high aspect ratio tabular grain silver iodochloride emulsion containing 0.01 mole % iodide. About 65% of the total projected grain area was provided by tabular grains having an average diameter of 1.5 μ and an average thickness of 0.18 μm.

Example 12

This example compares the photographic performance of a {100} silver chloride tabular emulsion according to the invention to a silver chloride cubic grain emulsion of similar average grain volume.

Emulsion A. Silver iodochloride tabular emulsion with {100} major faces

Precipitation (a remake of the Example 3 emulsion scaled up 3X)

A 6090 ml solution containing 3.52% by weight of low methionine gelatin, 0.0056 M sodium chloride and 1.48 x lO^-4 potassium iodide was provided in a stirred reaction vessel at 40°C. While the solution was vigorously stirred, 90 mL of 2.0 M silver nitrate and 90 mL of a 1.99 M sodium chloride and 0.01 M potassium iodide solution were added simultaneously at a rate of 180 mL/min each. The mixture was then held for 10 minutes with the temperature remaining at 40°C. Following the hold, a 0.5 M silver nitrate solution and a 0.5 M sodium chloride solution were added simultaneously at 24 mL/min for 40 minutes followed by a linear acceleration from 24 mL/min to 48 mL/min over 130 minutes, while maintaining the pCl at 2.35. The pCl was then adjusted to 1.30 with sodium chloride then washed using ultrafiltration to a pCl of 2.0 then adjusted to a pCl of 1.65 with sodium chloride. The resulting emulsion was a tabular grain silver chloride emulsion contained 0.06 mole percent iodide and had a mean equivalent circular grain diameter of 1.45 μm and a mean grain thickness of 0.13 μm.

Sensitization

An optimum green light sensitization was found for Emulsion A by conducting numerous small scale finishing experiments where the level of sensitizing dye, sodium thiosulfate pentahydrate, aurous dithiosulfate dihydrate and the hold time at 65°C were varied. ^• The optimum finish was as follows: to a 0.5 mole portion of Emulsion A melted at 40°C and well stirred, 0.800 mmol/mole of green light sensitizing dye A was added followed by a 20 minute hold. To this was added 0.10 mg/mole of sodium thiosulfate pentahydrate and 0.20 mg/mole of sodium aurous dithiosulfate dihydrate. The temperature was then increased to 65°C over 9 minutes and then held for 4 minutes at 65°C and rapidly cooled to 40°C. Sensitizing Dve A

Emulsion B. Silver chloride cubic grain emulsion (Control)

Precipitation

A onodisperse silver chloride cube with a cubic edge length of 0.59 μm was prepared by simultaneous addition of 3.75 M silver nitrate and 3.75 M sodium chloride to a well stirred solution containing 8.2 g/1 of sodium chloride, 28.2 g/1 of bone gelatin and 0.212 g/liter of 1,8-dithiadioctanediol while maintaining the temperature at 68.3°C and the pCl at 1.0. The temperature was reduced to 40°C and the emulsion was washed by ultrafiltration to a pCl of 2.0, then adjusted to a pCl of 1.65 with sodium chloride.

Sensitization

An optimum green light sensitization was found in the same manner as described for Emulsion A. The conditions for the optimum were as follows: to a 0.05 mole quantity of Emulsion B melted at 40^CC and well stirred, 0.2 mmol/mole of sensitizing dye A was added followed by a 20 minute hold. To this was added 0.25 mg/mole of sodium thiosulfate pentahydrate and 0.50 mg/mole of sodium aurous dithiosulfate dihydrate. The temperature was then increased to 65°C over 9 minutes and held for 10 minutes followed by rapid cooling to 40°C.

Photographic Performance

Each of the sensitized emulsions was coated on antihalation support at 0.85 g/m² of silver along with 1.1 g/m² of cyan dye-forming coupler C and 2.7 g/m² of gelatin. This was overcoated with 1.6 g/m² of gelatin and hardened with 1.7 weight percent, based on total gelatin, of bis(vinylsulfonylmethyl)ether. The coatings were evaluated for intrinsic sensitivity by exposing for 0.02 seconds in a step wedge sensitometer with the 365 nm line of a mercury vapor lamp as the light source. Sensitivity to green light was measured by exposing the coatings for 0.02 seconds using a step wedge sensitometer with a 3000°K tungsten lamp filtered to simulate a Daylight V light source and filtered to transmit only green and red light by a Kodak Wratten™ 9 filter (transmitting wavelengths longer than 450 nm) . The coatings were processed using the Kodak Flexicolor ™ C-41 color negative process, described in Brit. J. Photog. Annual 1988, pl96-198 , and the dye density was measured using status M red filtration.

Coupler C

OH

^C5^H11 The photographic results are summarized in Table I.

Table I

365 line exposure Wratten ™ 9 exposure

Emulsion Dmin Rsens contrast Dmin Rsens contrast

Emulsion A (tab.) unsensitized green sensitized .22 371 2 . 08

Emulsion B (cubic) unsensitized green sensitized 16 128 2 . 86

Table I shows that for intrinsic sensitivity as measured by the 365 line exposure, both Emulsions A and B are very similar as would be expected based on their similar grain volume. Comparing the green light sensitivity as measured by the Wratten ™ 9 exposures shows that the tabular emulsion is 2.9 times more sensitive to green light than the cubic emulsion. This clearly shows the advantage of the tabular morphology.

Example 13

This example describes the sensitization and photographic performance of a {100} silver chloride tabular emulsion and a silver chloride cubic emulsion of similar average grain volume sensitized using gold sulfide and a blue spectral sensitizing dye, and compared in low silver coatings on a resin coated paper support.

Precipitation of Silver chloride tabular emulsion with {100} major faces

This emulsion was prepared in an identical fashion to the {100} silver chloride tabular emulsion described in Example 10. Precipitation of Silver chloride cubic emulsion

This emulsion was prepared in a similar fashion to the cubic emulsion described in Example 10, except the ripener 1,8-dithiadioctanediol was omitted and flow rates and precipitation time were adjusted to achieve the same size emulsion.

Sensitization

Both emulsions were sensitized to blue light using the following procedures. A quantity of each emulsion was melted at 40°C, 660mg/mole Ag of sensitizing dye B was added to the {100} tabular emulsion and 220 mg/mole of the same dye was added to the cubic emulsion based on their specific surface area, followed by a 20 minute hold. 2.0 mg/mole of aurous sulfide was added to each emulsion followed by a 5 minute hold. The temperature was then raised to 60°C and held for 30 minutes after which 90 mg/mole of APMT was added and the emulsion was chill set.

Photographic Performance Each of the sensitized emulsions was coated on resin coated paper support at 0.28 g/m² of silver along with 1.1 g/m² of yellow dye forming coupler B and 0.82 g/m² of gelatin. The coatings were evaluated for intrinsic sensitivity by exposing for 0.1 seconds in a step wedge sensitometer with the 365 nm line of a mercury vapor lamp as the light source. Sensitivity to white light was measured by exposing the coatings for 0.1 seconds using a step wedge sensitometer with a 3000°K tungsten lamp. The coatings were processed using a standard RA-4 color paper process as described in Research Disclosure, Vol. 308, p.933, 1989. Dye density was measured using standard reflection geometry and status A filtration- The photographic results are summarized in

Table II.

Table II 365 line exposure 3000°K Tungsten exposure

Emulsion Dmin Rsens contrast Dmin Rsens contrast

{100} tabular 0.08 98 2.53 .08 154 2.53 cubic 0.11 100 2.64 .11 100 2.64

Table II shows that for intrinsic sensitivity as measured by the 365 line exposure, both the cubic and the tabular emulsion are similar in sensitivity, as would be expected based on their similar grain volume. Comparing the white light sensitivity as measured by the 3000°K tungsten exposures shows that the tabular emulsion is about 50% more sensitive to blue light than the cubic emulsion.

Example 14

This example shows how bromide can be added at the end of the precipitation or during the finish to produce emulsions with surface halide structure and/or growths. These emulsions show good photographic performance.

Emulsion A (Invention)

This emulsion was prepared identically to the {100} tabular emulsion described in Example 10. A quantity of this emulsion was then melted at 40°C and 1200 mg/mole of potassium bromide was rapidly added. 0.6 mmol of green sensitizing dye A per mole of emulsion was then added followed by a 20 minute hold. 1.0 mg/mole of sodium thiosulfate pentahydrate and 1.3 mg/mole of potassium tetrachloroaurate were then added followed by a temperature ramp to 60°C and a 10 minute hold. The emulsion was then cooled to 40°C and 70 mg/mole of APMT was added and the emulsion was chill set. Examination of the crystals by scanning electron microscopy revealed that the edges of the crystal had been roughened by the bromide deposition and some surface roughening was also present.

Emulsion B (Invention)

This emulsion illustrates the precipitation and sensitization of a {100} silver chloride tabular emulsion where potassium bromide was added during the final step of the precipitation to form an emulsion whereby the majority of the grains have epitaxial deposits located at 3 or 4 of the 4 available tabular grain corners.

Precipitation

A 1536 mL solution containing 3.52% by weight low methionine gelatin, 0.0056 M sodium chloride and 2.34 X 10^~ M potassium iodide was provided in a stirred reaction vessel at 40°C and pH 5.74. While this solution was vigorously stirred, 30 mL of 2.0 M silver nitrate and 30 L of 2.0 M sodium chloride were added simultaneously at a rate of 60 mL/min each. This achieved grain nucleation.

The mixture was then held for 10 seconds after which a 0.5 M silver nitrate and a 0.5 M sodium chloride solution were added simultaneously at 5.3 mL/min for 60 minutes with the pCl maintained at 2.35. The silver nitrate and sodium chloride solutions were then added using linearly accelerated flow rates from 5.3 mL/min to 15.6 mL/min over 150 minutes.

The pCl was then adjusted to 1.55 with sodium chloride and 25g of phthalated deionized gel was added and dissolved. The pH was then reduced to 3.85 and the stirring was stopped to allow the coagulum to settle. The supernatant was discarded and distilled water was added back to the coagulum to bring it to its original volume at the end of the precipitation. Stirring was resumed and the pH was adjusted back to 5.36 and the pCl was 2.45.

With vigorous stirring, 39 mL of 1.5 M potassium bromide solution was added over 30 minutes bringing the pCl to 1.8.

The pH was adjusted to 5.8 and 25g of phthalated deionized gel was added and dissolved. The pH was reduced to 3.85 and stirring was stopped to allow the coagulum to settle. The supernatant was removed, 27g of low methionine gel was added and the emulsion weight was raised to 800g with distilled water. The pH was adjusted to 5.77 and the pCl to 1.65 with 1.0 M sodium chloride solution. The resulting emulsion had a mean equivalent circular diameter of 1.67μm and a mean grain thickness of 0.135μm. The halide composition was 93.964% silver chloride, 6.0% silver bromide and 0.0036% silver iodide. Seventy-five percent of the grains had three or more minor edges with epitaxial deposits.

Sensitization

A 0.15 mole quantity of emulsion was melted at 40°C with stirring. To this was added 0.70 mmol/mole of green sensitizing dye A followed by a 20 minute hold. To this was added 1.0 mg/mole of sodium thiosulfate pentahydrate and 1.3 mg/mole of potassium tetrachloroaurate. The temperature was then increased to 60°C over 12 minutes and held for 5 minutes followed by rapid cooling to 40°C. 70 mg/mole of APMT was then added and the emulsion was chill set.

Emulsion C (Invention)

This example illustrates the precipitation and sensitization of a {100} silver chloride tabular emulsion where potassium bromide was added during the final step of the precipitation to form an emulsion where the majority of the grains had epitaxial deposits located at only 1 or 2 of the minor edges.

Precipitation A 1536 mL solution containing 3.52% by weight low methionine gelatin, 0.0056 M sodium chloride and 2.34 X 10^~ M potassium iodide was provided in a stirred reaction vessel at 40°C and pH 5.74. While this solution was vigorously stirred, 30 mL of 2.0 M silver nitrate and 30 mL of 2.0 M sodium chloride were added simultaneously at a rate of 60 mL/min each. This achieved grain nucleation.

The mixture was then held for 10 seconds after which a 0.5 M silver nitrate and a 0.5 M sodium chloride solution were added simultaneously at 8.0 mL/min for 40 minutes with the pCl maintained at 2.35. The silver nitrate and sodium chloride solutions were then added using linearly accelerated flow rates from 8.0 mL/min to 16.1 mL/min over 130 minutes. The pCl was then adjusted to 1.65 by running the sodium chloride solution at 20 mL/min for 8.0 min. This was followed by a 10 minute hold. The pCl was then increased back to 2.15 by running the silver nitrate solution at 5.0 mL/min for 27.7 min. This was followed by the addition of a 1.5 M potassium bromide solution at 2.0 mL/min over 20 minutes bringing the pCl to 1.70.

25 g of phthalated deionized gel was then added and dissolved. The pH was reduced to 3.85 and stirring was stopped to allow the coagulum to settle. The supernatant was removed and distilled water was added back to original volume. The pH was then adjusted back to 5.7 with vigorous stirring resumed. The pH was then adjusted back to 3.8 and the stirring was again stopped to allow the coagulum to form. The supernatant was again discarded and 20g of low methionine gel was added and the emulsion weight was raised to 800g with distilled water. The pH was adjusted to 5.77 and the pCl to 1.65 with 1.0 M sodium chloride solution.

The resulting emulsion had a mean equivalent circular diameter of 1.65μm and a mean grain thickness of 0.14μm. The halide composition was 93.964% silver chloride, 6.0% silver bromide and 0.0036% silver iodide. Examination of the emulsion by scanning electron microscopy showed that 97 percent of the grains had epitaxial depositions visible on two or fewer of the four available host tabular grain corners.

Sensitization The sensitization was identical to that used in Example 1 except the level of sodium thiosulfate pentahydrate and potassium tetrachloroaurate were increased by 50%.

Emulsion D (Control) This emulsion was composed of silver chloride cubic grains and was precipitated identically to the cubic emulsion in Example 10 and is of similar grain volume to the three tabular emulsions in this example. This emulsion was sensitized as follows: a quantity was melted at 40°C and 500 mg/mole of potassium bromide was added followed by 0.2 mg/mole of sensitizing dye A and a 20 minute hold. To this was added 0.25 mg/mole of sodium thiosulfate pentahydrate and 0.50 mg/mole of sodium aurous dithiosulfate dihydrate followed by a temperature ramp to 65°C and a 12 minute hold. The emulsion was then quickly chilled. Photographic Performance

Each of the sensitized emulsions was coated on antihalation support at 0.85 g/m² of silver along with 1.1 g/m² of cyan dye forming coupler C and 2.7 g/m² of gelatin. This was overcoated with 1.6 g/m² of gelatin and hardened with bis(vinylsulfonylmethyl)ether at 1.75% of the total coated gelatin weight. The coatings were evaluated for intrinsic sensitivity by exposing for 0.02 seconds in a step wedge sensitometer with the 365 nm line of a mercury vapor lamp as the source. Sensitivity to green light was measured by exposing the coatings for 0.02 seconds using a step wedge sensitometer with a 3000°K tungsten lamp filtered to simulate a Daylight V source and filtered to transmit only light with wavelengths longer than 400 nm by a Kodak Wratten ™ 2B filter. The coatings were then processed using the Kodak Flexicolor ™ C-41 color negative process. The dye density was measured using status M red filtration. The photographic results are tabulated and summarized in Table III.

Table III Wratten™2B exposure 365 line exposure

Emulsion Dmin Rsens contrast Dmin Rsens contrast

Emulsion A . 14 200 2 . 22 . 12 60 1.87

Emulsion B . 13 275 2 . 05 . 14 141 1 . 89

Emulsion C . 12 245 2 . 36 . 13 79 2 . 65

Emulsion D . 14 100 2 . 82 . 18 100 2 .48 (control)

Table III shows all of the {100} tabular grain emulsion examples are at least 2 times more sensitive to a white light exposure than the similarly sensitized cubic grain emulsion even though emulsion A and C showed less intrinsic sensitivity to the 365 mercury line exposure.

Sensitizing Dye B

( CH₂ ) ₃ S0₃ TBA⁺ = tributylammonium

Coupler Y

Example 15 (Comparison) The purpose of this Example is to demonstrate the inability of a ripening out procedure--specifically the procedure referred to in the 1963 Torino Symposium, cited above--to produce a tabular grain emulsion satisfying the requirements of the invention. To a reaction vessel containing 75 L distilled water, 6.75 g deionized bone gelatin and 2.25 mL of 1.0 M NaCl solution at 40°C were simultaneously added with efficient stirring 15 mL of 1.0 M AgNθ3 solution and 15 mL of 1.0 M NaCl solution each at 15 mL per minute. The mixture was stirred at 40°C for 4 minutes, then the temperature was increased to 77°C over a period of 10 minutes and 7.2 mL of 1.0 M NaCl soluton were added. The mixture was stirred at 77°C for 180 minutes and then cooled to 40°C.

The resulting grain mixture was examined by optical and electron microscopy. The emulsion contained a population of small cubes of approximately 0.2 μm edge length, large nontabular grains, and tabular grains with square or rectangular major faces. In terms of numbers of grains the small grains were overwhelmingly predominant. The tabular grains accounted for no more than 25 percent of the total grain projected area of the emulsion. The mean thickness of the tabular grain population was determined from edge-on views obtained using an electron microscope. A total of 26 tabular grains were measured and found to have a mean thickness of 0.38 μm. Of the 26 tabular grains measured for thickness, only one had a thickness of less than 0.3 μm, the thickness of that one tabular grain being 0.25 μm.

Example 16

This example has as its purpose to demonstrate successful preparation of an emulsion satisfying the requirements of the invention employing commercially available deionized gelatin as a starting material.

To a reaction vessel, equipped with a stirrer, were added 2865 g of distilled water containing 20 g of deionized gelatin (purchased from Rousellot™) . The initial calcium ion level was 8 X 10^~6 molar. Additional calcium ion was added to the reaction vessel as calcium chloride hydrate to compensate for calcium ion removal during deionization of the gelatin, thereby bringing the calcium ion concentration up to 2.36 millimolar. Adjustment of the dispersing medium within the reaction vessel was completed by adding 0.96 g of sodium chloride and 45 g of 0.012 molar potassium iodide solution. The pH was adjusted to 6.5 at 55°C and maintained at that value throughout the precipitation by addition of sodium hydroxide or nitric acid solutions.

A 4.0 M silver nitrate and a 4.0 M sodium chloride solution were added for 30 seconds at a rate consuming 5 percent of the total silver. The emulsion was then held at 62°C for 10 ninutes followed by the addition of 5000 g of a solution containing 1.6 percent of the deionized gelatin. This was followed by simultaneous addition of the silver nitrate and sodium chloride with the flow rates linearly increased by a factor of 2.58 over 70 minutes with the pAg maintained at 6.37. The total amount of silver iodochloride precipitated was 4.745 moles. Greater than 80 percent of total grain projected area was accounted for by tabular grains. The tabular grains exhibited an average ECD of 1.65 μm, an average thickness of 0.165 μm, and an average aspect ratio of 10. When the preparation procedure described above was repeated with calcium acetate substituted for calcium chloride hydrate, greater than 85 percent of total grain projected area was accounted for by tabular grains. The tabular grains exhibited an average ECD of 1.5 μm, an average thickness of 0.16 μm, and an average aspect ratio of 9.4. When magnesium, aluminum or iron ions were substituted for calcium ions in the dispersing medium, emulsions satisfying the requirements of the invention were also obtained.

Example 17 This example has as its purpose to demonstrate the thinning of high chloride {100} tabular grains through the introduction of bromide and/or iodide ions during the growth stages of precipitation.

Emulsion 17A. A silver iodochloride {100} tabular grain emulsion.

A 6000 L solution containing 3.52% by weight of low methionine gelatin, 0.0056 M sodium chloride was provided in a stirred reaction vessel at 40°C. While the solution was vigorously stirred, 90 mL of a 0.01 M potassium iodide solution was added followed by simultaneous addition of a 90 mL of 2.0 M silver nitrate and 90 mL of a 1.99 M sodium chloride, 0.01M potassium iodide solution at a rate of 180 mL/min each. The mixture was then held for 10 minutes with the temperature remaining at 40°C. Following the hold, a 1.0 M silver nitrate solution and a 1.0 M sodium chloride solution were added simultaneously at 12 mL/min for 40 minutes followed by a linear acceleration from 12 mL/min to 33.7 mL/min over 233.2 minutes, while maintaining the pCl at 2.25. The pCl within the reaction vessel was then adjusted to 1.65 with sodium chloride then the emulsion was washed and concentrated using ultrafiltration to a pCl of 2.0. The pCl was adjusted to 1.65 with sodium chloride and the pH to 5.7.

The resulting emulsion was a silver iodochloride {100} tabular grain emulsion containing 0.015 mole percent iodide. The emulsion grains exhibited a mean ECD 1.51 μ and a mean grain thickness of 0.21 μm.

Emulsion 17B. This example demonstrates that bromide ion in the halide salt solution at a 1 mole percent level during the final 89 percent of the precipitation significantly reduces the average grain thickness of the emulsion.

This emulsion was prepared identically to Emulsion 17A, except that the halide salt solution used during the 233.2 minute accelerated flow period was a 0.99 M sodium chloride and 0.01 M sodium bromide solution.

The resulting high chloride {100} tabular grain emulsion contained 0.015 mole percent iodide, 0.89 percent bromide and 99.095 mole percent silver chloride. The mean ECD was 1.69 μm and the average thickness was 0.17 μm.

Emulsion 17C. This example demonstrates that bromide ion in the salt solution at a 10 percent level during the final 89 percent of the precipitation significantly reduces the average grain thickness of the emulsion.

This emulsion was prepared identically to Emulsion 17A, except that the halide salt solution used during the 233.2 minute accelerated flow period was a 0.90 M sodium chloride, 0.10 M sodium bromide solution.

The resulting high chloride {100} tabular grain emulsion contained 0.015 mole percent iodide, 8.9 percent bromide and 91.085 mole percent silver chloride. The mean ECD was 1.69 μ and the average grain thickness was 0.17 μ .

Emulsion 17D A silver iodochloride {100} tabular grain emulsion with a bulk composition of 99.97 percent silver chloride and 0.03 percent silver iodide, where only silver chloride was precipitated during the growth stages.

A 1.5 L solution containing 3.52% by weight of low methionine gelatin, 0.0056 M sodium chloride and 0.3 mL of polyethylene glycol antifoamant provided in a stirred reaction vessel at 40°C. While the solution was vigorously stirred, 45 mL of a 0.01 M potassium iodide solution was added followed by 50.0 mL of 1.25 M silver nitrate and 50.0 mL of a 1.25M sodium chloride solution added simultaneously at a rate of 100 mL/min each. The mixture was then held for 10 seconds with the temperature remaining at 40°C. Following the hold, a 0.625 M silver nitrate solution containing 0.08 mg mercuric chloride per mole of silver nitrate and a 0.625 M sodium chloride solution were added simultaneously at 10 mL/min for 30 minutes followed by a linear acceleration from 10 mL/min to 15 mL/min over 125 minutes, then 30 minutes at a constant flow rate of 15 mL/min. The pCl was maintained at 2.35 during this time. The pCl was then adjusted to 1.65 with a sodium chloride solution. Fifty grams of phthalated gelatin were added and the emulsion was washed and concentrated using procedures of Yutzy et al U.S. Patent 2,614,928. The pCl after washing was 2.0. Thirty-four grams of low methionine gel were added, the pCl was adjusted to 1.65 with sodium chloride, and the pH was adjusted to 5.7.

The resulting high chloride tabular grain emulsion had an ECD of 1.86 μm and a mean grain thickness of 0.11 μm.

Emulsion 17E This emulsion demonstrates that the addition of low levels of iodide ion during the growth stage of precipitation results in lower average tabular grain thicknesses. This emulsion was precipitated identically to Emulsion 17D, except that the salt solution used during the accelerated growth stage and the final constant growth stage had a composition of 0.621 M sodium chloride and 0.004 M potassium iodide.

The resulting high chloride {100} tabular grain emulsion had an ECD of 1.8 μm and an average thickness of 0.09 μm.

Example 18 This example demonstrates advantages for introducing bromide ion rapidly during {100} tabular grain formation.

Emulsion Precipitations:

Emulsion 18A Silver iodobromochloride {100} tabular emulsion having a bulk halide composition of 96.964 mole percent chloride, 0.036 mole percent iodide, and 3 mole percent bromide, with slow addition of bromide over 30 minutes at a pCl at 1.6. A 1.5 L solution containing 3.52% by weight of low methionine gelatin, 0.0056 M sodium chloride, and 0.3 mL of polyethylene glycol antifoamant provided in a stirred reaction vessel at 40°C. While the solution was vigorously stirred, 36 mL of a 0.01 M potassium iodide solution was added followed by 50 mL of 1.25 M silver nitrate and 50 mL of a 1.25M sodium chloride solution added simultaneously at a rate of 100 mL/min each. The mixture was then held for 10 seconds with the temperature remaining at 40°C. Following the hold, a 0.5 M silver nitrate solution containing 0.08 mg mercuric chloride per mole of silver nitrate and a 0.5 M sodium chloride solution were added simultaneously at 10 mL/min for 30 minutes followed by a linear acceleration from 10 mL/min to 15 mL/min over 125 minutes, while maintaining the pCl at 2.35. The pCl was then adjusted to 1.60 by delivering the 1.25 M sodium chloride solution at 20 mL/min over 8 minutes followed by a 10 minute hold. A 0.5 M potassium bromide solution was then added at 3.0 mL/min over 20 minutes. 50 g of phthalated gelatin was added and the emulsion was washed and concentrated using procedures of Yutzy et al U.S. Patent 2,614,929. The pCl after washing was 2.0. Twenty-one grams of low methionine gelatin was added, the pCl was adjusted to 1.65 with sodium chloride, and the pH was adjusted to 5.7. The resulting emulsion was a {100} tabular grain emulsion had a mean ECD of 1.6 μ and a mean grain thickness of 0.125 μm.

Emulsion 18B

Silver iodobromochloride {100} tabular grain emulsion with a bulk halide composition of 96.964 mole percent chloride, 0.036 mole percent iodide, and 3 mole percent bromide with the bromide added rapidly at a pCl of 1.7.

This emulsion was precipitated identically to Emulsion 18A, except that at the end of the ramped growth portion, a 1.5 M sodium chloride solution was added at 20 mL/min for 15 minutes followed by the addition of 1.0 M silver nitrate at 5.0 mL/min for 30 minutes. This was followed by the addition of a 23 mL of 1.5 M potassium bromide solution over about 1 second. The emulsion then held for 10 minutes. The emulsion was washed and concentrated with the same pCl and pH adjustments as in the precipitation of Emulsion 18A. The ECD of the emulsion grains 1.6 μm, and average grain thickness was 0.14 μm.

Emulsion 18C Silver iodobromochloride {100} tabular grain emulsion with a bulk halide composition of 97.964 mole percent chloride, 0.036 mole percent iodide, and 2 mole percent bromide where the bromide was added slowly at a pCl of 1.6.

This emulsion was precipitated identically to Emulsion 18A, except that 0.625 M silver nitrate and 0.625 M sodium chloride solutions were used during the 30 minute constant flow growth and the 125 minute ramped flow growth. At the end of the ramped flow growth portion, a 1.25 M sodium chloride solution was added at 20 mL/min for 7.5 minutes followed by a 10 minute hold. This was followed by the addition of a 60 mL of 0.5 M potassium bromide solution over 20 minutes at 3 mL/min. The emulsion was washed and concentrated with the same pCl and pH adjustments as made in the preparation of Emulsion 18A. The emulsion grain ECD was 1.5 μ , and the average grain thickness was 0.12 μm.

Emulsion 18D Silver iodobromochloride {100} tabular grain emulsion with a bulk halide composition of 97.964 mole percent chloride, 0.036 mole percent iodide, and 2 mole percent bromide with the bromide added rapidly at a pCl of 2.3. This emulsion was precipitated identically to

Emulsion 18A, except 0.625 M silver nitrate and 0.625 M sodium chloride solutions were used during the 30 minute constant flow growth and the 125 minute ramped flow growth. At the end of the ramped flow growth portion, a 1.25 M sodium chloride solution was added at 20 mL/min for 7.5 minutes followed by a 10 minute hold. This was followed by the addition of the 1.25 M silver nitrate solution at 5.0 mL/min for 30 minutes. This was followed by the addition of a 60 mL of 0.5 M potassium bromide solution over about 1 second. The e ulsion was then held for 20 minutes. The emulsion was washed and concentrated with the same pCl and pH adjustments as made in Emulsion 18A. The emulsion grain ECD was 1.8 μm, and the average grain thickness was 0.14 μm.

Emulsion 18E Silver iodobromochloride {100} tabular emulsion with a bulk halide composition of 97.964 mole percent chloride, 0.036 mole percent iodide, and 2 mole percent bromide with the bromide added rapidly at a pCl of 1.6.

This emulsion was precipitated identically to Emulsion 18A, except that addition of 150 mL of 1.25M silver nitrate to adjust the pCl back to 2.3 before the addition of the potassium bromide was omitted so that potassium bromide solution was added at a pCl of 1.6. The emulsion was washed and concentrated with the same pCl and pH adjustments as made in Emulsion 18A. The emulsion grain ECD was 1.6 μm, and the average grain thickness was 0.13 μm.

Sensitization of Emulsions 18A and 18B to produce Examples 18/1 through 18/4.

The sensitizing procedure was as follows: A quantity of emulsion suitable for experimental coating was melted at 40°C. Red spectral sensitizing dye was then added at levels estimated from specific surface area measurements. The addition of each dye was followed by a 15 minute hold. The red sensitizing dyes were used as a set of two dyes. Set R-l consisted of red spectral sensitizing dyes Dye SS-23 and SS-25 in the mole ratio of 8 parts SS-23 per part SS-25. Sodium thiosulfate pentahydrate at a level of 1.0 mg/mole Ag was then added followed by potassium tetrachloroaurate at 0.7 mg/mole Ag. The temperature of the well stirred mixture was then raised to 60°C over 12 minutes and held at 60°C for a specified time. The emulsion was then cooled to 40°C as quickly as possible, and 70 mg/mole of APMT was then added and the emulsion was chill set.

Photographic Measurements

Each embodiment was coated on an antihalation support at 0.85 g/m2 of silver with 1.08 g/m2 of cyan dye-forming coupler C-l and 2.7 g/m2 of gelatin. This layer was overcoated with 1.6 g/m2 of gelatin, and the entire coating was hardened with bis (vinylsulfonyl- methyl)ether at 1.75 percent by weight of the total coated gelatin. Coatings were exposed through a step wedge for 0.02 second with a 3000°K tungsten source through Daylight V and Kodak Wratten ™ 2B filters. An additional set of coatings were also given a 0.02 sec exposure with a 365 nm line emission from a mercury vapor lamp. The coatings were processed in the Flexicolor ™ C-41 color negative process.

Table IV

C-1 Dye 60°C hold Wr 2B 365 Hg In.

Example Emulsion level time Dmin Rsens Rsens

18/1 18A 0.8 5 0.18 100 100

18/2 18A 0.8 10 0.18 110 98

18/3 18B 0.7 5 0.17 162 93

18/4 18B 0.7 10 0.21 186 115

As demonstrated in Table IV, although

Emulsion 18A provided thinner tabular grains and had a higher specific surface area, which allowed more sensitizing dye to be adsorbed, Emulsion 18B was significantly faster even though its projected area was the same and its intrinsic sensitivity as measured with the 365 Hg line exposure were about the same. This demonstrates that the spectral sensitization of Emulsion 18B was more efficient, which is in turn a function of the more rapid bromide addition described above.

Sensitization of Emulsions 18C though 18E to produce Examples 18/5 through 18/16

The sensitizing procedure was identical to that used for Examples 18/1 through 18/4 with the exception that Examples 18/11 through 18/16 used a different red sensitizing dye combination R-2, which consists of spectral sensitizing dye Dyes SS-23 and SS- 25 in a molar ratio of 2 parts Dye SS-23 to 1 part of Dye SS-25.

Photographic Measurements

Coatings were prepared, exposed and process as described for Examples 18/1 through 18/4 above.

Examples 18/5 through 18/10 of Table V show that Emulsions 18D and 18E, to which the bromide was added rapidly as compared to Emulsion 18C, show both improved spectral (Kodak Wratten ™ 2B filter) sensitivity as well as improved intrinsic sensitivity (365 Hg line exposure) . The fact that the spectral sensitivity increases are larger than the intrinsic sensitivity increases shows that the bromide band formed by rapid addition improves the interaction with the spectral sensitizing dyes so that transfer of the photoelectron from the excited sensitizing dye to the silver halide grain is more efficient.

Examples 18/11 though 18/16 show that this favorable interaction between emulsions with a high bromide band formed by rapid bromide addition and spectral sensitizing dyes is dependent on both the sensitizing dyes used and the pCl used for precipitation of the bromide band. Note that Emulsion 18D, the bromide band of which was precipitated at a pCl of 2.35, again showed much higher spectral and intrinsic speed relative to Emulsion 18C (slow bromide addition) , but Emulsion 18E, to which bromide was rapidly added at a pCl of 1.6, exhibited a speed in the region of spectral sensitization intermediate that of Emulsion 18C (slow bromide addition) and preferred examples Emulsion 18D.

From the examples that high chloride {100} tabular grain emulsions with bromide bands generally perform better when the bromide source is added rapidly. The performance of these emulsions is further enhanced in some cases when the rapid bromide addition is carried out at pCl values where the excess chloride ion in solution is relatively low.

Example 19

This example has as its purpose to demonstrate the effectiveness of various ripening agents in increasing the percentage of total grain projected area accounted for by {100} tabular grains. Ernulsion 19A : Control Emulsion

Solutions :

Solution A: 4 M silver nitrate solution. Solution B: 4 M sodium chloride solution. Solution C: 0.012 M potassium iodide solution.

Solution D: 6.5 L of distilled water containing 2.1 g of sodium chloride. Solution E: 2.865 L of distilled water containing 0.96 g sodium chloride, 25 g of gelatin and 90 mL of solution C.

Precipitation:

Solution E was charged in a reaction vessel equipped with stirrer. The content of the vessel was maintained at pH 6.5 and 55 °C. While the solution was vigorously stirred, solutions A and B were added at 120 mL/min. each for 30 seconds.

Solution D was then added to the mixture. At the same time the mixture temperature was raised to 62°C, pCl adjusted to 1.91, and pH was maintained at 6.5 throughout the precipitation process. The mixture was then allowed to sit for 5 min. Following the hold, solutions A and B were then added simultaneously at linearly accelerated rates from 10 mL/min to 24 mL/min in 56 min. with the pCl maintained at 2.14.

The resulting emulsion had 50 % of its total grain projected area accounted for by {100} tabular grains having a mean ECD of ca. 1.4 μm and a mean aspect ratio of 8. The emulsion contained a large quantity of fine grains. Emulsion 19B: Methionine as a growth accelerator.

Solution AA: 4 M silver nitrate containing 2325 ppm of methionine.

Precipitation: This emulsion was precipitated the same way as emulsion 19A, except that solution AA was used, instead of solution A, for the growth period (the period after the hold) . The resulting emulsion was essentially free of fine particles with greater than 65 % of total grain projected area accounted for by {100} tabular grains having a mean thickness of 0.16 μm and a mean ECD of 1.5 μm.

Emulsion 19C: 1,10-Dithia-4,7,13,16-tetraoxacyclodecane as a growth accelerator. This emulsion was the same as Emulsion 19B, except that Solution AA, instead of containing methionine, contained 1162 ppm of 1,10-dithia- 4,7,13,16-tetraoxacyclodecane. The resulting emulsion was essentially free of fine particles with greater than 65 % of its total grain projected area accounted for by {100} tabular grains having a mean thickness of 0.14 μm and a mean ECD of 1.2 μm.

Emulsion 19D: 1,8-Dihydroxy-3, 6-dithiaoctane as a growth accelerator. This emulsion was the same as Emulsion 19B except that Solution AA, instead of containing methionine, contained 23 ppm of 1,8-dihydroxy-3,6- dithiaoctane. The resulting emulsion was essentially free of fine particles with greater than 65 % of total grain projected area accounted for by {100} tabular grains having a mean thickness of 0.14 μm and a mean ECD of 1.2 μm. Emulsion 19E: 2,5-Dithiasuberic acid as a growth accelerator This emulsion was the same as Emulsion 19B, except that Solution AA, instead of containing methionine, contained 58 ppm of 2,5-dithiasuberic acid. The resulting emulsion was essentially free of fine particles with greater than 65 % of total grain projected area accounted for by {100} tabular grains having a mean thickness of 0.13 μm and a mean ECD of 1.2 μm.

Emulsion 19F: Glycine as a growth accelerator

This emulsion was the same as Emulsion 19B, except that Solution AA, instead of containing methionine, contained 5813 ppm of glycine. The resulting emulsion was essentially free of fine particles with greater than 70 % of total grain projected area accounted for by {100} tabular grains having a mean thickness of 0.14 μm and a mean ECD of 1.1 μm.

Emulsion 19G: Sodium sulfite as a growth accelerator This emulsion was the same as Emulsion 19B, except that Solution AA, instead of containing methionine, contained 174 ppm of sodium sulfite. The resulting emulsion was essentially free of fine particles with greater than 65 % of total grain projected area accounted for by {100} tabular grains having a mean thickness of 0.14 μm and ECD of 1.2 μm.

Emulsion 1 H: Thiocyanate as a growth accelerator

This emulsion was the same as Emulsion 19B, except that Solution AA, instead of containing methionine, contained 79 ppm of sodium thiocyanate. The resulting emulsion was essentially free of fine particles with greater than 65 % of total grain projected area being accounted for by {100} tabular grains having a mean thickness of 0.15 μm and a mean ECD of 1.1 μm.

Emulsion 191: Imidazole as a growth accelerator

This emulsion was the same as Emulsion 19B, except that Solution AA, instead of containing methionine, contained 581 ppm of imidazole. The resulting emulsion was essentially free of fine particles with greater than 60 % of total grain projected area being accounted for by {100} tabular grains having a mean thickness of 0.14 μm and ECD of 1.4 μm.

Examples 20 to 23

Iridium dopants in concentrations of from 1 X 10^-9 to 1 X 10^-6, preferably 1 X 10^-8 to 1 X 10^~7, mole per silver mole are contemplated for the purpose of reducing reciprocity failure in the emulsions of the invention. Photographic exposure is the product indicated by the equation:

E = I X ti where

E is exposure,

I is exposure intensity, and ti is exposure time. Reciprocity failure is the term applied to failures of equal exposures to produce the same photographic response when they are constituted by different exposure intensities and times. Iridium dopants are particularly contemplated to reduced low intensity reciprocity failure (LIRF)--that is, departures from exposure reciprocity in the exposure time range of from

10^~2 to 10 seconds. Example 20

Emulsion 20A. Silver chloride {100} tabular grain emulsion with potassium hexachloroiridate added after 80% of the precipitation to give a bulk concentration of 0.05 mg/mole of emulsion.

A 4900 mL solution containing 3.52% by weight of low methionine gelatin, 0.0056 M sodium chloride and 1.0 mL of polyethylene glycol antifoamant provided in a stirred reaction vessel at 40 C. While the solution was vigorously stirred, 149 mL of a 0.01 M potassium iodide solution was added followed by 95 mL of 1.25 M silver nitrate and 95 L of a 1.25M sodium chloride solution added simultaneously at a rate of 180 mL/min each. The mixture was then held for 10 seconds with the temperature remaining at 40°C. Following the hold, a 0.5 M silver nitrate solution and a 0.5 M sodium chloride solution were added simultaneously at 25 mL/min for 40 minutes followed by a linear acceleration from 25 mL/min to 40.3 mL/min over 107 minutes, while maintaining the pCl at 2.35. At this point 30 mL of a solution containing 5.12 mg potassium hexachloroiridate per liter was added over a 1.2 minute period while the 0.5 M silver and salt solutions continued to run from 40.3 to 40.5 mL/min. Following the addition of the iridium salt, the addition of the 0.5 M silver nitrate and the 0.5 M sodium chloride solutions was continued for 33.0 minutes with the flow rates linearly ramped from 40.5 mL/min to 45.0 mL/min. The pCl was then adjusted to 1.65 with sodium chloride then the emulsion was washed and concentrated using ultrafiltration to a pCl of 2.0. 16 g of low methionine gelatin was added then the pCl was adjusted to 1.65 with sodium chloride and the pH to 5.7. The resulting emulsion was a tabular grain silver chloride emulsion containing 0.048 ole percent iodide and had a mean ECD of 1.64 μm and a mean grain thickness of 0.146 μm.

Emulsion 20B Silver chloride {100} tabular grain emulsion with potassium hexachloroiridate added after 80% of the precipitation to give a bulk concentration of 0.005 mg/mole of emulsion.

This emulsion was prepared identically to Emulsion 20A, except that the solution containing the iridium salt had a concentration of 0.512 mg potassium hexachloroiridate per liter. The resulting emulsion was a tabular grain silver chloride emulsion containing 0.048 mole percent iodide and had a mean ECD of 1.8 μm and a mean grain thickness of 0.148 μ .

Emulsion 20C Silver chloride {100} tabular grain emulsion lacking an iridium dopant.

This emulsion was prepared identically to emulsion A except no iridium salt solution was added. The resulting emulsion was a tabular grain silver chloride emulsion containing 0.048 mole percent iodide and had a mean equivalent circular grain diameter of 1.7 μm and a mean grain thickness of 0.145 μm

Sensitization

Type I Embodiments 1 through 26

This type of sensitization used sodium thiosulfate pentahydrate and potassium tetrachloroaurate as chemical sensitizing agents. A variety of sensitization embodiments were prepared where the level of potassium bromide, the type of sensitizing dye and the hold time at 60°C were varied. The sensitizing procedure was as follows: A quantity of emulsion suitable for experimental coating was melted at 40°C. Potassium bromide was added followed by a total of 0.7 mmol of green or red sensitizing dye per mole of emulsion. The green spectral sensitizing dye consisted of a Dye SS-21. The red sensitizing dyes were used as a set of two dyes. Set R-l consisted of red spectral sensitizing dyes Dye SS-23 and Dye SS-24 in the ratio of 8 parts SS-23 to 1 part SS-24. Set R-2 consisted of Dye SS-23 and Dye SS-25 in the ratio of 2 parts Dye SS-23 to 1 part Dye SS-25. The dye addition was followed by a 20 minute hold. One mg per mole of sodium thiosulfate pentahydrate, and 0.7 mg/mole of potassium tetrachloroaurate were then added. The temperature of the well stirred mixture was then raised to 60°C over 12 minutes and held at 60°C for a specified time. The emulsion was then cooled to 40°C as quickly as possible and 70 mg/mole of APMT was then added and the emulsion was chill set.

This type of sensitization used a colloidal aurous sulfide suspension as the chemical sensitizing agent added after the addition of sensitizing dye and potassium bromide.

The general sensitizing procedure was as follows: A quantity of emulsion suitable for experimental coating was melted at 40°C. Embodiments 27 and 28 used emulsion C and embodiments 29 and 30 used emulsion A. 0.7 mmol/mole Ag of green sensitizing SS-21 was added to each emulsion. The dye addition was followed by a 20 min hold. 600 mg/mole of potassium bromide was then added to embodiments 24 and 26 followed by a 10 minute hold. 2.5 mg/mole of aurous sulfide was then added followed by a 5 minute hold. The temperature of the well stirred mixture was then raised to 60°C over 12 minutes and held at 60o for 30 minutes. The emulsion was then cooled to 40°C as quickly as possible and 90 mg/mole of APMT was then added and the emulsion was chill set. Type III - Embodiment numbers 31 through 34

This type of sensitization used a colloidal aurous sulfide suspension as the chemical sensitizing agent added at 40°C before the addition of the sensitizing dye. The general sensitizing procedure was as follows. A quantity of emulsion suitable for experimental coating was melted at 40°C. Embodiments 31 and 32 used emulsion C and embodiments 33 and 34 used emulsion A. 0.25 mg/mole g of aurous sulfide was added followed by a 5 minute hold. In embodiments 27 and 29 the temperature was ramped to 60°C over 12 minutes and held at 60°C for 30 minutes then ramped back to 40°C over 12 minutes. Embodiments 28 and 30 were held constant at 40°C during this same time. 0.7 mmol/mole Ag of sensitizing dye SS-21 was added to each emulsion followed by a 20 min hold and the addition of 90 mg/mole of APMT followed by chill set.

Photographic Results:

Each embodiment was coated on an antihalation support at 0.85 g/m² of silver with 1.08 g/m² of cyan dye forming coupler C and 2.7 g/m2 of gelatin. This layer was overcoated with 1.6 g/m2 of gelatin and the entire coating was hardened with bis (vinylsulfonyl- methyl)ether at 1.75% of the total coated gelatin. Coatings were exposed with a Xenon lamp filtered with a Kodak Wratten ™ 2B filter. The intensity of the lamp was varied with inconel filter so that different exposure times received the same total exposure. The coatings were processed in a Kodak Flexicolor ™ C-41 process. 0^"4 - 10 sec IC² - 10 sec sensitivity sensitivity difference difference

32 20

32 32

10 29

12 26

17 38

0 35

51 51

45 41

58 41

51 45

38 17

29 20

10 10

-7 7

26 12

23 10

7 7

10 5

17 5

15 7

45 29

23 2

15 2

45 5

41 7

66 74

2 48

20 12 20 20

Comparing the iridium containing embodiments with the embodiments lacking iridium, it can be seen that the iridium containing emulsion show improved reciprocity for both the overall 10^-4 to 10 sec range as well as the 10^~2 to 10 second (low intensity) range. Furthermore by investigating the effects of the iridium over a wide range of sensitizations, it can be seen that the iridium improves the robustness of the reciprocity behavior as a function of the extent of finish.

Examples 21 and 22

These examples demonstrate the effectiveness of iridium as a dopant to reduce low intensity reciprocity failure (LIRF) when the iridium is located very near the grain surface. In these examples LIRF was measured by comparing 1/10 and 10 second exposures. Three individual silver iodochloride {100} tabular grain emulsions were prepared for use in these examples. Table VIII describes the grain dimensions and iodide content.

Emulsion Iodide

The dopants used in combination with the emulsions S-l, 2 and 3 to improve LIRF are given in Table IX. Table IX

Dopant Chemical Formula

D-l K₃irCl₆

D-2 K4lr₂Cl₁₀

D-3 K₆Ir₆Cl₂₄

The examples that follow describe the use of these dopants in various amounts and in various locations during the sensitization of emulsions S-l to S-3.

The sensitized emulsions were coated onto cellulose acetate film support. The coating format was an emulsion layer comprised of 200 mg/ft² (21.5 mg/dm²) of the tabular silver chloride emulsion dispersed in 500 mg/ft² (53.8 mg/dm²) of gelatin; an overcoat comprised of 100 mg/ft² (10.8 mg/dm²) gelatin and a hardener, bis(vinylsulfonylmethyl)ether at a level of 0.5% by weight, based on total gelatin.

The coated photographic elements were evaluated for reciprocity response by giving them a series of calibrated (total energy) exposures ranging from 1/10 of a second to 10 seconds. The exposed film was processed for 6 minutes in a hydroquinone-Elon™ (p- N-methylaminophenol hemisulfate) developer. Example 21

This example demonstrates the usefulness of dopant D-2 added during spectral sensitization by means of a pCl cycle which is comprised of sequential addition of chloride ion, D-2, and silver ion. The introduction of the dopant in the pCl cycle produces an emulsion with improved LIRF behavior as compared to either an emulsion that is spectrally sensitized without use of the dopant or the pCl cycle or an emulsion that is spectrally sensitized with the pCl cycle, but with the dopant omitted, where the emulsions are otherwise the same.

Emulsion S-l was spectrally sensitized by treating a portion with 550 mg per mole of silver of blue spectral sensitizing dye Dye SS-1 followed by heat digestion. APMT was added thereafter at an amount of 90 mg per silver mole. This represents the control emulsion.

Other portions of S-l were spectrally sensitized in a similar manner, except that a pCl cycle of 2 mole % chloride ion and D-2 addition followed by 2 mole % silver ion addition was performed to effect the incorporation of D-2. Such a pCl cycle was accomplished either before or after the treatment of S- 1 with the sensitizing dye. These samples constitute examples of the invention.

A final example was prepared in which a pCl cycle without dopant was performed to demonstrate the effect of the 2 mole % cycle, free of any dopant effects.

Table X summarizes the photographic results of various amounts of D-2 added via a pCl cycle technique.

From Table X it is apparent that the use of D-2 reduces LIRF of the emulsion. Speed as reported in Tables X, XI, XIII and XXIII is 100 times the log of the exposure required to provide a density of 0.15 above the minimum density.

Example 22

This example demonstrates the usefulness of dopants D-l, D-2 and D-3 in reducing LIRF when added via a pCl cycle technique to the spectral and chemical sensitization of emulsions S-2 and S-3.

Separate portions of S-2 and S-3 were spectrally and chemically sensitized by treating each portion with 550 mg per mole of silver of blue spectral sensitizing dye Dye SS-1 followed by a heat digestion. Then 2 mg per mole of a colloidal gold sulfide reagent were added followed by heat digestion for 30 minutes at 60°C. Thereafter, the temperature was adjusted to 40°C, and 90 mg per mole of APMT were added. The resulting parts represent the undoped emulsions for comparison to the doped emulsions.

Another undoped example was prepared in a similar manner, except a 2 mole % pCl cycle consisting of chloride ion followed by silver ion was performed after the dye addition and digestion steps, but before the chemical sensitization step.

Other portions were spectrally and chemically sensitized, given a pCl cycle with various amounts of dopant added, then treated with APMT as described above. The photographic results showing the LIRF improvements of the parts containing the dopants D-l, D-2 and D-3 is documented in Table XI. Also noteworthy is the significant speed increases that are obtained with certain amounts of D-l and D-3.

This example demonstrates the effectiveness of iridium to reduce LIRF when incorporated during precipitation with a silver bromide Lippmann emulsion.

The host high chloride {100} tabular grain emulsion employed Emulsion S-3, described in Example 23.

Lippmann silver bromide emulsions (of approximately 0.08 μm edge length) were prepared with and without incorporated dopants. Table XII lists the Lippmann emulsions used and the dopant type and amount contained in each emulsion. By blending doped and undoped Lippmann emulsions a variety of dopant concentrations were available for incorporation onto the host AgCl {100} T-grain emulsion.

Portions of host emulsion S-3 were spectrally and chemically sensitized by treating each portion with 550 mg per mole of silver of blue spectral sensitizing dye Dye SS-1 followed by a heat digestion. Two mg per silver mole of a colloidal gold sulfide reagent were added followed by heat digestion for 30 minutes at 60°C. Thereafter, the temperature was adjusted to 40°C and 90 mg per silver mole of APMT were added. The resulting parts represent the undoped emulsions provided for comparison.

Another comparative emulsion was prepared in a similar manner to that described above, except that 2 mole % of an undoped Lippmann silver bromide emulsion were added after the colloidal gold sulfide and heat digestion. Once the Lippmann emulsion was added an additional heat digestion of 10 minutes at 60°C was performed. Then the temperature was lowered to 40°C, and 90 mg per silver mole of APMT was added. This comparative example was provided to demonstrate the effect of an undoped Lippman bromide on the S-3 host emulsion.

Other portions of the S-3 host emulsion were sensitized as the above comparative example, except that doped Lippmann silver bromide emulsions or blends of doped and undoped Lippmann silver bromide emulsions were added and digested for 10 minutes at 60°C. Table XIII shows the LIRF benefit when the doped Lippmann additions were made. Coating, exposure and process were undertaken as described in Example 22 .

Table XIII

Ex. 23 2% Amount of Part Lippmann Dopant dopant White light bromide Type (PPM) speed LIRF

221 20 233 20

230 16 238 10

244 12

237 17 231 15 239 11

As demonstrated in Table XIII, the treatment of the high chloride {100} tabular grain host emulsion with iridium doped Lippmann silver bromide emulsions results in a significant reduction in LIRF.

Example 24

Compounds that release selenium, such as potassium selenocyanate, can be used to sensitize high chloride {100} tabular grain emulsions, both as a replacement for sulfur and as an enhancement to a sulfur and gold sensitization. Advantages include lower fog at similar speed and high speed at equal fog levels.

Emulsion Precipitation:

Silver iodochloride {100} tabular grain emulsion with a bulk halide composition of 99.954% chloride and 0.048 % iodide on a mole basis.

A 4900 mL solution containing 3.52% by weight of low methionine gelatin, 0.0056 M sodium chloride and

1.0 mL of polyethylene glycol antifoamant provided in a stirred reaction vessel at 40 C. While the solution was vigorously stirred, 149 mL of a 0.01 M potassium iodide solution was added followed by 95 mL of 1.25 M silver nitrate and 95 mL of a 1.25M sodium chloride solution added simultaneously at a rate of 180 mL/min each. The mixture was then held for 10 seconds with the temperature remaining at 40°C. Following the hold, a 0.5 M silver nitrate solution and a 0.5 M sodium chloride solution were added simultaneously at 25 mL/min for 40 minutes followed by a linear acceleration from 25 mL/min to 45 mL/min over 140 minutes, while maintaining the pCl at 2.35. The pCl was then adjusted to 1.65 with sodium chloride, then the emulsion was washed and concentrated using ultrafiltration to a pCl of 2.0. Sixteen grams of low methionine gelatin were added, then the pCl was adjusted to 1.65 with sodium chloride, and the pH was adjusted to 5.7.

The resulting emulsion was a silver chloride {100} tabular grain emulsion containing 0.048 mole percent iodide that had a mean grain ECD 1.64 μm and a mean grain thickness of 0.146 μm.

Sensitization:

Samples of the emulsion were melted at 40°C. Potassium bromide was added followed by a total of 0.7 mmol of green spectral sensitizing dye SS-21 per mole of emulsion. The dye addition was followed by a 20 min hold. Sodium thiosulfate pentahydrate was then added (to some samples only) followed by 0.7 mg/Ag mole of potassium tetrachloroaurate. This was followed by the addition of potassium selenocyanate (to some samples only) . The temperature of the well stirred mixture was then raised to 60°C over 12 minutes and held at 60°C for a specified time. The emulsion was cooled to 40°C as quickly as possible, 70 mg/mole of APMT was added, and the emulsion samples were chill set. A sample of each emulsion was coated on a support having an antihalation backing at 0.85 g/m² of silver with 1.08 g/m² of cyan dye-forming coupler C-l and 2.7 g/m² of gelatin. The emulsion layer was overcoated with 1.6 g/m² of gelatin, and the entire coating was hardened with bis(vinylsulfonylmethyl) ether at 1.75 percent by weight of the total coated gelatin.

Photographic Evaluation The photographic elements were exposed for

1/50 second through a step wedge with a tungsten lamp filtered with a Kodak Wratten ™ 2B filter. The coatings were processed in the Kodak Flexicolor ™ C-41 color negative process.

TABLE XIV

Na2S2θ3 level mg/ Sample _{Ag mo}|_e

24 A 1.0

24 B 1.0

24 C 0.5

24 D 0.5

24 E 0

24 F 0

24 G 1.0

24 H 1.0

The samples containing selenium included the sample that produced the lowest minimum density and the sample that produced the highest sensitivity. Overall, it is apparent that the use of selenium improved performance when both sensitivity and minimum density were taken in account. Examole 25

This example demonstrates the effect of introducing K2 u(CN)g during precipitation as a grain dopant. A silver iodochloride (0.05 mole percent iodide) {100} tabular grain emulsion according to the invention was prepared in which 10 mppm of K2Ru(CN)g was added along with the silver accounting for the segment of the run between 85 and 95 percent of total silver added. In the resulting emulsion greater than 50 percent of total grain projected area was accounted for by tabular grains having {100} major faces. The mean grain ECD was 1.44 μm and mean grain thickness was 0.147 μ . The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.6, respectively. This emulsion is hereafter designated

Emulsion 25/D.

A comparison emulsion, hereinafter designated Emulsion 25/UD was similarly precipitated, except that the K2Ru(CN)g dopant was omitted during the precipitation. In the resulting emulsion greater than

50 percent of total grain projected area was accounted for by tabular grains having {100} major faces. The mean grain ECD was 1.61 μm and mean grain thickness was 0.150 μm. The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.6, respectively.

Each emulsion was combined with a yellow dye-forming coupler stabilized with benzenosulfonic acid. Each emulsion was coated at 2.8 mg/dm² silver,

10.8 mg/dm² dye-forming coupler, and 8.3 mg/dm² gelatin on a resin coated paper support.

Samples of the emulsion coatings were given equal exposures at 100, 1/2 and 1/100 second. HIRF was measured as a difference between photographic speed at 1/100 and 1/2 second exposures, while LIRF was measured as a difference between photographic speed at 100 and 1/2 second exposures. Latent image keeping was measured as a speed difference between strips developed at 30 seconds and 30 minutes after exposure. Heat sensitivity was measured as a speed difference between exposures at 40°C and room temperature. The rapid access Kodak RA-4 ™process was used.

While both emulsions demonstrated photographic utility, the principal advantage for

Emulsion 25/D over Emulsion 25/UD was found in faster speed, improved toe sharpness and higher contrast at comparable latent image keeping and heat sensitivity levels. Emulsion 25/D also exhibited higher sensitivity at shorter exposure times and lower sensitivity at longer exposure times, both of which can be advantageous for particular photographic uses.

Example 26

This examples has as its purpose to demonstrate the effectiveness of iron hexacyanide as a dopant in high chloride {100} tabular grain emulsions to reduce high intensity reciprocity failure (HIRF) . Emulsion 26/1

Six solutions were prepared as follows:

Solution 1 (26/1) Gelatin (bone) 211 gm

NaCl 1.96 gm

D.W. 5800 mL

Solution 2 (26/1) KI 0.15 gm

D.W. 90 mL

Solution 3 (26/1) NaCl 207 gm

D.W. 7000 mL

Solution 4 (26/1) NaCl 13.1 gm

D.W. 108 mL

Solution 5 (26/1) Ag 03 soln. 5.722 molar 922 gm D.W. 5425 mL

Solution 6 (26/1) AgNO soln. 5.722 molar 922 gm

D.W. 73.7 mL

Solution 7 (26/1) Gelatin (phthalated) 100 gm D.W. 1000 mL

Solution 8 (26/1) Gelatin (bone) 80 gm

D.W. 1000 mL

Solution 1 (26/1) was charged into a reaction vessel equipped with a stirrer at 40°C. Solution 2 (26/1) was added to the reaction vessel, and the pH was adjusted to 5.7. While vigorously stirring the reaction vessel, Solution 4 (26/1) and Solution 6

(26/1) were added at 180 mL/min. for 30 seconds. The reaction vessel was held for 10 min. Following this hold, Solution 3 (26/1) and Solution 5 (26/1) were added simultaneously at 24 mL/min. for 40 minutes with the pCl maintained at 1.91. The rate was then accelerated to 48 mL/min. over 130 minutes. The mixture was then cooled to 40°C and Solution 7 (26/1) added and stirred for 5 minutes. The pH was then adjusted to 3.8 and the gel allowed to settle. At the same time the temperature was dropped to 15°C before decanting the liquid layer. The depleted volume was restored with D.W. The pH was adjusted to 4.5, and the mixture held at 40°C 20 minutes before the pH was adjusted to 3.8 and the settling and decanting steps repeated. Solution 8 (26/1) was added and the pH and pCl adjusted to 5.6 and the pCl to 1.6, respectively.

Emulsion 26/2

A second emulsion (26/2) was prepared like the first emulsion (26/1), but with 36 mg K4Fe(CN)g in 278 gm of a solution otherwise like Solution 3 (26/1) added at 4 mL/min at the same time as Solutions 3 and 5 were accelerated. This addition lasted for 70 min._. Emulsions 26/1 and 26/2 were finished by treating them with 0.5 % NaBr holding for 5 minutes, adding a combination of spectral sensitizing dyes (Dye SS-21 and Dye SS-26 in a 3:1 molar ratio), holding for 10 minutes, adding Na2S2θ3.5H2θ at 1.2 mg/mole and KAUCI4 at 1.6 mg/mole and heating for 10 minutes at 60°C. APMT at 90 mg/mole was added after the heating step. The finished emulsions were coated at 50 mg Ag/ft² (5.38 mg/dm²) with a mixture of magenta dye- forming couplers at 50 mg/ft² (5.38 mg/dm²). The coatings were overcoated with gel and hardened. Samples of the coatings were equally exposed at decade intervals ranging from 1 X 10^"5 to 0.1 second and processed for 2' 15" in the Kodak Flexicolor ™ C-41 color negative process. The results are summarized in Table XV. Speed is measured at a density of 0.35 above fog.

Example 27 This example illustrates the use of desensitizing dopants with high chloride {100} tabular grain emulsions.

Emulsion 27/1

Six solutions were prepared as follows:

Solution 1 (27/1)

Gelatin (bone) 75 gm NaCl 2.88 gm

D.W. 4300 mL

Solution 2 (27/1)

KI 0.44 gm D.W. 220 mL

Solution 3 (27/1)

NaCl 397.4 gm

D.W. to total volume 1700 mL

Solution 4 (27/1)

NaCl 4.3 gm D.W. 6500 mL

Solution 5 (27/1)

AgNO_β 5.722 M soln. 2110 gm

D.W. 518 mL

Solution 6 (27/1)

Gelatin (phthalated) 200 gm

D.W. 1500 mL

Solution 7 (27/1

Gelatin (bone) 130 gm D.W. 1500 mL

Solution 1 (27/1) was charged into a reaction vessel equipped with a stirrer. Solution 2 (27/1) was added to the reaction vessel, the pH was adjusted to 6.5, and the temperature was raised to 55°C. While vigorously stirring the reaction vessel, Solution 3 (27/1) and Solution 5 (27/1) were added at 45 mL/min. for one minute. Solution 4 (27/1) was then added to the mixture. The temperature was raised to 62°C, the pCl was adjusted to 1.91, and the pH maintained at 6.5. The mixture was held for five minutes. Following this hold, Solution 3 (27/1) and Solution 5 (27/1) were added simultaneously each at a linearly accelerated rates ranging from 15 mL/min. to 37 mL/min. in 56 minutes with the pCl maintained at 1.91. The mixture was then cooled to 40°C, and Solution 6 (27/1) was added and stirred for 5 minutes. The pH was then adjusted to 3.2, and the gel was allowed to settle. At the same time the temperature was dropped to 15°C before decanting the liquid layer. The depleted volume was restored with D. W. The pH was adjusted to 4.5 and the mixture held at 40°C for 20 minutes before the pH was adjusted to 3.2 and the settling and decanting steps were repeated. Solution 7 (27/1) was added and the pH and pCl adjusted to 6.5 and 1.6, respectively. Emulsion 27/2

A second emulsion (27/2) was prepared like 27/1 but with K₃0s(N0)Cl₅ added at a formal total concentration of 0.1 mppm in a band from 70 to 80% of the salt and silver addition.

Emulsion 2212.

A third emulsion was prepared like 27/1 but with K3Ru(NO)Cl5 added at a formal total concentration of 0.1 mppm in a band from 70 to 80% of the salt and silver addition.

Emulsion 27/4M

A fourth emulsion (27/4) was prepared like 27/1 but with K₃RhCl₆ added at a formal total concentration of 0.1 mppm in a band from 70 to 80% of the salt and silver addition. Emulsions 27/1, 27/2 and 27/3 were chemically and spectrally sensitized by treating them with 1.5% NaBr holding for 5 minutes, adding spectral sensitizing dye Dye SS-22, holding for 10 minutes, adding Na2S2θ3*5H2θ) at 1.6 mg/mole and KAUCI4 at 1.0 mg/mole and heating for 10 minutes at 60°C. APMT at 100 mg/mole was added after the heating step. The emulsions were coated at 5.4 mg Ag/dm² with 5.4 mg/dm² of a magenta dye-forming coupler. The coatings were overcoated with gel and hardened. The coatings were given a daylight with a Wratten ™ W9 filter exposure for 0.02 second and processed for 3'15" in the Kodak Flexicolor ™ C-41 color negative process. The results are summarized in Table XVI. Speed was measured at a density of 0.20 above fog. A portion of Emulsion 27/1 not previously sensitized (hereinafter referred to as Emulsion 27/1M) and Emulsion 27/4M were chemically and spectrally sensitized by treating them with 2% NaBr holding for 5 minutes, adding a spectrally sensitizing dye mixture (Dye SS-23 and Dye SS-25 in a 2:1 molar ratio), holding for 10 minutes, adding Na2S2θ3*5H2θ at 1.6 mg/mole, adding KAUCI4 at 1.0 mg/mole, and heating for 10 minutes at 60°C. APMT at 100 mg/mole was added after the heating step. The finished emulsions were coated at 5.4 mg Ag/dm² with 5.4 mg/dm² of a magenta dye- forming coupler. The coatings were overcoated with gel and hardened. The coatings were given a daylight with a Wratten ™ 9 filter exposure for 0.02 second and processed for 3 '15" in the Kodak Flexicolor ™ C-41 color negative process. The results are summarized in Table XVI. Speed was measured at a density of 0.20 above fog.

Example 28

This example illustrates the use of shallow electron trapping dopants with high chloride {100} tabular grain emulsions.

Emulsion 28/1

Eight solutions were prepared as follows:

Solution 1 (28/1)

Gelatin (bone) 211 gm

NaCl 1.96 gm

D.W. 5798 mL

Solution 2 (28/1)

KI 0.15 gm

D.W. 90 mL Solution 3 (28/1)

NaCl 206.7 gm

D.W. 7000 mL

Solution 4 (28/1)

NaCl 13.1 gm

KI 0.19 gm

D.W. 108 mL

Solution 5 (28/1)

AgN03 5.722 M soln. 70 gm

D.W. to total volume 112 mL

Solution 6 (28/1)

AgNθ3 5.722 M soln. 922 gm

D.W. 542.6 mL

Solution 7 (28/1)

Gelatin (phthalated) 100 gm

D.W. 1000 mL

Solution 8 (28/1)

Gelatin (bone) 80 gm

D.W. 1000 gm

Solution 1 (28/1) was charged into a reaction vessel equipped with a stirrer. Solution 2 (28/1) was added to the reaction vessel. The pH was 5.7, and the temperature was raised to 40°C. While vigorously stirring the reaction vessel, Solution 4 (28/1) and

Solution 5 (28/1) were added at 130 mL/min for one half minute. The pCl was adjusted to 2.3. The mixture was held for ten minutes. Following this hold, Solution 3 (28/1) and Solution 6 (28/1) were added simultaneously at 24 mL/min for 40 minutes, then the flow was linearly accelerated from 24 mL/min to 48 mL/min in 130 minutes with the pCl maintained at 2.3. Solution 7 (28/1) was added and stirred for 5 minutes. The pH was then adjusted to 3.8 and the gel allowed to settle. At the same time the temperature was dropped to 15°C before decanting the liquid layer. The depleted volume was restored with D.W. The pH was adjusted to 4.5 and the mixture held at 40°C for 5 minutes before the pH was adjusted to 3.8 and the settling and decanting steps repeated. Solution 8 (28/1) was added and the pH and pCl adjusted to 5.6 and 1.6, respectively. Emulsion 28/2

A second emulsion (28/2) was prepared like Emulsion 28/1, but with K4Ru(CN)g added at a formal total concentration of 25 mppm in a band extending from 70 to 80 percent of the halide and silver addition. Emulsion 28/3

A third emulsion (28/3) was prepared like 28/1, but with K Ru(CN)g added at a formal total concentration of 50 mppm in a band extending from 70 to 80 percent of the halide and silver addition. Emulsions 28/1, 28/2 and 28/3 were finished by treating them with 1% NaBr holding for 5 minutes, adding a spectral sensitizing dye (Dye 1-22), holding for 10 minutes, adding Na2S2θ3*5H2θ at 0.8 mg/mole and KAUCI4 at 1.0 mg/mole and heating for 10 minutes at 60°C. APMT at 120 mg/mole was added after the heating step. The finished emulsions were coated at 5.4 mg Ag/dm² with a magenta dye-forming coupler at 5.4 mg/dm². The coatings were overcoated with gel and hardened. Samples of the coatings were equally exposed at decade time intervals ranging from 1 X 10^~5 to 1/10 second and processed for 2' in the Kodak Flexicolor ™ C-41 color negative process. The results are summarized in Table XVII. Speed is measured at a density of 0.35 above fog. Table XVII dopant Δ speed log E Emul. level ₍₁₀_₅ __{Q 1 SQC )}

28/1 undoped -0.08 28/2 25 mppm +0.05 28/3 50 mppm +0.12

Example 29

The addition of mild silver oxidizing agents during the precipitation and or precipitation under oxidizing conditions such as low pH have shown significant reduction in fog level without speed loss after spectral and chemical sensitization. The mild silver oxidants include inorganic salts such as a mercuric salt or an alkali tetrahaloaurate as well as organic compounds which release silver oxidizing species such as elemental sulfur, such as 4,4'-phenyl disulfide diacetanalide.

Emulsion 29A. (no oxidizing feature) A silver bromochloride (3% bromide) {100} tabular grain emulsion to which no oxidizing agents were added or precipitation modifications made to reduce fog.

A 4.5 liter solution containing 3.52% by weight low methionine gelatin, 0.0056 M sodium chloride and 1.0 mL of polyethylene glycol antifoamant was provided in a stirred reaction vessel at 40°C. While the solution was vigorously stirred, 135 mL of a 0.01 M potassium iodide solution was added followed by 150 mL of 1.25 M silver nitrate and 150 mL of a 1.25M sodium chloride solution added simultaneously at a rate of 300 mL/min each. The mixture was then held for 10 seconds with the temperature remaining at 40°C. Following the hold, a 0.625 M silver nitrate solution and a 0.625 M sodium chloride solution were added simultaneously at 30 mL/min for 30 minutes followed by a linear acceleration from 30 mL/min to 45 mL/min over 125 minutes, while maintaining the pCl at 2.35. At this point 480 mL of 1.25M sodium chloride was added over 8 minutes, followed by a 10 minute hold. The 1.25 M silver nitrate solution was then added at 15 mL/min for 30 minutes after which 180 mL of 0.5 M sodium bromide was added and the emulsion was held for 20 minutes. The pCl was then adjusted to 1.65 with sodium chloride then the emulsion was washed and concentrated using^' ultrafiltration to a pCl of 2.0. Ten grams of low methionine gelatin where added then the emulsion was adjusted to a pCl of 1.65 with sodium chloride and a pH of 5.7. The resulting emulsion was a tabular grain silver chloride emulsion containing 3% silver bromide and 0.032 mole percent iodide. The emulsion exhibited a mean grain ECD of 1.8 μ and a mean grain thickness of 0.15 μm.

Emulsion 29B (oxidizing feature)

This emulsion was prepared identically to Emulsion 29A, except that mercuric chloride was added to the silver nitrate solutions at a concentration of 0.08 mg mercuric chloride per mole of silver nitrate.

Emulsion 29C (oxidizing feature)

This emulsion was prepared identically to Emulsion 29A, except that potassium tetrachloroaurate was added to the silver nitrate solution at a concentration of 0.2 mg per mole of silver during the 125 ramped flow growth period in which 69 percent of total silver was precipitated.

Emulsion 29D (oxidizing feature) This emulsion was prepared identically to

Emulsion 29A, except that 4, 4 ' -diphenyl disulfide acetanalide was added to the silver nitrate solution at a concentration of 1.0 mg per mole of silver during the 125 minute ramped flow growth period in which 69 percent of total silver was precipitated.

Emulsion 29E (oxidizing feature)

This emulsion was prepared identically to Emulsion 29A, except that the pH of the emulsion was adjusted from 5.7 to 4.5 with nitric acid after 17 percent of the total silver had been precipitated. The pH remained at 4.5 throughout the completion of the precipitation, but was adjusted back to 5.7 after the emulsion was washed and the final gelatin was added.

Sensitization and Coating

A quantity of emulsion suitable for coating was melted at 40°C. Potassium bromide was added followed by spectral sensitizing dye Dye SS-21. The dye addition was followed by a 20 minute hold. Sodium thiosulfate pentahydrate, a sulfur sensitizer, and potassium tetrachloroaurate, a gold sensitizer, were then added. The temperature of the well stirred mixture was then raised to 60°C over 12 minutes and held at 60°C for a time shown below. The emulsion was then cooled to 40°C as quickly as possible, 70 mg/mole APMT was then added, and the emulsion was chill set. A sample of each emulsion was coated on a support having an antihalation backing at 0.85 g/m² of silver with 1.08 g/m² of cyan dye-forming coupler C-l and 2.7 g/m² of gelatin. The emulsion layer was overcoated with 1.6 g/m² of gelatin, and the entire coating was hardened with bis(vinylsulfonylmethyl) ether at 1.75 percent by weight of the total coated gelatin.

Photographic Evaluation

The photographic elements were exposed for 1/50 second through a step wedge with a tungsten lamp filtered with a Kodak Wratten ™ 2B filter. The coatings were processed in the Kodak Flexicolor ™ C-41 color negative process.

From Table XVIII it is apparent that the presence of mild oxidants or oxidizing conditions during emulsion precipitation is capable of reducing fog while retaining essentially similar photographic sensitivities.

Example 30

This example demonstrates that the addition of a benzothiazolium salt during sensitization produces a high chloride {100} tabular grain emulsion exhibiting higher speed and lower fog. The emulsion was precipitated as described in Example 24.

Sensitization:

Samples of the emulsion were melted at 40°C. Potassium bromide was added followed by a total of 0.7 mmol of green spectral sensitizing dye Dye SS-21 per mole of emulsion. The dye addition was followed by a 20 min hold. Sodium thiosulfate pentahydrate was then added at a level of 1.0 mg/Ag mole followed by 0.7 mg/Ag mole of potassium tetrachloroaurate. This was followed by the addition of 5 mg of 3- (2-methylsulfo- nylethyl)benzothiazolium tetrafluoroborate (hereinafter referred to as BTZTFB) per mole of silver (in some samples) . The temperature of the well stirred mixture was then raised to 60°C for a time specified below in Table XIX. The emulsion was cooled to 40°C as quickly as possible, 70 mg/mole of APMT was added, and the emulsion samples were chill set.

A sample of each emulsion was coated on a support having an antihalation backing at 0.85 g/m² of silver with 1.08 g/m² of cyan dye-forming coupler C-l and 2.7 g/m² of gelatin. The emulsion layer was overcoated with 1.6 g/m² of gelatin, and the entire coating was hardened with bis(vinylsulfonylmethyl) ether at 1.75 percent by weight of the total coated gelatin.

Photographic Evaluation

The photographic elements were exposed for 1/50 second through a step wedge for with a 3000°K tungsten lamp filtered with a Daylight V filter and a Kodak Wratten ™ 9 filter. The coatings were processed in the Kodak Flexicolor ™ C-41 color negative process.

From Table XVII it is apparent that the addition of the benzothiazolium salt during sensitization not only increased sensitivity but additionally lowered minimum density.

Example _______

This example demonstrates the effectiveness of a variety of spectral sensitizing dyes to increase the speed of high chloride {100} tabular grain emulsions.

A silver iodochloride (0.05 mole percent iodide) {100} tabular grain emulsion containing 3 x 10" ⁷ mole mercury per silver mole added with the silver salt during precipitation was employed. Tabular grains with {100} major faces accounted for greater than 50 percent of total grain projected area. The emulsion grain ECD was 1.37 μm and mean grain thickness was 0.148 μm. The emulsion was washed by ultrafiltration, its pH was adjusted to 5.6, and its pCl was adjusted to 1.6.

The emulsion was chemically and spectrally sensitized according to the following scheme: Table XX

Finish Profile

Temperature Addendum HoldTime

40°C 1.5 mole % KBr 10 minutes

Dye or Optical 20 minutes

Brightener OB-1 plus dye

Sodium Thiosulfate (1.6 2 minutes mg/mole Ag)

Potassium 2 minutes

Tetrachloroaurate (0.8 mg/mole Ag)

Ramp 5°C/3 10 minutes min to 60°C

Ramp 5°C/3 none min to 40°C

40°C APMT (60 mg/Ag mole) 10 minutes

OB-1 4,4^■-{2-[4- (2-chloroanilino)-6-chloro-l,3,5- triazinyl]amino}-2,2'-disulfostilbene, disodium salt

The samples were coated at 1.61 g Ag/m² and 3.23 g gel/m² on an unsubbed 7 mil (178 μm) polyacetate butyrate film support. Surfactants were added as coating aids, and bis (vmylsulfonylmethyl) ether at 1.5 percent by weight was used as a hardener.

Absorptance measurements on the coatings were used to determine the wavelength of maximum light absorption for the dyes. Exposure and processing consisted of 1/5" 5500°K exposure followed by 6" development in a hydroquinone-Elon™ (p-N-methyl- aminophenol hemisulfate) developer (Kodak DK-50™), a stop bath, a fix (Kodak F-5 ™) , and wash. The speeds for the coatings were measured as the exposure necessary to produce a density of 0.15 above the minimum density. An undyed comparison coating was assigned a sensitivity value of 100 for purposes of comparison and all the dyed examples are expressed relative to the undyed. The data is summarized in Table XXI.

Table XXI

The following example illustrates the use of blue spectral sensitizing dye combinations to spectrally sensitize high chloride {100} tabular grain emulsions.

The same emulsion employed as in Example 31.

The emulsion was chemically and spectrally sensitized according to the following scheme:

Table XXII

Finish Profile

Temperature Addendum HoldTime

40°C 1.5 mole % KBr 10 minutes

Single dye or dye 20 minutes for combination one dye 10 minutes each for two dyes

Sodium Thiosulfate 2 minutes (1.6 mg/mole Ag)

Potassium 2 minutes

Tetrachloroaurate (0.8 mg/mole Ag)

Ramp 5°C/3 10 minutes min to 63°C

Ramp 5°C/3 min to 40°C

40°C APMT (80 mg/Ag mole) .0 minutes

Each spectrally sensitized emulsion sample was dual melted with a common dye-forming coupler dispersion melt containing dispersion A, dispersion B, and surfactants. The samples were coated on a 5 mil (125 μm) cellulose triacetate support that had been backed with a carbon black (Remjet ™) antihalation backing and subbed with 4.88 g/m² of gelatin. The emulsion and couplers were laid down at a level of 968 mg/m² silver, 484 mg/m² dye-forming coupler Y-l, and 484 mg/m² coupler Y-2. Surfactants were added as coating aids. The emulsion layer was overcoated with 1.08 g/m² gelatin and hardened with 1.75 percent by weight bis(vinylsulfonyl)methane, based on total gelatin.

Dispersion A contained 9% by weight yellow dye-forming coupler Y-l, 6% by weight deionized gelatin, 0.44% a sodium triisopropylnaphthalene sulfonate (anionic surfactant), 1.1% 2N propionic acid.

Dispersion B had the following composition: 9% by weight yellow dye-forming coupler Y-2, 4.5% dibutyl phthalate, 6.5% gelatin, 0.6% a sodium triisopropylnaphthalene sulfonate (anionic surfactant) , and adjusted to pH 5.1 with 2N propionic acid. Coupler Y-l

Coupler Y-2

Strips from these coatings were given a 1/50' stepped wedge exposure from a 5500°K light source through a Wratten ™ 2B filter. The samples were processed using the Kodak Flexicolor ™C41 color negative process, but with the composition of the bleach solution modified to include propylene- diaminetetraacetic acid. The minimum density was measured and the photographic speed determined as 100 times the log of the exposure required to give a density 0.15 above the minimum density. The data are summarized in Table XXIII.

The data in Table XXIII show not only that the dye combinations are useful for the spectral sensitization of high chloride {100} tabular grain • emulsions, but also that the combinations have a synergistic effect. The combination of dyes imparts more sensitivity to the emulsion than either dye alone.

Example 33

This example has as its purpose to demonstrate the effectiveness of combinations of spectral sensitizing dyes in high chloride {100} tabular grain emulsions.

Emulsion preparation:

A 1.5 L solution containing 3.52% by weight of low methionine gelatin, 0.0056 M sodium chloride and 0.3 ml of polyethylene glycol antifoamant provided in a stirred reaction vessel at 40°C. While the solution was vigorously stirred, 45 ml of a 0.01 M potassium iodide solution was added followed by 50 mL of 1.25 M silver nitrate and 50 mL of a 1.25M sodium chloride solution added simultaneously at a rate of 100 mL/min each. The mixture was then held for 10 seconds with the temperature remaining at 40°C. Following the hold, a 0.625 M silver nitrate solution containing 0.08 mg mercuric chloride per mole of silver nitrate and a 0.625 M sodium chloride solution were added simultaneously at 10 mL/min for 30 minutes followed by a linear acceleration from 10 mL/min to 15 mL/min over 125 minutes, then a constant flow rate growth for 30 minutes at 15 mL/min while maintaining the pCl at 2.35. The pCl was then adjusted to 1.65 with sodium chloride. Fifty grams of phthalated gelatin were added, and the emulsion was washed and concentrated using procedures of Yutzy et al U.S. Patent 2,614,928. The pCl after washing was 2.0. Twenty-one grams of low methionine gel were added, the pCl was adjusted to 1.65 with sodium chloride, and the pH was adjusted to 5.7. The resulting emulsion was a silver iodochloride {100} tabular grain emulsion containing 0.036 mole percent iodide. The emulsion had a mean grain ECD of 1.6 μm and a mean grain thickness of 0.125 μm.

Sensitization:

A sample series of different emulsion sensitizations was undertaken. In each sensitization a quantity of emulsion suitable for coating was melted at 40°C. Potassium bromide was added followed by a total of 0.7 mmol of green spectral sensitizing dye per Ag mole. When two green spectral sensitizing dyes were added, the ratio of the principal and secondary dye was as shown in Table XXIV. The dye addition was followed by a 20 min hold. This was followed by 1.0 mg/mole of sodium thiosulfate pentahydrate then 0.7 mg/mole of potassium tetrachloroaurate. The temperature of the well stirred mixture was then raised to 60°C over 12 minutes and held at 60° for a specified time. The emulsion was then cooled to 40°C as quickly as possible, 70 mg/mole of APMT was added, and the emulsion was chill set.

Photographic Results:

Each sample was coated on a support having an antihalation layer at 0.85 g/m² of silver, 1.08 g/m² of cyan dye-forming coupler C-l, and 2.7 g/m² of gelatin. This layer was overcoated with 1.6 g/m² of gelatin, and the entire coating was hardened with bis(vinyl- sulfonylmethyl)ether at 1.75 percent by weight of the total coated gelatin.

Coatings were exposed through a step wedge for 0.02 second with a 3000°K tungsten source filtered with Daylight V and Kodak Wratten ™9 filters. The coatings were processed in the Kodak Flexicolor ™ C-41 color negative process.

From Table XXIV it is apparent that the spectral sensitizing dye combinations produce higher level of response than when the same amount of only one of the dyes is employed.

Example 34

This example demonstrates the photographic performance of blue, green and red spectrally sensitized high chloride {100} tabular grain emulsions in yellow, magenta and cyan dye-forming layer units, respectively. The emulsions were then coated on a resin coated paper support and processed.

Blue Sensitized Emulsion (B-SensEm) An iodochloride (0.05 mole percent iodide)

{100} tabular grain emulsion was employed having a mean grain ECD of 1.61 μm and a mean thickness 0.150 μm. The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.5, respectively. This emulsion was sensitized by addition of blue spectral sensitizing dye SS-1 followed by the addition of gold sulfide and heat digestion, after which APMT was added to the emulsion melt.

Green Sensitized Emulsion (G-SensEm)

An iodochloride (0.05 mole percent iodide) {100} tabular grain emulsion was employed having a mean grain ECD of 1.38 urn and a mean thickness 0.148 μm. The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.5, respectively. The emulsion was sensitized by addition of red spectral sensitizing dye SS-21 followed by the addition of gold sulfide and heat digestion, after which APMT was added to the emulsion melt.

Red Sensitized Emulsion (R-SensEm)

An iodochloride (0.05 mole percent iodide) {100} tabular grain emulsion was employed having a mean grain ECD of 1.61 μm and a mean thickness 0.150 μm. The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.5, respectively. The emulsion was sensitized by addition of red spectral sensitizing dye SS-19 followed by the addition of gold sulfide and heat digestion, after which APMT was added to the emulsion melt.

Dye-Forming Coupler Dispersions

One of the dye-forming coupler dispersions shown in Table XXV was introduced as a disperse phase in a sample of one of the blue, green and red sensitized emulsions. Table XXV

Dispersion No. Disperse Phase Composition

34A C-5, 61.3%; S-l, 33.7%; S-5, 5.0% 34B C-55, 41.0%, S-2, 29.5%, S-4, 29.5% 34C C-6, 86.2%; S-l, 6.9%, S-6, 6.9% 34D C-20. 49.1%; ST-1, 20.9%; ST-3, 4.9%;

S-l, 25.1%

34E C-56. 50%; S-4, 50% 34F C-57,. 30%; ST-5, 35%; ST-4, 5%; S-2, 30% 34G C-14,, 33.3%; ST-2, 16.7%; S-l, 50.0% 34H C-13,. 33.3%; ST-2, 16.7%; S-l, 50.0% 341 C-58.. 25.0%; ST-2, 12.5%; S-4, 62.5% 34J C-15, 66.7%; S-2, 33 3% 34K C-25. 66.7%; S-l, 16 7%; S-5, 16.7% 34L C-26, 50% ST- 6 , 22% S-l, 22% 34M C-57, 30% ST-2, 40% S-2, 30% 34N C-57, 30% ST-1, 40% S-2, 30% 340 C-57, 30% ST-5, 40% S-2, 30% 34P C-57, 30% ST-2, 20% ST-7, 20%; S-2, 30% 34Q C-57, 30% ST-2, 20% ST-5, 20%; S-2, 30% 34R C-57, 30% ST-2, 30% ST-8, 10%; S-2, 30% 34.S C-57, 30% ST-2, 35% ST-4, 5%; S-2, 30% 34T C-57, 30% ST-5, 35% ST-4, 5%; S-4, 30%

Dye-Forming Couplers

C-5

_0-Bu -S = 0 II

C-6

Cl C-14 C-25

NHSO _ He C-26

C-57

Stabilizers

ST-4

ST- 7

Solvents

S-l Dibutyl phthalate

S-2 Tritolyl phosphate

S-3 N,N-Diethyldodecanamide

S-4 Tris (2-ethylhexyl)phosphate

S-5 2- (2-Butoxyethoxy)ethyl acetate

S-6 2,5-Di-t-pentylphenol Photographic Elements 34/1-34/12

The photographic elements were prepared by coating the following layers in the order listed on a resin-coated paper support:

1st Laver

Gelatin 3.23 g/m²

2nd Layer

Gelatin 1.61 g/m²

Coupler Dispersion (See Table XXIV)

Emulsion (See Table XXIV)

3rd Layer

Photographic Elements 34/13-34/22

1st Laver

Gelatin 3.23 g/m²

2nd Layer

Gelatin 1.61 g/m²

Coupler Dispersion (See Table XXV)

Emulsion (See Table XXV)

3rd Laver

Exposure and Processing

The photographic elements were given stepwise exposures and processed as follows at 35°C:

Developer 45 seconds

Bleach-Fix 45 seconds

Wash (running water) 1 minute, 30 seconds

The developer and bleach-fix were of the following compositions:

Developer Water

Triethanolamine Blankophor RED^" ™ (Mobay Corp.)

Lithium polystyrene sulfonate (30%) N,N-Diethylhydroxylamine (85%) Lithium sulfate N-{2-[ (4-amino-3-methylphenyl)ethyl- amino]ethyl}methanesulfonamide, sesquisulfate 1-Hydroxyethyl-l,1-diphosphonic acid 0.81 g

(60%) Potassium carbonate, anhydrous 21.16 g

Potassium chloride 1.60 g

Potassium bromide 7.00 mg

Water to make 1.00 L pH @ 26.7°C adjusted to 10.4 ± 0.05

Bleach-Fix

Water 700.00 mL

Solution of ammonium thiosulfate 127.40 g

(56.4%) + Ammoniumsulfite (4%) Sodium metabisulfite 10.00 g

Acetic acid (glacial) 10.20 g

Solution of ammonium ferric ethylene- 110.40 g diaminetetraacetate (44%) + ethylene diaminetetraacetic acid (3.5%) Water to make 1.00 L pH @ 26.7°C adjusted to 6.7

Photographic Results

Cyan, magenta, or yellow dyes were formed upon processing. The following photographic characteristics were determined: D-max (the maximum density to light of the color complementary to the dye color) ; D-min (the minimum density) ; and Speed (the relative log exposure required to yield a density of 1.0). These values for each example are tabulated in Table XXVIII.

Table XXVIII demonstrates the usefulness of the high chloride {100} tabular grain emulsions with a variety of couplers in dispersions commonly used for color paper reflection print materials. Examples 35-37

These examples demonstrate the reduced high intensity reciprocity failure (HIRF) of the high chloride {100} tabular grain emulsions of the invention as compared to high chloride cubic grain emulsions. Example 35

A comparison cubic grain high chloride emulsion, hereinafter referred to as Emulsion 35/C, was precipitated by equimolar addition of silver nitrate and sodium chloride into a well stirred reactor containing gelatin peptizer and thioether ripener. The resulting emulsion contained cubic grains with a mean edge length of 0.74 μm.

A silver iodochloride (0.05 mole percent iodide) {100} tabular grain emulsion according to the invention was prepared in which greater than 50 percent of total grain projected area was accounted for by tabular grains having {100} major faces. The mean grain ECD was 1.55 μm and mean grain thickness was 0.155 μm. The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.6, respectively. This emulsion is hereafter designated Emulsion 35/T.

Each of the emulsions was divided into separate aliquots for spectral and chemical sensitization. Portions of Emulsion 35/C were optimally sensitized by the addition of gold sulfide and increased in temperature to 60°C during which time APMT, potassium bromide and one of the blue spectral sensitizing dyes SS-1, SS-50 or SS-51 were added.

These emulsion portions are hereinafter referred to as 35/C1, 35/C2 and 35/C3, respectively. Portions of Emulsion 35/T were optimally sensitized by the addition of SS-1, SS-50 or SS-51 followed by the addition of gold sulfide and heat digestion, after which APMT was added to the melt. These emulsion portions are hereinafter referred to as 35/T1, 35/T2 and 35/T3, respectively.

All of the emulsions were coated on resin coated paper support at 1.8 mg/drn^ silver and 7.5 mg/dιr.2 gelatin along with a yellow dye-forming coupler to form a blue recording layer unit. Both green and red recording layer units were also coated to form a multicolor pack. Samples of the multicolor pack were subjected to equal exposures of 10-1 and 10-5 second using an optical reciprocity sensitometer. The exposed samples were processed in a Kodak RA-4 ™ color print developer. Photographic speed was taken at minimum density plus a density of 0.35.

The results are summarized in Table XXIX.

Table XXIX

From Table XXIX the higher speed and reduced HIRF of the samples of Emulsion 35/T are apparent.

Example

A comparison cubic grain high chloride emulsion, hereinafter referred to as Emulsion 36/C, was precipitated by equimolar addition of silver nitrate and sodium chloride into a well stirred reactor containing low methionine gelatin peptizer. The resulting emulsion contained cubic grains with a mean edge length of 0.42 μm.

A silver iodochloride (0.05 mole percent iodide) {100} tabular grain emulsion according to the invention was prepared in which greater than 50 percent of total grain projected area was accounted for by tabular grains having {100} major faces. The mean grain ECD was 1.38 μm and mean grain thickness was

0.148 μm. The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.6, respectively. This emulsion is hereafter designated

Emulsion 36/T.

Portions of each of Emulsions 36/C and 36/T were sensitized by the addition of gold sulfide and spectral sensitizing dye SS-21 and heat digestion, followed by the addition of APMT and potassium bromide.

The sensitized portions of the emulsions were coated, exposed and processed as described above in Example 35, except that the sensitized emulsion portions were mixed with a magenta dye-forming coupler and coated as the green recording layer unit of a multicolor pack. The results are summarized in Table XXX.

Table XXX

Relative Speed Part at 10"¹s at 10-5s Delta Dmin

From Table XXX the higher speed and reduced HIRF of the samples of Emulsion 36/T are apparent.

Example 37

A comparison cubic grain high chloride emulsion, hereinafter referred to as Emulsion 37/C, was precipitated by equimolar addition of silver nitrate and sodium chloride into a well stirred reactor containing gelatin peptizer and thioether ripener. The resulting emulsion contained cubic grains with a mean edge length of 0.40 μm.

A silver iodochloride (0.05 mole percent iodide) {100} tabular grain emulsion according to the invention was prepared in which greater than 50 percent of total grain projected area was accounted for by tabular grains having {100} major faces. The mean grain ECD was 1.61 μm and mean grain thickness was 0.15 μm. The emulsion was washed by ultrafiltration, and its pH and pCl were adjusted to 5.6 and 1.6, respectively. This emulsion is hereafter designated Emulsion 37/T.

A portion of Emulsion 37/C was optimally chemically and spectrally sensitized by the addition of gold sulfide and heat digestion followed by the addition of AMPT, potassium bromide and red spectral sensitizing dye SS-19. A portion of Emulsion 37/T was optimally chemically and spectrally sensitized similarly as Emulsion 37/C.

The sensitized portions of the emulsions were coated, exposed and processed as described above in Example 35, except that the sensitized emulsion portions were mixed with a cyan dye-forming coupler and coated as the red recording layer unit of a multicolor pack. The results are summarized in Table XXXI.

Table XXXI

Relative Speed ^Part at 10"¹s at 1Q-5_s Del a Dmin

From Table XXIX the higher speed and reduced HIRF of the samples of Emulsion 37/T are apparent.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A process of preparing silver halide emulsions in which tabular grains of less than 0.3 μm in thickness exhibiting {100} major faces with adjacent edge ratios of less than 10 account for at least 50 percent of total grain projected area and internally at their nucleation site contain iodide and at least 50 mole percent chloride, comprised of the steps of

(1) introducing silver and halide salts and a dispersing medium into a continuous double jet reactor so that nucleation of the tabular grains occurs in the presence of iodide with chloride accounting for at least 50 mole percent of the halide present in the dispersing medium and the pCl of the dispersing medium being maintained in the range of from 0.5 to 3.5 and

(2) following nucleation completing grain growth in a reaction vessel which receives emulsion from the continuous double jet reactor under conditions that maintain the {100} major faces of the tabular grains.

2. A process according to claim 1 wherein bromide ion is present in the dispersing medium

following grain nucleation.

3. A process according to claim 1 wherein grain nucleation is undertaken in the presence of halide ions consisting essentially of chloride and iodide ions with the pCl of the dispersing medium being in the range of from 1.0 to 3.0 and a gelatino peptizer being present having a methionine content of less than 30 micromoles per gram of peptizer.

4. A process according to claim 3 wherein grain nucleation is undertaken in the presence of halide ions consisting essentially of chloride and iodide ions with the pCl of the dispersing medium being in the range of from 1.5 to 2.5 and a gelatino peptizer being present having a methionine content of less than 12 micromoles per gram of peptizer.

5. A process according to claim 1 wherein silver and halide salt solutions are introduced into the dispersing medium during grain nucleation and growth.

6. A process according to claim 5 wherein the addition of the silver and halide salt solutions is suspended after grain nucleation to allow Ostwald ripening of grain nuclei and then resumed.

7. A process according to claim 6 wherein chloride and iodide salt solutions are introduced into the dispersing medium during grain nucleation.

8. A process according to claim 7 wherein bromide salt solution is introduced into the dispersing medium after salt solution introduction is resumed after the addition of the silver and halide salt solutions has been suspended to allow Ostwald ripening of grain nuclei.

9. A process according to claim 1 wherein grain growth is continued until said portion of the tabular grains have an average aspect tabularity of greater than 25.

10. A process according to claim 1 wherein a silver halide ripening agent is introduced into the dispersing medium in the growth reaction vessel.

11. A process according to claim 11 wherein the ripening agent is a thioether.

12. A process according to claim 12 wherein the thioether wherein the thioether is a crown

thioether.

13. A process according to claim 11 wherein the ripening agent is a thiocyanate.

14. A process according to claim 11 wherein the ripening agent is methionine.

15. A process according to claim 11 wherein the ripening agent contains a primary or secondary amino moiety.

16. A process according to claim 16 wherein the ripening agent is an imidazole ripening agent.

17. A process according to claim 16 wherein the ripening agent is a glycine.

18. A process according to claim 1 wherein bromide ion in a concentration of from 0.5 to 15 mole percent is present in the reaction vessel during grain growth.

19. A process according to claim 18 wherein bromide ion in a concentration of from 1 to 10 mole percent is present in the reaction vessel during grain growth.

20. A process according to claim 1 wherein iodide ion in a concentration of from 0.001 to less than 1 mole percent is present in the reaction veseel during grain growth.

21. A process according to claim 1 wherein precipitation occurs in a pH range of from 2.0 to 8.0.

22. A process according to claim 22 wherein precipitation occurs at a pH of less than 7.0.

23. A process according to claim 23 wherein precipitation occurs in a pH range of from 2.0 to 5.0.

24. A process according to claim 1 wherein precipitation occurs in the presence of a mild

oxidizing agent chosen from the class consisting of a mercuric salt, an alkali tetrahaloaurate and an elemental sulfur releasing compound.