US3574628A

US3574628A - Novel monodispersed silver halide emulsions and processes for preparing same

Info

Publication number: US3574628A
Application number: US701092*A
Authority: US
Inventors: Evan T Jones
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1968-01-29
Filing date: 1968-01-29
Publication date: 1971-04-13
Anticipated expiration: 1988-04-13
Also published as: DE1904148A1; GB1250717A; DE1904148B2; BR6905935D0; BE727548A; FR2000911A1

Abstract

NOVEL MONODISPERSED SILVER HALIDE EMULSIONS COMPRISE SILVER HALIDE GRAINS FORMED IN THE PRESENCE OF AN ORGANIC THIOETHER SILVER HALIDE SOLVENT.

Description

United States Patent 01 ice NOVEL MONODISPERSED SILVER HALIDE EMULSIONS AND PRUCESSES FOR PRE- PARING SAME Evan T. Jones, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y. No Drawing. Filed Jan. 29, 1968, Ser. No. 701,092 Int. Cl. G03c 1/28 US. Cl. 96107 25 Claims ABSTRACT OF THE DISCLOSURE Novel monodispersed silver halide emulsions comprise silver halide grains formed in the presence of an organic thioether silver halide solvent.

This invention relates to photographic materials, processes for preparing photographic materials and the use of said materials. In one aspect this invention relates to the preparation of monodispersed silver halide crystals. In another aspect this invention relates to novel processes which can be used to provide new direct-positive emulsions.

Various ways have been described in the art to prepare monodispersed silver halide crystals. It also is known that the photographic sensitivity of silver halide crystals can be increased by increasing the crystal size. The methods previously described in the art, however, do not produce relative large monodispersed crystals which have high photographic sensitivity. It is evident, therefore, that a means which can be used to obtain monodispersed emulsions with relatively large grains would be desirable.

Accordingly, it is an object of this invention to provide a novel means for obtaining monodispersed silver halide crystals.

It is another object of this invention to provide monodispersed silver halide emulsions having increased photographic sensitivity.

It is another object of this invention to provide new monodispersed silver halide emulsions.

It is another object of this invention to provide directpositive silver halide emulsions having increased photographic sensitivity.

It is another object of this invention to provide coarse grain monodispersed silver halide emulsions.

In accordance with this invention it has been found that the above and other objects of this invention can be obtained by forming the monodispersed silver halide crystals in the presence of an organic thioether silver halide solvent. The emulsions are provided by a process which comprises reacting a water-soluble silver salt and an alkali metal halide in an aqueous hydrophilic colloid media containing an organic thioether solvent, wherein the pH is maintained at less than about 7 and the pAg is maintained between about 7.1 and about 9.6. The silver halide grains formed by this reaction can then be chemically sensitized to form negative type silver halide emulsions which sepectrally sensitize efficiently as described in lllingsworth, French Pat. 1,497,202 issued Aug. 28, 1967, or can be fogged to form a direct-positive emulsion. Direct-positive photographic emulsions prepared using this process exhibit a significant increase in photographic speed or sensitivity in comparison to direct-positive photographic silver halide emulsions prepared by conventional means.

One embodiment of this invention relates to relatively coarse grain photographic silver halide emulsions comprising silver halide grains which have a uniform diameter frequency distribution, i.e., silver halide grains which have substantially the same diameter, and to a process 3,574,628 Patented Apr. 13, 1971 for forming said grains in the presence of an organic thioether silver halide solvent. Such emulsions are termed monodispersed silver halide emulsions. Preferably, the silver halide grains of said emulsion have a mean grain diameter greater than about 0.25 micron.

One embodiment of this invention relates to new emulsions comprising silver halide grains having a mean grain diameter greater than 0.25 micron and at least 95%, by Weight, of said grains having a diameter within about 40% of the mean grain diameter.

Another embodiment relates to a new emulsion comprising silver halide grains, at least by Weight, of said grains being regular, and wherein said grains are formed in the presence of an organic thioether silver halide solvent. Silver halide grains which have a regular shape, as contrasted to an irregular crystal shape, are well known as shown, e.g., in chapter 2 of The Theory of Photographic Process, Mees and Jones, third edition (1966), published by Macmillan Company.

Another embodiment relates to new direct-positive photographic emulsions comprising fogged monodispersed silver halide grains having a mean grain diameter greater than 0.25 micron.

Aqueous solutions of suitable organic thioether silver halide solvents utilized during the grain growth or formation of the silver halides of the present emulsions have greater solubility for silver chloride than water. More specifically, such thioether silver halide solvents are those which, when utilized in aqueous solutions (60 C.) at 0.02 molar concentrations, are capable of dissolving more than twice the amount (by weight) of silver chloride than that which can be dissolved by water at 60 C.

The thioether silver halide solvent can be added to the silver halide emulsion at any stage of the preparation thereof before the silver halide grains have attained their ultimate size and shape, such as to the colloidal material in which the silver halide is precipitated, in combination with one of the water-soluble salts utilized to form the silver halide such as with the water-soluble silver salt (i.e., silver nitrate) or with a water-soluble halide such as an alkali metal halide, to the silver halide prior to or during the ripening of the silver halide, or during one or more of such emulsion preparation steps. Preferably, the thioether is added during the formation of the silver halide crystals.

The amount of the thioether silver halide solvent utilized can be widely varied depending on the effect desired, the nature of the thioether utilized and related variables. Concentrations of about .01 to about 10 grams of thioether per mole of silver halide are generally utilized, with about 0.1 to about 5 grams per mole of silver halide being preferably utilized.

Typical organic thioether silver halide solvents that can be suitably utilized in preparing the emulsions of the invention contain at least one moiety wherein oxygen and sulfur atoms are separated by an ethylene radical (e.g., O-CH CH S). Generally, the subject silver halide solvents have 1 to 3 thioether atoms (S although silver halide solvents having more than 3 thioether atoms can be utilized.

Certain of the present organic thioether silver halide solvents can be represented by the formulas:

and

wherein: r and m are integers of to 4; n is an integer of 1 to 4; p and q are integers of 0 to 3; X is an oxygen atom (-O), a sulfur atom (-S), a carbamyl radical or a oxycarbonyl radical R and R are ethylene oxide radicals (-O-CH CH Q and Z are hydroxy radicals (-OH), carboxy radicals, or alkoxy radicals (O-alkyl) wherein the alkyl group has 1 to carbon atoms; and Q and Z can also be substituents described for X linked to form a cyclic compound.

In one preferred embodiment the organic thioether silver halide solvent is a straight chain thioether, i.e., such as those represented by the formulae:

a carbonyl radical HO (-R -S-R -O*-'R 0 wherein r is an integer of 1 to 3 and .R is an alkylene radical having 1 to 5 carbon atoms and is preferably ethylene (-CH CH In another preferred embodiment the organic thioether silver halide solvent is a cyclic organic thioether, i.e., such as those represented by the formula:

ably regular in shape and have a rather uniform diameter frequency distribution. For example, at least 95 by Weight or number, of the photographic silver halide grains can have a diameter which is within about 40% and preferably within 30%, of the mean grain diameter. Mean grain diameter, i.e., average grain size, can be determined using conventional methods, e.g., as shown in an article by Trivelli and Smith entitled Empirical Relations Between Sensitometric and Size-Frequency Characteristics in Photographic Emulsion Series, in The Photographic Journal, vol. LXXIX, 1939, pages 330-338. The emulsions of this invention can comprise silver halide grains which either (1) are regular in shape or structure or (2) have a substantially uniform diameter. The emulsions of this invention can be Washed to remove excess soluble salts. For example, they may be chill-set, shredded and washed by leaching in cold water, or they may be Washed by coagulation. Suitable coagulation procedures are described in Yutzy and Frame, US. Pat. 2,614,928; Hewitson and McClintock, US. Pat. 2,618,556; Yackel, US. Pat. 2,565,418 and Waller et al. US. Pat. 2,489,341. Preferred photographic silver halide emulsions comprise at least 50 mole percent bromide, the most preferred emulsions being silver bromoiodide emulsions, particularly those containing less than about mole percent iodide. The photographic silver halides can be coated at silver 4 coverages in the range of about 50 to about 500 milligrams of silver per square foot of support.

The photographic emulsions of this invention generally comprise silver halide grains having a substantially uniform diameter. Generally, in such emulsions, no more than about 5%, by weight, of the silver halide grains smaller than the mean grain size and/or no more than about 5%, by number, of the silver halide grains larger than the mean grain size, vary in diameter from the mean grain diameter by more than about 40%. Preferred photographic emulsions of this invention comprise silver halide grains, at least 95%, by weight, of said grains having a diameter which is within 40%, preferably within about 30%, of the mean grain diameter. Mean grain diameter, i.e., average grain size, can be determined using conventional methods, e.g., as shown in an article by Tri-velli and Smith entitled Empirical Relations Between Sensitometric and Size-Frequency Characteristics in Photographic Emulsion Series in The Photographic Journal, vol. LXXIX, 1939, pages 330-338. The aforementioned uniform size of distribution of silver halide grains is a characteristic of the grains in monodispersed photographic silver halide emulsions. Silver halide grains having a narrow size distribution can be obtained by controlling the conditions at which the silver halide grains are prepared using a double run procedure. In such a procedure, the silver halide grains are prepared by simultaneously running an aqueous solution of a Water-soluble silver salt, for example, silver nitrate, and a water-soluble halide, for example, an alkali metal halide such as potassium bromide, into a rapidly agitated aqueous solution of a silver halide peptizer, preferably gelatin, a gelatin derivative or some other protein peptizer. The pH and the pAg employed in this type of procedure are interrelated. For example, changing one while maintaining the other constant at a given temperature can change the size frequency distribution of the silver halide grains which are formed, as shown in Example 1. However, generally the temperature is about 30 to about 90 degrees C., the pH is up to about 9, preferably 7 or less, and the pAg is up to about 9.8. Suitable methods for preparing photographic silver halide emulsions generally having uniform particle size are disclosed in an article entitled Ia: Properties of Photographic Emulsion Grains, by Klein and Moi'sar, The Journal of Photographic Science, vol. 12, 1963, pages 242-251; an article entitled The Spectral Sensitization of Silver Bromide Emulsions on Different Crystallographic Faces by Markocki, The Journal of Photographic Science, vol. 13, 1965, pages -89; an article entitled Studies on Silver Bromide So ls, Part I. The Formation and Ageing of Monodispersed Silver Bromide Sols by Ottewill and Woodbridge, The Journal of Photographic Science, vol. 13, 1965, pages 98-103 and an article entitled Studies on Silver Bromide Sols, Part II. The Effect of Additives on the S01 Particles by Ottewill and Woodbridge, The Journal of Photographic Science, vol. 13, 1965, pages 104-107.

Direct-positive photographic silver halide emulsions according to this invention contain silver halide grains which are fogged. Fogging can be effected by chemically or physically treating the photographic silver halides by methods previously described in the prior art. Such fogging can be accomplished by various techniques such as chemical sensitization to fog, particularly good results being obtained with techniques of the type described by Antoine Hautot and Henri Sauvenier in Science et Industries Photographiques, vol. XXVHI, January 1957, pages 57 to 65. The silver halide grains can be fogged with the high intensity light, reduction fogged with a reducing agent such as thiourea dioxide or stannous chloride or fogged with gold or noble metal compounds. Combinations of reduction fogging agents with gold compounds or compounds of another metal more electro-positive than silver, e.g., rhodium, platinum, or iridium, can be used in fogging the silver halide grains. The fogged silver halide grains in the direct-positive photographic emulsions of this invention give a density of at least 0.5, when developed without exposure for five minutes at 68 degrees F. in Kodak DK-50 developer when a direct positive emulsion containing such grains is coated at a coverage of 50 to about 500 mg. of silver per square foot of support.

Direct-positive photographic emulsions according to this invention can comprise reduction and gold fogged silver halide grains, i.e., silver halide grains which are fogged with a combination of a reduction fogging agent and a gold fogging agent. The use of low concentrations of reduction and gold fogging agents in such a combination give unique fogged silver halide grains which are characterized by a rapid loss of fog upon chemical bleaching, as known in the art. It is known that one equivalent weight of a reducing agent will reduce one equivalent weight of silver halide to silver. To obtain the fogged silver halide grains which are characterized by a rapid loss of fog upon bleaching, much less than one equivalent Weight of reduction fogging agent is employed. Thus, less than about 0.06 milliequivalent of reduction fogging agent per mole of silver halide is employed in fogging the silver halide grains. Generally, about 0.0005 to about 0.06, preferably about 0.001 to about 0.03 milliequivalent of reduction fogging agent per mole of silver halide is employed in fogging the silver halide grains in the practice of this invention. Higher concentrations of reduction fogging agent can result in a substantial loss in photographic speed. Examples of suitable reduction fogging agents which can be employed in the practice of this invention include hydrazine, phosphonium salts such as tetra (hydroxy methyl)phosphonium chloride, thiourea dioxide, as disclosed in Hillson, U.S. Pat. 3,062,651, isued Nov. 6, 1962, and Allen et al., U.S. Pat. 2,983,609, issued May 9, 1961, reducing agents such as the stannous salts, e.g., stannous chloride, as disclosed in Carroll, U.S. Pat. 2,487,850, is-

sued Nov. 15, 1939, polyamines such as diethylene triamine, as disclosed in Lowe et al., U.S. Pat. 2,519,698, issued Aug. 15, 1950, polyamines such as spermine as disclosed in Lowe et al., U.S. Pat. 2,521,925, issued Sept. 12, 1950, bis(fl-aminoethyl sulfide and its water-soluble salts as disclosed in Lowe et al., U.S. Pat. 2,521,926, issued Sept. 12, 1950, and the like.

The gold fogging agents employed in practicing this invention can be any gold salt suitable for use in fogging photographic silver halide grains and includes the gold salts disclosed in Waller et al., U.S. Pat. 2,399,083, issued Apr. 23, 1946, and Damschroder et al., U.S. Pat. 2,642,361, issued June 16, 1953. Specific examples of gold fogging agents are potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride, 2- aurosulfobenzothiazole metho chloride, and the like. The concentration of gold fogging agent employed in the practice of this invention is subject to variation, but is generally in the range of about 0.001 to about 0.01 millimole per'mole of silver halide. Potassium chloroaurate is a preferred gold fogging agent and is often used at concentrations of less than about 5 mg. per mole of silver halide and preferably at concentrations in the range of about 0.5 to about 4 milligrams per mole of silver halide. When a gold fogging agent is used in combination with a reduction fogging agent, the gold fogging agent preferably comprises a major portion of the fogging combination with the ratio of gold fogging agent to reduction fogging agent generally being in the range of about 1:3 to about 20:1, often about 2:1 to about 20:1. The silver halide grains are preferably fogged using the reduction fogging agent initially and subsequently using the gold fogging agent. However, the reverse order of agents can be used or the reduction and gold fogging agents can be used simultaneously.

In practicing this invention, the silver halide grains can be fogged prior to coating or they can be coated prior to fogging. The reaction conditions during fogging of the silver halide grains are subject to wide variation although the pH is generally in the range of about 5 to about 7, the pAg is generally in the range of about 7 to about 9 and the temperature is generally in the range of about 40 to about degrees C., most often about 50 to about 70 degrees C. During fogging the silver halide grains can be suspended in a suitable vehicle such as gelatin which is generally employed at a concentration in the range of about 10 to about 200 grams per mole of silver halide.

One embodiment of this invention relates to directpositive relatively coarse grain monodispersed emulsions which comprise not only silver halide grains having a substantially uniform size frequency distribution, but also a compound which accepts electrons. Suitable electron accepting compounds include the photoelectron accepting compounds or desensitizing dyes often used in photographic reversal systems. Compounds of this type include the known desensitizers which trap electrons, as disclosed in Illingsworth and Spencer, Belgian Pat. 695,364, granted Sept. 11, 1967.

The compounds which accept electrons in the directposi'tive photographic silver halide emulsions of this invention can be employed in widely varying concentrations. However, such compounds are preferably employed at concentrations in the range of about 100 milligrams to about 2 grams of electron acceptor per mole of silver halide. Typical examples of suitable electron acceptors include Intro-substituted azoles, nitro-substituted cyanine and merocyanine dyes, indolocarbocyanine dyes, phenosafranine, pinacryptol yellow, and the like.

Direct-positive emulsions of the type disclosed herein can be used in electron recording elements such as are disclosed in Brooks et al., Belgian Pat. 695,369 issued Sept. 11, 1967; Millikan, French Pat. 1,458,303 granted Oct. 3, 1966 and Trevoy, U.S. Pat. 3,245,833 issued Apr. 12, 1966.

Photographic elements made with the emulsions of this invention can be processed in conventional liquid solutions or can be processed with viscous processing solutions as described in The Journal of the Society of Motion Picture and Television Engineers, vol. 70, pages 875-881, 1961.

This invention can be further illustrated by the following examples of preferred embodiments thereof although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

EXAMPLE 1 A monodispersed silver bromoiodide photographic emulsion containing approximately 2.5% iodide and having an average grain size of about 0.2 micron is prepared by adding (1) an aqueous solution of potassium bromide and potassium iodide and (2) an aqueous solution of silver nitrate, simultaneously to a rapidly agitated aqueous gelatin solution at a temperature of 70 C. over a period of about 35 minutes. The pH is maintained at about 2 during precipitation and the pAg is about 8.9. The emulsion is chill-set, shredded and washed by leaching with cold water in the conventional manner.

Similar emulsions are made except that the thioether silver halide solvent, 1,8-dihydroxy 3,6 dithiaoctane (Compound I) is added at the concentrations listed in Table I to the gelatin solution before the precipitation, and the precipitation is carried on at the temperature and for the length of time also listed in Table I.

Each of the emulsions is reduction and gold fogged by first adding 01-025 mg. of thiourea dioxide (addendum 1) and heating for 60 minutes at 65 C., and then adding 0.5-2.0 mg. of potassium chloroaurate (addendum 2) and heating for 60 minutes at 65 C.; 25-100 mg. of an electron accepting compound S-m-nitrobenzalrhodanine (Dye 1) per mole of silver halide is added to each emulsion; specific concentrations are listed in Table I. Samples of the emulsions are coated on cellulose acetate film support at coverages of silver and gelatin listed in the table. Photomicrographs of the emulsions show the increase in grain size when the thioether is present during emulsion precipitation. A sample of each coating is exposed on an Eastman 1B sensitometer, developed for 6 minutes in an Elon-hydroquinone developer, fixed, washed and dried. The maximum density and photographic speed at a given density below maximum density are determined using the same procedure for each coating. The results in Table I show the tremendous increases in speed in coatings of emulsions precipitated in the presence of the thioether.

silver and 111 mg. of gelatin per ft. A sample of the coating is exposed on an Eastman 1B sensitometer, developed for 6 minutes in an Elon-hydroquinone developer, fixed, washed and dried. This emulsion prepared in the presence of the thioether has a considerably higher sensitivity than a similar emulsion prepared in the absenceof a thioether silver halide solvent.

The invention has been described in considerable detail with reference topreferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the TABLE I Amount of Mean Mg.

Compound grain Addendum, .Ag/ft. Dye 1, Relat e lia gi i nake iiigi with 1/2 lsf l i 12 7 5 x zib ie sp eii Dmax.

as at o. .35 25 0 100 204 46 as at 70 4 10/1 0 x 100 214 1. as

at 0 0 0. 135 so 1, 25. 0 2, 240 1. 2e

EXAMPLE 2 invention as described hereinabove and as defined in TABLE I Time and Addendum 1/2 Mean Temp. of mgJAg grainsize, make Thloether (gmsJAg mole) mole micron 70 at 70 C- 1,10-dlthla-4,7,l3,l6-tetra- 0. 1/1. 0 1. 2

oxacyelooctadecaue (0.8). Do 2, 2 -thlodlethano1 (30.0)- 0. 1/1. 0 0.8

The emulsions made with these thioethers have higher sensitivities than similar emulsions prepared in the absence of a thioether silver halide solvent.

Similar results are obtained when 1,17-di(N-ethylcarbamyl)-6,l2-dithia 9 oxaheptadecane, 3,15-dioxa- 6,9,12-trithio heptadecane and 6,9-dioxa-3,IZ-dithia-tetradecane-1,14-diol are utilized in the preparation of the silver halide grains.

EXAMPLE 3 Thioethers can also be advantageously used in the preparation of negative type regular grain emulsions, such as those described in Illingsworth, French Pat. 1,497,202.

A monodispersed silver bromoiodide photographic emulsion containing approximately 2.5 mole percent iodide and having an average grain size of about 0.25 micron is prepared by simultaneously adding an aqueous solution of potassium bromide and potassium iodide and an aqueous solution of silver nitrate in equal molar amounts to a rapidly agitated aqueous gelatin solution maintained at a temperature of 40 C. and containing 0.4 g. of 1,8-di-hydroxy-3,6dithiaoctane per mole of silver halide; the addition being carried out over a period of about 35 minutes. The pH of the aqueous gelatin solution is maintained at 5.6 and the pAg is maintained at 9.0 during the addition. The emulsion is washed to remove soluble salts and additional gelatin is added. The emulsion is sulfur and gold sensitized as described in Waller, US. Pat. 2,399,083. The emulsion is panchromatically sensitized with cyanine dyes and is then coated on a cellulose ester support at a coverage of 150 mg. of

the appended claims.

I claim:

1. A monodispersed photographic emulsion comprising silver halide grains formed in the presence of an organic thioether silver halide solvent, wherein said thioether silver halide solvent at a 0.02 molar concentration in water is capable of dissolving more than twice the amount of silver chloride than that which can be dissolved by water at 60 C. and wherein at least 95%, by weight or number, of said silver halide grains have a size which is within 40% of the mean grain size.

2. An emulsion according to claim 1 wherein said silver halide grains have been fogged to give a density of at least 0.5, when developed without exposure for 5 minutes at 68 F. in Kodak DK-50 Developer when coated on a support at a coverage of between about 50 to about 500 mg. of silver per square foot of support.

3. An emulsion according to claim 1 wherein said silver halide grains have a mean grain size greater than about 0.25 micron.

4. An emulsion according to claim 1 wherein said silver halide. grains are regular.

5. An emulsion according to claim 1 wherein at least by weight or by number, of said grains are regular.

6. An emulsion according to claim 1 wherein the halide of said silver halide is predominantly bromide.

7. A direct-positive photographic emulsion according to claim 1 wherein at least by weight or number, of said grains have a size which is within about 30% 0f the mean grain size.

8. An emulsion according to claim 1 wherein said silver halide grains are cubic-regular grains having a mean grain size in the range of about 0.25 to about 2 microns.

9. A direct-positive photographic emulsion according to claim 1 wherein said grains are fogged with a reduction fogging and a gold fogging agent.

10. The photographic emulsion of claim 1 wherein said organic thioether contains at least one moiety of the following formula:

11. The photographic emulsion of claim 1 wherein said thioether is a straight chain compound.

12. The photographic emulsion of claim 1 wherein said thioether is a cyclic compound.

13. A process for preparing a photographic emulsion comprising (1) reacting an alkali metal halide and a water-soluble silver salt in the presence of an aqueous hydrophilic colloid and an organic thioether silver halide solvent, wherein the pH is controlled at less than 7 and the pAg is controlled between 7.1 and 9.6, wherein said thioether silver halide solvent at a 0.02 molar concentration in water is capable of dissolving more than twice the amount of silver chloride than that which can be dissolved by water at 60 C., and (2) fogging the silver halide grains to give a density of at least 0.5, when developed without exposure for minutes at 68 F. in Kodak DK-50 Developer when coated on a support at a coverage of between about 50 to about 500 mg. of silver per square foot of support.

14. The process of claim 13 wherein said thioether contains at least one moiety of the formula:

15. The process of claim 13 wherein said thioether is a cyclic compound.

16. The process of claim 13 wherein said thioether is a straight chain compound of the formula:

wherein r is an integer of from 1 to 3; R and R are alkylene radicals having 1 to 5 carbon atoms.

17. The process of claim 13 wherein said thioether is present during at least part of the precipitation at a concentration of at least about 0.1 gram to about 5 grams per mole of silver halide.

18. The process of claim 13 wherein the fogging step comprises contacting the silver halide grains with a reduction fogging agent and a gold fogging agent.

19. The process of claim 13 wherein the fogging step comprises contacting the silver halide grains with about 0.0005 to about 0.06 milliequivalent per mole of silver halide of a reduction fogging agent and about 0.001 to about 0.01 millimole per mole of silver halide of a gold fogging agent.

20. The process of claim 13 wherein the fogging is carried out at a temperature in the range of about 40 to 100 C., and the ratio of gold fogging agent to reduction fogging agent being in the range of 1:3 to about 20:1.

21. A process for preparing monodispersed silver halide grains, wherein at least 95%, by weight or number, of said silver halide grains have a size which is within 40% of the mean grain size, comprising reacting an alkali metal halide and a water-soluble silver salt in the presence of an organic thioether silver halide solvent, Wherein the pH is controlled at less than 7 and the pAg is controlled between 7.1 and 9.6, wherein said thioether silver halide solvent at a 0.02 molar concentration in water is capable of dissolving more than twice the amount of silver chloride than that which can be dissolved by water at C.

22. The process according to claim 21 wherein at least by weight or number, of said grains have a size which is within about 30% of the mean grain size.

23. The process according to claim 21 wherein said silver halide grains have a mean grain size greater than about 0.25 micron.

24. A monodispersed, direct-positive, photographic emulsion comprising fogged silver halide grains wherein said silver halide grains were precipitated in the presence of an organic thioether solvent, which at a 0.02 molar concentration in Water is capable of dissolving more than twice the amount of silver chloride than that which can be dissolved by water at 60 C., and wherein at least 95%, by weight or number, of said silver halide grains have a size which is within 40% of the mean grain size.

25. A direct-positive emulsion according to claim 24 wherein said grains have been chemically fogged after precipitation thereof to give a density of at least 0.5, when developed without exposure for 5 minutes at 68 F. in Kodak DK-SO Developer when coated on a support at a coverage of between about 50 to about 500 mg. of silver per square foot of support.

References Cited UNITED STATES PATENTS 3,021,215 2/1962 Williams et al. 96107 3,271,157 9/1966 McBride 96-108 FOREIGN PATENTS 1,027,146 4/1966 Great Britain 96-94 NORMAN G. TORCHIN, Primary Examiner R. E. FIGHTER, Assistant Examiner