US3637392A - Direct-positive silver halide emulsions containing boron hydrides - Google Patents

Abstract

Direct-positive photographic elements having good sensitometric characteristics and aging stability are prepared by incorporating in the silver halide emulsion from 5 X 10 7 to 2.4 X 10 4 mole per 1.5 moles of silver halide of at least one boron hydride in which the skeletal framework forms a polyhedron or a fragment thereof, and which may contain heteroskeletal atoms selected from the groups C, S, and N, as a chemical fogging agent.

Description

United States Patent Bigelow [151 3,637,392 [4 1 Jan. 25, 1972 DIRECT-POSITIVE SILVER HALIDE EMULSIONS CONTAINING BORON HYDRIDES lnventor: John Howard Bigelow, Rochester, NY.

Assignee: E. I. du Pont de Nemours and Company,

Wilmington, Del.

Filed: Aug. 10, 1970 Appl. No.: 62,650

Related US. Application Data Continuation-impart of Ser. No. 8,417, Feb. 3 1970.

US. Cl ..96/108, 96/109 Int. Cl. ..G03c l/28,G03c l/34 Field of Search ..96/108, 109, 101

[56] References Cited UNITED STATES PATENTS 3,361,564 l/1968 Bigelow ..96/101 3,445,235 5/1969 Burt ..96/101 Primary Examiner-Norman G. Torchin Assistant Examiner-Richard E. Fichter Attorney-Lynn Barratt Morris [5 7] ABSTRACT 13 Claims, N0 Drawings DIRECT-POSITIVE SILVER HALIDE EMULSIONS CONTAINING BORON HYDRIDES CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 8,417, filed Feb. 3, 1970, and is related to Assignees Burt, U.S. Ser. No. 736,276, filed June 12, 1968, and Burt & Ciavarri, U.S. Ser. No. 865,976, filed Oct. 13, 1969.

BACKGROUND OF THE INVENTION This invention relates to direct positive colloid silver halide photographic emulsions. More particularly it relates to such emulsions which are prefogged by a chemical fogging agent.

Direct positive emulsions are, of course, well known and there are many different methods of producing direct positive images. For example, a silver halide emulsion may be given a short overall exposure of high intensity radiation and then given a longer imagewise exposure of lower intensity. Upon development, a direct positive will be obtained. Another method is to expose imagewise and develop and bleach out developed silver and then flash expose and redevelop. A still further method is to use emulsion coated elements which have been chemically fogged with, for example, formaldehyde, hydrazine, sodium arsenite, silver-ions and other nonsulfide fogging agents thus eliminating a solarizing exposure. Upon imagewise exposure and development of such chemically fogged elements, a positive image is obtained. In direct positive photographic elements utilizing this latter method, there is usually incorporated a desensitizing compound, usually a colored desensitizing dye. A significant improvement was made in the art by the use of amine boranes as chemical fogging agents in direct positive emulsion systems. Direct positive emulsions and elements comprising such emulsions are described and claimed in assignees Bigelow & Burt, U.S. Pat. Nos. 3,361,564 and Burt, U.S. Pat. Nos. 3,445,235 and assignees Overman, U.S. Ser. No. 861,501, Sept. 26, 1969. The Burt patent discloses and claims the use of rhodium and iridiurn salts in direct positive emulsions described in the above Bigelow and Burt patent and the Overman Application disclose the use of bismuth as antikink agents. The direct positive emulsions made as described in the Bigelow & Burt and Burt patents are quite fast as compared to direct positive elements of the prior art and suffer none of the disadvantages of long exposure times, low reversal densities, undesirable stain due to desensitizing dyes, as well as undesirable contrast, and a multiplicity of exposure steps and processing operations. Improvements in speed and other sensitometric properties of the direct positive layers of the above patents are taught by the above Burt and Burt and Ciavarri applications.

SUMMARY OF THE INVENTION It has now been found that the shelf life or aging stability and contrast of the direct positives disclosed in the above patents and applications can be substantially improved by the use of a new class of borane compounds as chemical fogging agents. The new compounds are boron hydrides in which the skeletal framework forms a polyhedron or fragment thereof and may contain heteroskeletal atoms selected from the group C, S, and N. They may be added to the emulsions in amounts ranging from 5X10" to 2.4Xl" mole per 1.5 moles of silver nitrate. They may be added to the emulsion as a solution with a suitable solvent, i.e., water, ethyl alcohol, benzene, acetone, dioxan, etc. The useful boron hydrides in aqueous solutions have greater stability than the amine boranes of the above patents. The effectiveness of some of the boron hydrides in fogging a silver halide emulsion is not affected by the pH of the systems and they show good fogging properties under acid as well as alkaline conditions. Of course, certain of the boron hydrides are more effective than others when used in equivalent quantities. The fogging amounts of particular boron hydrides within the above limits can easily be deter mined empirically. The fogging effect produced by the boron hydride is bleachable by light exposure prior to development.

A suitable developer may be any conventional, alkaline, photographic developing solution which would be used for standard processing of direct-positive emulsions in the absence of the boron hydrides. Although the mechanism is not clearly understood, the light-bleaching effect is believed due to solarization. While the silver halide system used in this invention is generally silver chlorobromide, other types, e.g., silver chloride, silver iodobromide, silver iodochloride, and silver iodobromo chloride, may be used with the boron hydrides.

Among the boron hydrides which have been found useful as chemical fogging agents in this invention are the boron hydrides of the series B l-[ to 8 1-1 and the boron hydrides comprising the polyhedral azaboranes, polyhedral thiaboranes and polyhedral carboranes. These boranes and their preparation are described in: POLYHEDRAL BORANES by Earl L. Mueterties and Walter H. Knoth, published by Marcel Dekker, Inc., New York.

For most efficient fogging action it is preferred that the boron hydrides of this invention be added to the silver halide emulsion after it has been made, ripened and washed to remove the excess soluble salts resulting from the precipitation of the silver halides. Conveniently, the borane compounds are added just prior to or during the digestion or after ripening period. The pH of the emulsion is adjusted to between 3.0 and 9.0 and preferably between 5 and 8.5 and maintained at a preselected level in this range during digestion. After digestion, the pH is adjusted to the desired coating pH and the conventional coating aids are added. Coating aids such as saponin, sodium salts of polyether sulfonates, alkyl sulfonates, cetyl betaine, etc., can be utilized. The emulsion is then coated and dried in the manner known to those skilled in photographic manufacturing methods.

The sensitometric characteristics of the direct positive emulsions may be determined by exposing in an intensity scale sensitometer (described on page 616, Mees, The Theory of the Photograph Process, MacMillan Company, New York, 1942) using a suitable step wedge and light intensity depending upon the inherent speed of the emulsion. Direct positive emulsions are of course tailored to meet certain needs in the industry and are manufactured accordingly. After exposure in the sensitometer the strips are developed in suitable developers as shown below, immersed in a shortstop bath and fixed for 3 minutes in a conventional fixer and then washed and dried.

In evaluation of the processed strips, the toe is equal to the step number at density 0.25 minus step number at density 0.01. The maximum density is the highest density value obtained on the image of the step wedge. The speed of a typical low speed commercial direct positive in terms of /E X 10 is 103 and the conventional material has a B of 3.5.

The invention will now be illustrated in and by the following examples:

EXAMPLE I A direct positive emulsion was made in the following manner. To an aqueous solution of gelatin containing 1 mole of potassium chloride and 6.8 10 mole of rhodium trichloride there was rapidly added 1 mole of silver nitrate in an aqueous solution. To the resulting mixture there was added 0.4 mole of potassium bromide in aqueous solution and the resulting emulsion was then ripened by heating for 10 minutes as 160 F. A second portion of 0.6 mole of potassium bromide was added and the emulsion was further ripened for 10 minutes at 160 F.

The resulting emulsion was then cooled, washed and redispersed in the manner disclosed in Moede, U.S. Pat. No. 2,772,165, issued Nov. 27, 1956.

To 340 grams of 10 percent aqueous gelatin solution there was added 0.375 mole of the above silver chlorobromide emulsion. The suspension is stirred, heated to F the pH adjusted to 8.1 with borax. The chemical fogging agents,

morpholine borane as a control and various azaboranes as shown in the following table, were added and the emulsion was digested at I30 F. for 40 minutes. Morpholine borane is disclosed and claimed in assignees patents noted above as (CH NNB H, shows better aging stability than morpholine borane in 5-day oven aging, and (CH ).,N(CHQ NN'B H shows equal or better aging stability on 7-day oven aging. These foggers also increased contrast (less toe) at equal or chemical fogging agents and is used here as a control sample. 5 better D-maX. After digestion, the emulsion was cooled to 95 F the pH was adjusted to 7.0 with acetic acid and a wetting agent, i.e., EXAMPLE H saponin, vvas added- The Viscositlf Suitably adjusted and A direct positive emulsion was made as described in examlhe emulsion was coated at approximately 700 AgNOa P ple l and coatings were made of emulsion samples containing square foot on a Photographlc film PP and drled a the borane compounds shown in the table below for the purventional manner. sensltometric tests were made on freshly pose demonstrating h ili f h solutions f the coated material and material which had been aged an 7 ounds which are added to the emulsions. All testing was cardays at 120 F. and 62 p rc n r la ive humid y. h is ried out as in example i. No testing was carried out of emulgeneral y quiv n to about months normal g gl 5 sions containing morpholine borane from solutions older than Th n l m m SHIPS were eXPOSed In the above- 36 hours because of deterioration of the solution as shown by described sensitometer with Luxometer units through a analytical tests.

TABLE II Amount Develmole/1.5 Age of Sp Dmaz. Toe oper .AgHal Compound solution Ctg. N0

1 1,280 5.67 2.6 II 1groin ZAXIO Morphollne borane 0.001Min ethanol gSfi-hr.

ample 2 777 5. 58 2.7 .ldo- 1.2XI0' (CHahNNBpHp 0.001 M in ethanol. Day of use. 3 1,040 5.56 1.5 do. 1.2Xl0' d0- 8-day. 4 3,770 6.13 2.3 do 1.2)(10 d0 27-day. 5 l, 150 6.58 2.3 d0 1.8)(10' (CH3)4N(CI Day of use.

in ethanol. 6 1,110 5.56 2.4 do 1.8X104 d0 27-day.

N 0TE.See Table I footnote.

2 wedge with 21 steps. The strips were developed in the two (CH ).,N(Cl-l l NNB H is more stable than (CH -,NNB H following developers for 1% minutes at 68 F.

Developer I (High-contrast type) Water to make LG I.

because there is little difference between a fresh (day of use) solution and 27-day old solution. (Cl-l -,);,NNB,,l-l solution shows little change in 8-day age. This is better than morpholine borane (which is only allowed 36-hr. age) as explained above.

EXAMPLE Ill mole of (CH NNB H, /unit even with a solution that is 26 days old. A fresh solution requires a nominal l.2 l()' mole/1.5 moles AgHal (see coating 2 of Ex. I) to give results similar to l.6 l0 mole/1.5 moles AgHal of a 26 day old Sensitometric evaluation adduced the following data: solution.

TABLE I Amount mole] Devel- 1.6 moles Test Sp. Din" Toe oper AgHal Compound age Ctg. No

1 1, 580 5. 73 1. 7 II 2.4X10- Mgrplholine borane 0011- Fresh.

:0 I, 930 5. 74 2. 0 I 2.4Xl0- 2, 590 5.59 2.2 II 2.4)(10- 2, 690 5. 40 2. 3 I 2.4)(10- 2 2, 070 5. 72 1. 3 II 1.2X10' 2, 5. 76 1. 6 I 1.2)(10- 2,260 5.84 1.6 II 1.2XI0- 2, 070 5. 91 I. 5 I 1.2X1U- 3 1, 380 6. 31 1. 3 II 2.4Xl0' (CH3)4N(CH )2 Fresh NNBQHH I, 280 6. 06 1.1 I 2.4X10" Same Do. 1,680 5.74 1.9 II 2.4)(10 ..do 7 days. 1, 870 5. 74 1. 9 I 2.4)(10 ...do Do.

NoTE.Sp.=Speed=l00lEX10-* at density 0.25; D L=Maximum density;

0.25 minus step number at density 0.01. This the greater the contrast.

Toe=Step number at density number gives a measure of contrast in which thesmaller the number TABLE III Amount Develmole/1.5 oper moles Sp Dmax Toe Ex. I AgHal Compound Ctg. N o

1 3, 640 3.07 3 2 II 1.2X10 (CIImNNB H solution aged 26 days.

4, 420 3. 31 3 3 I 1. 2X10' Do. 2 1,630 6. 31 2.8 II 1.6Xl- Do. 2,000 6.31 2.4 I 1.6Xl0- Do. 3 777 6.34 2 2 II 2.4X10- Do. 889 6.33 2 1 I 2.4X10 D0. Norm-See Table I footnote.

EXAMPLE IV These data show that CSB H S is a more effective compound A direct positive emulsion was made as described in example I and morpholine borane and the boron hydride, CsB l-l s, were added as chemical fogging agents as indicated in the following table. All tests and operations were carried out as described in example I. As indicated in example I, the emulsion containing morpholine borane is used as a control. Semitometric evaluation provided the following data:

at low pH than morpholine borane, and maintains D-max more effectively in oven aging at low pH.

EXAMPLE VI Direct positive emulsions were made as described in example l to which various boron hydrides were added to demonstrate the wide number of these compounds which can be used NoTE.See Table I footnote.

These data show the compound, CSB H S, to give greater speed and contrast (low toe) at approximately equal D-max, and less change on oven aging than the control compound, morpholine borane, and requiring only 0.1 as much fogging agent as the control.

EXAMPLE V A direct positive emulsion was made as described in example I and morpholine borane as a control and the boron hydride, CsB H S, was added to emulsion adjusted to various pH's as indicated in the following table. The emulsions were otherwise treated as in example I and developed in Developer as chemical fogging agents for the production of bleachable fog.

EXAMPLE vn Example 11 was repeated using 2.4 l0 mole/1.5 moles silver halide, the boron hydride, CsB H, S, as the chemical fogging agent which was added from 0.001 molar aqueous solution aged as indicated in the following table. Approximately the same response is obtained regardless of fogger solution age.

These data show that within the limits of experimental error, 13 weeks old 0.001 Molar CSB H S solution in water ll. produces the same result as day-of-use solution. With 0.00]

TABLE V Amount mole/1.6 Romelt moles Tost Sp. Din-x. Too pH AglIal Foggor typo Ctg. No.2

1 No dens 3.0 24X10- Morpholine boruno. Fresh. .d0 3.0 24x10 do 5 days. 2 3, 900 3.86 1.4 3.0 2:900" CSBQII S Fresh. 3,640 4.18 2.2 3.0 2.4)(10- Same as above fidays. 3 No dens 4.0 24x10- Morpholino b0tan0 Fresh. do... 4.0 24 l0- -do-... 6days. 4 2, 260 5.31 1.8 4.0 2.4)(10- CSBQI'IHS Fresh. 1,930 5.08 2.3 4.0 2.4)(10' Same as above 5 days. 5 4, 680 3.51 2.0 5.0 24X10' Morphollno borano Fresh. 4,680 1.71 3.0 5.0 24)(10- do 5 days. 6 497 6.1 1.3 5.0 2.4xl0- CsB H S Fresh. 627 6.1 2.1 5.0 2.4)(10- Same as above fidays.

NoTE.-Seo Table 1 footnote.

molar morpholine borane water solution, it is necessary to EXAMPLE Vlll limit solution age to a maximum of 36 hours in order to obtain A standard slow speed, fine grain silver chloride emulsion uniform results. was prepared in a conventional manner. To portions of the TABLE VI Amount Dovelmole/ Sp. Dun. Toe Ex.I AgHal Compound Ctg. No.:

l 2,000 0.23 2.4 I 2.4X10 /UO2CH2 II N=BII1 CH2 CH3 Morpholine borane Control.

2, 070 a. 27 2. 2 i1 2. 4x10- Same as above.

2 702 6. 1. 4 I 2. 4X10- BnH13S(C2H5)2 752 0.21 1. 6 II 2. 4X10- Same as above.

3 1. 230 6. 22 2.0 I 2.4)(10 B10 lZ[s(c2H5)g]g 1, 230 6. 2. 5 II 2. 4X10 Same as above.

4 827 6.14 1.6 I 2. 4X10 BiuHizCNHa 927 6. 24 1.0 II 2. 4X10- Same as above.

5 1, 330 5. 22 2. 2 I 2. 4x10- (CH3)4N(CH3)3CNHCB10H13 1, 280 6. 11 2. I II 2. 4000- Same as above.

6 1, 870 6.21 2. 2 I 2. 4X10 Morpholine bomne control.

I. 480 6. 21 2. 5 II 2. 4X10 Do.

7 880 6. 23 1. 0 I 2. 4X10 B10H122(C H928 1 027 6. 24 1. 9 II 2. 4X10- Same as above.

a 611 6.18 3.0 1 2.4x10- cnnunliws 603 6.17 2. 0 II 2. 4X10" Same as above.

0 852 6.18 3.0 I 2.4)(10- (CH3)4NBmHnS 852 0. 17 2. 5 II 2. 4X10- Same as above.

10 8.080 3. 18 2.5 I 2. 4Xl0 (GH3)3N1IB10III2CII V 7. 780 3. 32 2. 4 II 2. 4X10" Same as above.

NOTE. See Table I footnote. 7 i

TABLE VII Cataloging weight mg. AgNOa/ Devel- Fogglng agent Sp. Toe Dmlx. sq. ft. Fog oper solution ago Ctg. No

1 4, 290 2.7 4.64 573 .00 I Day-of-use".

4,190 '2. 2 4. 32 573 .00 II Do.

2 5,450 2.4 3.97 503 .00 I 2 days.

5,640 3.4 3. 503 .00 II Do.

3 4,290 2.2 4.83 605 .00 I Sdags.

4 5,640 2.5 4.23 537 .00 I 13 days. 6,030 2.2 3.00 537 .00 II Do.

5 4,290 2. 7 5.02 698 .00 I 15 days. 4,680 2.0 5. 23 698 .00 II Do.

6 5, 450 2. 4 4.00 815 .00 I 13 weeks plus 1 day. 5,640 2. 2 4. 70 816 00 II Do.

Nora-See Table I footnotes.

TABLE VIII Fog Fogglng Sp. Dmn. 1X 3X agent Amount Catalog No.:

1 5.11 .00 .05 HCHO oon- 0.1 mole/1.5 Fresh.

trol. moles AgHal. 106 4.65 .00 .04 do "do 2M0.NA.

30 1.70 .00 .00 do "do 7days 0'1.

2 142 4. 71 .00 .00 GsB H S 7X10- mole] Fresh.

1.6 moles AgHal. 142 5.05 .00 .04 Same .do 2 M0. NA. 126 5.17 .00 .00 do "do 7 days OT.-

Natural ng (months). 2 Oven te l mn accelerated aging test considered to equal approximately 10 months natural ago, see Example I).

emulsion there was added formaldehyde and the boron hydride, CsB H S, in the amounts indicated in Tubl e V lll.

EXAMPLE 1X An emulsion was made as described in example I and morpholine borane as a control and the boron hydride, [(C6H,) .CH.P].B,.H were added in an amount of 2.4x 10*? mole per 1.5 moles of silver halide. The pH of the emulsion containing the control amine borane as adjusted to 8.0, and that to which the polyhedral borane was added, was adjusted to 7.0. The results are shown in the table.

TABLE IX Develp per, Sp Dmux. Toe x;I Compound Ctg. No

1 2,490 4.16 3.1 I Morp)hollne borane.

2,590 4.25 2.2 II o. 2 4,680 4.57 2.5 I gcuHshcHaPhBnIIu 4, 420 4.63 2.8 II ame as above.

NoTE.See Table I footnote.

The boron hydride shows higher speed and maximum density than the control.

EXAMPLE X An emulsion was prepared as described in example I and various quantities of the boron hydride, CSBBHHS, were added at pH 8.1 to 8.3 to give results as shown in the table.

TABLE X Mole Devel- CsB H S/ paper. 1.5 moles Sp. Durex. Toe Fog x.I AgHal Ctg No 1 12,300 3.49 2.7 .00 I 2.4)(10' 11,900 3.27 2,-7 .00 II 2.4X10- 2 2,890 5.97 1.5. .00 I 4X10" 2,990 5.65' 1.6 .00 II 4X10" 3 1,280 5.81 2.1 .00 I 8X10 1,330 5.64 1.2 .00 II 8X10- 4 965 6.24 1.3 .00 I 12X10- 965 6.25 1.8 .00 II 12X10- 5 777 5.60 1.7 .00 I 16X10' 752 5.67 1.6 .00 II 16x10- 727 5.53 1.5 .00 II 20X10- 7 677 5.63 1.3 .02 I 24X10- 725 5.53 1.5 .02 II 24x10- Vslow. VV slow.

7 Nora- 82 1121131 1 footnote.

EXAMPLE XI To demonstrate the usefulness of the boron hydrides as chemical fogging agents under varying low pH conditions an emulsion was made described in example I. The boron hydride, CsB H S, was added in the amounts per 1.5 moles of silver halide and under the pH conditions indicated in the following table.

TABLE XI Mole Devel- Emul- CSB IInS/ opar, slon 1.5 moles Sp. Dmu. Too Fog Ex. 1 p11 AglIul Ctg.No

1 3,640 4.28 3.2 .00 I 3.0 2.4)(10 3,000 3.86 1.4 .00 II 3.0 2.4X10" 2 2,000 2.50 4.4 .00 I 3.0 12x1o- 2,000 2.67 3.4 .00 II 3.0 12X10- 3 927 2.97 2.2 .00 I 3.0 36x10- 965 2.66 1.8 .00 II 3.0 36X10- 4 2,200 4.9 2.7 .00 I 4.0 2.4)(10- 2,260 5.3 1.8 .00 II 4.0 2.4X10- 607 5.24 1.8 .00 II 4.0 12X10- 6 585 1.05 2.4 .01 I 4.0 30x10- 677 .02 2.0 .00 II 4.0 36 1CH 7 479 6.06 1.2 .00 I 5.0- 2.4X10- 497 6.12 1.3 .00 II 5.0 2.4X10- 8 268 4.67 .6 .25 I 5.0 12x10- 243 5.77 .3 .00 II 5.0 12X10- 9 252 4.11 .4 .84 I 5.0 36X10 293 3.63 .9 .00 II 5.0 36 10- Norm-See Table I footnote.

The preferred concentrations are 2.4X10 to 2.4Xl0' and some of the boron hydrides are considerably more active as chemical fogging agents than morpholine borane which is used as a control in many of the above examples and is representative of the amine boranes used as chemical fogging agents in Bigelow and Burt US. Pat. No. 3,361,564. Many of the boron hydrides act efficiently as chemical fogging agents under conditions of low pH i.e., 3-5 whereas amine boranes require pHs generally above 7 and preferably 8.0 to 8.5 to give satisfactory fogging.

Boron hydrides generally show less loe in the sensitometric curve hence greater contrast than the amine boranes. The boron hydrides produce direct positive elements having improved fresh and oven aged response which is related to shelfaging over that obtained from amine boranes. A further advantage is that the boron hydrides are more stable in the stock solutions used to add them to the silver halide emulsion systems than is the case with the amine boranes some of which can not be made up in solution form for more than 36 hours before use.

As is evident from the sensitometn'c data the use of the boron hydrides as chemical fogging agents provide the above advantages without any loss in speed as compared to the amine boranes thus providing extremely fast direct positive elements. The elements of this invention, as shown in the examples, may be developed in any standard developing solution either of the continuous tone type or the high contrast lithographic type using standard technique. Variations in the developing solution will have much the same effect as it would in developing nonreversal emulsions. No preexposure operations or auxiliary processing procedures are necessary or described in using the novel elements of this invention. It is also unnecessary to utilize stain producing nonsensitive dyes or other desensitizing compounds in the emulsion. The boron hydrides are far superior to formaldehyde, the principle fogging agent of the prior art, because the boranes do not have a hardening effect on the gelatin layer.

The emulsions of this invention may be coated on any suitable base including paper and transport film supports. For exterephthalic acid and dimethyl terephthalate with propylene glycol, dimethylene glycol, tetramethylene glycol or cyclohexane-l,4-dimethanol(hexahydro-p-xylene alcohol). The films of Bauer et al., US. Pat. No. 3,059,543 may also be used. The above polyester films are particularly suitable because of their dimensionable stability. The emulsions are generally coated on the supports in quantities to give a coating weight of about 700 mg. AgNO /ftF.

Many boron hydrides other than those listed above can be used as chemical fogging agents for direct positive photographic elements in accordance with the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A direct-positive, water-permeable colloid-silver halide emulsion containing a fogging amount of a boron hydride in which the skeletal framework forms a polyhedron or a fragment thereof and may contain heteroskeletal atoms selected from the group C, S, and N.

27 An emulsion according to claim 1, wherein the amount ranges from X10 to 24x10 mole per 1.5 moles of silver.

3. An emulsion according to claim 2, wherein the silver halide is silver chloride.

4. An emulsion according toclaim 2, wherein the silver halide is silver chlorobromide.

S. An emulsion according to claim 2, wherein said colloid is gelatin.

6. An emulsion according to claim 2, wherein the boron polyhedral homonuclear borane of the series ing to claim 2, wherein the boron hydride is a polyhedral heteronuclear borane.

8. A direct-positive photographic element comprising a sheet support bearing a l defined in claim 2.

ayer of a silver halide emulsion as 9. An element according to claim 8, wherein the silver halide is silver chloride.

BaHu' IO B zHiz 12. An element accord hydride is a polyhedral het 13. An element accord hydride is CSB H S.

ing to claim 8, wherein said boron eronuclear borane. ing to claim 8, wherein said boron star-11*

Claims (12)

1. 2. An emulsion according to claim 1, wherein the amount ranges from 5 X 10 7 to 2.4 X 10 4 mole per 1.5 moles of silver.
2. 3. An emulsion according to claim 2, wherein the silver halide is silver chloride.
3. 4. An emulsion according to claim 2, wherein the silver halide is silver chlorobromide.
4. 5. An emulsion according to claim 2, wherein said colloid is gelatin.
5. 6. An emulsion according to claim 2, wherein the boron hydride is a polyhedral homonuclear borane of the series B6H6 2 to B12H12 2.
6. 7. An emulsion according to claim 2, wherein the boron hydride is a polyhedral heteronuclear borane.
7. 8. A direct-positive photographic element comprising a sheet support bearing a layer of a silver halide emulsion as defined in claim 2.
8. 9. An element according to claim 8, wherein the silver halide is silver chloride.
9. 10. An element according to claim 8, wherein the silver halide is silver chlorobromide.
10. 11. An element according to claim 8, wherein the boron hydride is a polyhedral homonuclear borane of the series B6H6 2 to B12H12 2.
11. 12. An element according to claim 8, wherein said boron hydride is a polyhedral heteronuclear borane.
12. 13. An element according to claim 8, wherein said boron hydride is CsB9H12S.