US3394005A - Increased development rate of photosoluble silver halide emulsions by the action of water-soluble iodide - Google Patents

Description

United States Patent O 3,394,005 INCREASED DEVELOPMENT RATE OF PHOTO- SOLUBLE SILVER HALIDE EMULSEONS BY THE ACTION OF WATER-SOLUBLE IODHDE Ralph Kingsley Blake, Westfield, N.J., assignor to E. 11. du Pont de Nemours and Company, Wilmington, Del, a corporation of Delaware No Drawing. Filed Oct. 16, 1964, Ser. No. 404,482 8 Claims. (Cl. 96--64) ABSTRACT OF THE DISCLOSURE Photosoluble silver halide emulsions containing in substantially greater than fog inhibiting amounts, mercapto and selenomercapto compounds, e.g., compounds as described in Blake US. Patent 3,155,507 are exposed to actinic radiation, developed with a developer containing a phenolic hydroxyl group, treated with a fixing solution and, after exposure, but prior to fixing, are treated in the presence of a Water-soluble iodide.

This invention relates to photography and more particularly to a process for making useful negative images from novel elements normally designed for the production of positive images.

A new type of photographic process has been described in Blake US. Patent 3,155,507, Nov. 3, 1964. The novel process of said application, characterized as photosolubilization, requires the use of a specially prepared silver halide emulsion layer containing a stipulated amount of an organic compound which modifies the silver halide solubility so that, in conventional silver halide solvents, said organic compound causes the silver halide grains to dissolve more slowly than normal. Such an element is given an imagewise exposure and the exposed areas can then be treated in a solution of a silver halide solvent to yield a positive, silver halide image (the silver halide remaining undissolved in the unexposed areas). As an optional additional processing step, the silver halide image may be intensified, e.g., by reduction, to convert it into a black silver image. Photosoluble elements suitable for use in such a process have been disclosed in the above application as well as in other copending applications of the assignee as follows: Blake, Ser. No. 236,412, filed Nov. 8, 1962, US. Patent 3,155,514; Blake, Ser. No. 236,418, filed Nov. 8, 1962, U.S. Patent 3,155,516; Blake, Ser. No. 317,817, filed Oct. 21, 1963, U8. Patent 3,155,518; Blake, Ser. No. 317,824, filed Oct. 21, 1963, U.S. Patent 3,155,519; Blake, Ser. No. 390,460, filed Aug. 18, 1964; Blake, Ser. No. 403,661, filed Oct. 13, 1964, US. Patent 3,284,206; Celeste et al., Ser. No. 236,417, filed Nov. 8, 1962, US. Patent 3,155,515; Fan, Ser. No. 403,632, filed Oct. 13, 1964; Fan, Ser. No. 403,631, filed Oct. 13, 1964; Blake and Fan, Ser. No. 403,660, filed Oct. 13, 1964.

The elements disclosed above yield excellent silver halide positive images when exposed imagewise and then treated with a silver halide solvent which dissolves silver halide more rapidly in exposed than in nonexposed areas. The remaining positive silver halide images may be intensified, e.g., by treating with a photographic developing solution to reduce the image to metallic silver. However, when such elements are exposed and processed according to conventional photographic procedures, i.e., developing first and then fixing, the results are generally unsatisfactory in at least two respects: (a) the development of exposed silver halide crystals is hindered by the organic compound which modifies the silver halide solubility, and (b) the subsequent operation of removing the unexposed and undeveloped silver halide (fixing) i made slower so ICC that it may be impossible to obtain proper clean-out withm a reasonable time. Of the two, the latter is relatively the more serious problem.

It is an object of this invention to provide a process whereby elements, manufactured specifically to yield positive images by the photosolubilization process, may be processed to yield satisfactory negative images. More specific objects are to increase the rate of negative development and/ or to increase the rate of fixing of negative images prepared from photosoluble elements. Still other objects will be apparent from the following description of the invention.

The above objects are attained in the process of this invention for the formation of negative images which comprises, in the order stated, the steps of:

(a) Exposing, imagewise, to actinic radiation a photosoluble layer containing light-sensitive crystals of silver bromide, silver chloride or silver chlorobromide having associated therewith a silver salt of an organic mercapto or selenyl compound that dissolves slowly in aqueous alkaline thiosulfate.

(b) Developing the exposed layer in an aqueous developer solution containing a silver halide developing agent and having in one of its tautomeric forms at least one phenolic hydroxyl group and enough alkali to maintain the pH of at least 8.5, and

(c) Removing the undeveloped silver halide by treatin g the layer with an aqueous silver halide fixing solution; said process being further characterized in that said layer is treated in the presence of a water-soluble iodide after exposure but prior to completion of step (c). In a preferred embodiment, said water soluble iodide is present at least during the step (c).

In US. Patent 3,155,507 and the other patents and applications of the assignee referred to above, the photosoluble layers are described as containing silver salts (silver mercaptides or silver selenides) which are of lower solubility in water than silver chloride and the silver halide crystals so associated with these silver salts are disclosed as dissolving more slowly in 10% aqueous sodium thiosulfate solution, at a predetermined pH, that untreated silver halide crystals.

Also, as disclosed in the cited applications, it is preferred that the silver mercaptide or silver selenide be present in such amount, in terms of the ratio of its weight to the surface area of said silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide (70/30 mole percent) gelatin dispersion containing 10 g. of gelatin per mole of Ag and .57 mg. of Ag per ml., and said silver chlorobromide dispersion is treated with 10% by weight, aqueous sodium thiosultate (so that the resulting mixture contains 0.29 mg. of silver and mg. of sodium thiosulfite), at least three times the amount of silver chlorobromide remains undissolved as in a similar dispersion successively treated with 5%, by weight, aqueous sodium hypochlorite and 10% by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate), after vigorous agitation of both dispersions for 30 seconds at 25 C.

In the above applications, various practical tests are provided which include specific concentrations of solutions, times, etc., so that suitable mercaptans or selenols may be readily and positively identified.

Preferred pohtosoluble layers are those which are prepared fr'om me-rcaptans of the formula:

wherein R is an unsubstituted hydrocarbon radical of 6 10 carbon atoms and has a cyclic hydrocarbon radical of 6 carbon atoms attached through a cyclic carbon of said radical to the 4-carbon atom of the heterocyclic ring and X is selected from the group consisting of S, Se, and NH. Suitable radicals of the latter type include cyclohexyl, phenyl, and alpha-naphthyl. Especially preferred layers are those disclosed and claimed in Blake U.S. 3,155,519 in which X of the above formula is sulfur.

Preferably, the silver halide crystals are dispersed in a Water-permeable organic colloid to form a light-sensitive photographic emulsion. The selected mercapto or selenyl compound can be added to the silver halide emulsion while the latter is in the liquid state or the emulsion may be coated on a suitable support and the resulting element bathed or impregnated with a solution, e.g., an alcoholic solution of the organic compound. The desired amount of the organic compound in the silver halide emulsion may vary with a number of factors such as the nature of the compound and the size of the silver halide crystal (and thus the surface area of the crystal per mole of silver halide).

When the organic compound is added to the emulsion in the liquid state it is most efficiently adsorbed to the silver halide crysal by digesting the emulsion, e.g., heating the emulsion between 140 and 180 F. Generally the organic mercap t-o or selenyl compound is used in a range of from 0.3 to 1.5 gram per mole of silver halide and, more preferably, from 0.4 to 1.2 grams per mole of silver halide. The optimum concentration of organic compound is decreased somewhat, e.g., about 10%, when the emulsion is sensitized with a photographic optical sensitizing dye as disclosed in assignees copending application of Blake, Ser. No. 390,460, filed Aug. 8, 1964.

The binder-silver halide ratio is not critical and may vary from 3 :1 to 1:20 depending on the particular organic compound and silver halide crystal.

In carrying out the process of the present invention, a suitable photosoluble layer as described above is given an imagewise exposure appropriate for the light-sensitivity of the particular element. The exposed element is then developed in a photographic developing solution in which the pH has been adjusted to at least 8.5 by the addition of alkali, e.g., sodium carbonate, sodium or potassium hydroxide, etc. One or more developing agents may be used but at least one of said agents must be of the phenolic type, e.g., hydroquinone, catechol, p-methylaminophenol, aminophenol, pyrogallol, 2,4-diaminophen-ol, etc. Other ingredients comomnly used in photographic developing solutions may also be present, e.g., other developing agents such as the pyrazolidone type or ascorbic acid; sodium sulfite; boric acid; potassium or sodium carbonate; potassium bromide; wetting agents such as sodium dodecyl sulfate or saponin; polyethylene oxides; sodium hydroxide; antifoggants, etc.

The water-soluble iodide, e.g., sodium, potassium or amomnium iodide, which must be used in treating the exposed element, is conveniently included in the developer solution in a concentration of 10- to moles per liter, preferably in a concentration of 10 to 10 moles per liter of Solution. Alternately, the iodide may be used in a similar concentration in a separate bath either preceding or following the development step. According to still another embodiment of the invention, the iodide may be added to the fixing solution, e.g., from 10 to 10- but preferably 10 to 10 moles per liter of fixing solution. Obviously, the rate of development may be accelerated only when iodide is present in the developer solution or in a developer pre'bath. However, the rate of fixing may be accelerated when the iodide is present in any of the processing solutions. In certain instances it may be beneficial to employ soluble iodide in more than one of the processing solutions. In any of the processing solutions, the preferred concentration of iodide will vary considerably depending upon the type of element being processed (particularly the silver halide crystal type and size), the solution temperature, length of time of treatment, etc.

The fixing solutions, which optionally contain the soluble iodide, may contain other ingredients commonly employed in the processing of photographic films. The preferred silver halide solvent is a water soluble thiosulfate such as sodium, potassium, or ammonium thiosulfate, etc. The latter are generally present in the amounts that are the same as those used in conventional aqueous fixing solutions.

In addition to the thiosulfate, various conventional agents may be present to adjust and maintain the pH at the desired level, e.g., boric acid, acetic acid, sodium bisulfate, sodium bisulfite, sodium sulfite, sodium acetate, sodium hydroxide, sulfuric acid, etc. Also, there may be present gelatin hardening agents, e.g., chrome alum or potassium alum.

The invention will be further illustrated by, but is not intended to be limited to, the following examples.

Example I A lithographic-type, gelatino-silver halide emulsion mole percent silver chloride and 30 mole percent silver bromide) was prepared according to the conventional procedures of precipitation, Ostwald ripening and coagulation washing to remove soluble salts. An amount of the coagulate remaining after washing which contained one mole of silver halide was redispersed with an aqueous solution containing 48 g. of photographic gelatin. After redispersion, there were added 0.44 g. of 2-mercapto-4-phenylthiazole and 0.025 g. of a merocyanine optical sensitizing dye of the structure disclosed in Example I of assignees copending application of Blake, U.S. Ser. No. 390,460, filed Aug. 18, 1964. The emulsion was digested for 20 minutes at 160 F., the usual adjuvants were added, and the emulsion was applied at a coating weight of 101 mg./dm. (calculated as AgBr) on a photographic film base as described in Example IV of Alles, US. 2,779,684.

After drying, samples of the photosoluble element thus prepared were exposed through a 21-step square-root-oftwo photographic step wedge for 1 second to the radiation from a high intensity, tungsten filament, lamp (General Electric incandescent lamp ASA code DXC) at a distance of 26 inches, the lamp operating at volts.

The Samples were then placed in a photographic developer solution comprising 3.0 g. of p-methylaminophenol sulfate, 9.0 g. of hydroquinone, 50 g. of potassium carbonate, 4.5 g. of potassium bromide and 50 g. of anhydrous sodium sulfite per liter of solution. After 1 minute in this developer solution the element was washed for 15 seconds in water and placed in a 12.8% by weight aqueous solution of sodium thiosulfate until the undeveloped silver halide was dissolved, then washed in water for 10 After drying, the optical densities in regions of high and low exposure were recorded on a conventional photographic densitometer (Western Electric RA-11000 C Densitometer). The above procedure was repeated, except that small amounts of a solution of sodium iodide were added to the developer solution. The addition of NaI to the developer resulted in increased optical transmission density as indicated in Table 1.

TABLE 1 Total Optical Density Exposure Step 21 Exposure Step 1 (High Exposure) (Low Exposure) Molarity of Nal in developer:

The procedure of Example I was repeated except that, in place of the sodium iodide, the below indicated amounts were used. The addition of KI to the developer resulted in increased optical density as indicated in Table 2.

TABLE 2 Total Optical Density Exposure Step 21 Exposure Step 1 (High Exposure) (Low Exposure) Molarity of KI in developer:

Example III TABLE 3 Total Optical Density Exposure Step 21 Exposure Step 1 (High Exposure) (Low Exposure) Molarity of NaI in the Predevelopment bath:

Example IV Using the same 2-mercapto-4-phenylthiazole insolubilized lithographic film of Example I, it was demonstrated that an increase in the rate of fixing will result from the use of 1) an iodide treatment before developing, (2) iodide in the developer, (3) an iodide treatment after development but before fixing, or (4) iodide in the fixer. Five samples of the element were exposed for 1 second in the manner described in Example I and (1) treated for 15 seconds in an aqueous predevelopment bath, (2) developed for 60 seconds in the developer of Example I, (3) treated for 15 seconds in an aqueous post-development bath, and (4) placed into a 12.8% by weight aqueous solution of sodium thiosulfate fixer. Potassium iodide was added to only one of the above processing solutions for the processing of each sample as indicated in Table 4 with Sample No. 1, serving as a control, being processed without the addition of iodide to any of the four solutions. The data recorded below, for clearing times in the fixer, demon strate that the iodide treatment is effective at any step of the processing for increasing the fixing rate.

Example V A gelatino-silver chloride emulsion was prepared employing a twin-stream precipitation which maintained a slight excess of chloride ion. The grains were coagulation washed to remove the soluble salts. An amount of the coagulate remaining after washing, which contained one mole of silver halide crystals, was thereafter redispersed with an aqueous solution containing 117 g. of photographic gel atin. After redispersion there was added 0.80 gram of 2mercapto-4-phenylthiazole. The emulsion Was digested for 20 minutes at 160 F., the usual adjuvants were added, and the emulsion was applied at a coating weight of 70.5 mg./dm. (calculated as AgBr) on a photographic film base as disclosed in Example I to form a photosoluble element.

After drying, five samples of the film were exposed for one second in the manner described in Example I and 1) treated for 15 seconds in an aqueous predevelopment bath, (2) developed for 20 seconds in the developer of Example I, (3) treated for 15 seconds in a aqueous postdevelopment bath, and (4) placed in a 12.8% by weight aqueous solution of sodium thiosulfate fixer. Potassium iodide was added to only one of the above processing solutions for the processing in each sample as indicated in Table 5 with Sample No. 1, serving as a control, being processed without the addition of iodide to any of the four solutions. The data recorded below, for clearing times of the fixer, demonstrates that the iodide treatment is effective at any step of the processing for increasing the fixing rate.

A litho]gra:pshio emulsion of the type described in Example I was treated with 0.67 g. per mole of silver halide of 2-mercapto-4-cyclohexylthi-azole (instead of 2mercapto-4-phenylthiazole) and was coated at 60 mg./dm. of silver halide. Samples of this photosoluble element were exposed and processed in the manner described in Example I. The addition of KI to the developer, in varying concentrations, resulted in increased optical density as indicated in the table below.

TABLE 6 Total Optical Density Exposure Step 21 Exposure Step 1 (High Exposure) (Low Exposure) Moltrity of KI in Developer:

Example VII A lithographic emulsion of the type described in Example I was treated with 1.33 g. per mole of silver halide of dodecyl mercaptan (instead of 2-mercapto-4- phenylthiazole) and was coated at mg./dm. of silver halide. Samples of this photosol-uble element were exposed and processed in the manner described in Example I. The addition of KI to the developer resulted in increased optical density as indicated in the table below.

TABLE 7 Total Optical Density Exposure Step 1 (High Exposure) Molarity of K1 in Developer:

Example VIII A lithographic emulsion of the type described in Example I was treated with 0.84 g. per mole of silver halide of 2-seleno-benz-othiazole (instead of 2-mercapto-4- phenylthiazole) and was coated at mg./dm. of silver halide (calculated as AgBr) to form a photosoluble element. Samples of this element were then exposed and processed in the manner described in Exanrple I. The addition of K1 to the developer resulted. in increased optical density as indicated in Table 8. With the addition of NaI to a 4.3% by weight aqueous sodium thiosulfate fixer solution, the fixing time was decreased as indicated in Table 8A.

TABLE 8 Molarity of KI Density at in Developer Low Exposure (Step No. 1)

TABLE 8-A Molarity of Neil Clearing Time,

in Fixer min.

Example IX Developer A B C Water 800 800 800 p-Methylaminophenol sulfate (g) 3 Na SO (anhyd) (g.) 50 80 80 Hydroquinone (g) 9 16 16 4-methyl-l-phenyl-Spyrazolidone (g 1 5. 5 5. 5

. 24 24 1,000 1, 000 1,000 pH of Solution 11.0 13+ 13+ 6-nitrobenzimidazole-nitrate (1% ethanol, by vol). b l-phenyl-5-mercaptotetrazole (1% ethanol, by vol.).

Samples of the above film were exposed for seconds through a neutral density step wedge in which the density of each step increased by the fourth-root-of-two using a I-B intensity scale sensitometer as described in Mees, The Theory of the Photographic Process, published by MacMillan Co., New York, 1942, page 607 et seq. The exposure at step 11 was 0.049 m.c.s.

The exposed films were treated for one and a half .minutes at 68 F. in the above developer solutions to which potassium iodide was added in the concentrations indicated in the table below. The developed films were then treated for 3 minutes in a conventional white alum fixing bath (Photo Lab Index, 1959, p. 526), washed for 10 minutes in water and dried. Their optical densities were read as described in Example 1 giving the results tabulated below:

TABLE 9 Optical Densities at Highest and Lowest Exposure Levels (Steps 21 and 1) Developer Solution A B C High-Low High-Low HighLow Example X A number of samples of a photosoluble film similar to that described in Example I were half covered with an opaque sheet while the other half of each sample was exposed to a high-intensity, tungsten filament lamp (General Electric reflector photofiood lamp ASA N0. PH/RFLZ) operating at 19 volts. The exposure was for 10 seconds with a lamp distance of 2 feet.

A separate exposed sample of the film was taken developed for 75 seconds at F. in each of the three developer solutions D, E and F shown below. Another separate exposed sample of the film was also developed, at the same temperature and for the same time, in each of the three developer solutions but to which 10 ml. of 0.1 M KI had been added per liter of solution. In all three developing solutions in which the iodide was absent, no image could be detected by visual observations. With the addition of potassium iodide, however, negative images were obtained by treatment in each of the developer solutions. Formulations of the three developer solutions were as follows:

g. Water to (ml)..- 1, 00

Example XI Example I was essentially repeated except for variation in the pH of the developer, using the developer solutions at the 0 and the 0.1 molar concentration of NaI disclosed in that example, These developer solutions, as used in Example I, had a pH of slightly over 13 but were adjusted downward for the present example to a pH of 8.0. When exposed film samples were developed in either of these reduced pH developer solutions, no visible image could be seen.

The pHs of both developer solutions were then raised to 8.5 by the addition of sodium hydroxide. This time, a faint but definite negative image was evident in the film sample developed in the solution which was 0.1 molar in NaI. No such image was discernible in the sample developed in the absence of Nal. Better negative images are obtained as the pH is increased further.

The photosoluble elements which are useful according to the present invention are those containing silver halide crystals wherein the halide is chloride, bromide or a mixture of the two.

In place of part of the gelatin in these elements, other natural or synthetic water-permeable organic colloid binding agents can be used and in some cases such binders can be used alone. Such agents include water-permeable or water-soluble polyvinyl alcohol and its derivatives, e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers and acetals containing a large number of intralinear -CH CHOH groups, hydrolyzed interpolymers of vinyl acetate and unsaturated addition polymerizable compounds such as maleic anhydride, acrylic, and methacrylic acid esters, and styrene. Suitable such colloids of the lastmentioned type were disclosed in U.S. Patents 2,276,322, 2,276,323 and 2,397,866. The useful polyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinyl butyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal. Other useful colloid binding agents which can be used include the poly-N-vinyllactams of Bolton U.S. Patent 2,945,918, various polysaccharides, e.g., dextran, dextrin, etc., the hydrophilic copolymers in Shacklett U.S. Patent 2,833,650, hydrophilic cellulose ethers and esters, and polymers of acrylic and methacrylic esters and amides. Also, it has been found practical to treat silver halide layers on a base material in the essential absence of a binder, e.g., those prepared by chemical or vacuum deposition.

The emulsion may optionally contain any of the usual adjuvants customarily employed in silver halide systems so long as they do not interfere with the adsorption and complexing action of the organic mercaptans of the invention.

The emulsion can be coated on any suitable support, e.g., cellulose esters, cellulose mixed esters; superpolyrners, e.g., poly(vinyl chloride covinyl acetate); polyvinyl acetals, butyrals; polystyrene; polyamides, e.g., polyhexamethylene adipamide, polyesters, e.g., polycarbonates, polyethylene terephthalate/isophthalate, esters formed by condensing terephthalic acid and its derivatives, e.g., dimethyl terephthalate with propylene glycol, diethylene glycol, tetramethylene glycol, cyclohexane-1,4-dimethanol (hexahydro-p-xylene dialcohol); paper, metal, glass, etc.

The process of this invention has the advantage that it increases the versatility of the special elements designed for the photosolubilization process. Thus it is possible to prepare satisfactory negative as Well as positive images from the same film elements. The presence of the iodide ion, in negative processing solutions, causes the formation of higher densities in the exposed areas and thus increases effective photographic speed. In addition, total processing time can be shortened appreciably by the presence of iodide ion at any stage of processing. Still additional advantages will be apparent from the above description of the invention.

I claim:

1. A process for the formation of negative images which comprises, in the order stated, the steps of:

(a) exposing, imagewise, to actinic radiation :1 photosoluble layer containing light-sensitive silver halide crystals selected from the group consisting of silver bromide, silver chloride, and silver chlorobromide having associated therewith in substantially greater than fog-inhibiting amounts a silver salt of an organic compound selected from the group consisting of mercapto and selenomercapto compounds, said compounds being of lower solubility in water than silver chloride, the silver halide crystals so associated with the silver mercaptide dissolving more slowly in 10% aqueous sodium thiosulfate than untreated silver halide crystals at a predetermined pH, the mercapto compound of said formula being present in such amount, in terms of the ratio of its weight to the surface area of said silver halide crystals, that when admixed in such ratio with an aqueous silver chlorobromide (70/30 mole percent) gelatin dispersion containing 10 g. of gelatin per mole of Ag and .57 mg. of Ag per ml., and said silver chlorobromide dispersion is treated with 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver and 100 mg. of sodium thiosulfate), at least three times the amount of silver chlorobromide remains undissolved as compared with a similar dispersion successively treated with 5%, by weight, aqueous sodium hypochlorite and 10%, by weight, aqueous sodium thiosulfate (so that the resulting mixture contains 0.29 mg. of silver, 25 mg. of sodium hypochlorite and 100 mg. of sodium thiosulfate), after vigorous agitation of both dispersions for 30 seconds at 25 C.

(b) developing the exposed layer in an aqueous developer solution containing a silver halide developing agent having in one of its tautomeric forms at least one phenolic hydroxyl group and suificient alka li to maintain a pH of at least 8.5, and

(c) removing the undeveloped silver halide by treating the layer with an aqueous silver halide fixing solution;

said process being characterized in that the photosoluble layer is treated after exposure but prior to completion of step (c) in the presence of a water-soluble iodide.

2. A process according to claim 1 wherein the fixing solution contains a water-soluble thiosulfate fixing agent.

3. A process according to claim 1 wherein the photosoluble layer contains gelatin as a binding agent for said crystals.

4. A process according to claim 1 wherein said organic compound is a mercaptan of the formula Ro-N 114% i JSH wherein R is an unsubstituted hydrocarbon radical of 6l0 carbon atoms and has a cyclic hydrocarbon radical of 6 carbon atoms attached through a. cyclic carbon of said radical to the cyclic carbon atom of the heterocyclic ring and X is a member selected from the group consisting of S, Se, 0, and NH.

5. A process according to claim 1 wherein the organic compound is present in the amount of 0.3-1.5 grams per mole of silver halide.

6. A process according to claim 1 wherein said mercaptide is of 2-mercapto-4-phenylthiazole.

7. A process according to claim 1 wherein said mercaptide is of 2-mercapto-4-cyclohexylthiazole.

8. A process according to claim 1 wherein the water soluble iodide is present in a concentration of 10 to 10 moles per liter of treating solution.

References Cited UNITED STATES PATENTS 2,017,167 10/1935 Russell -88 2,346,775 4/1944 Maller 9661 X 2,707,681 5/1955 Raasch 96-66.4 3,155,507 11/1964 Blake 96-64 NORMAN G. TORCHIN, Primary Examiner. R. E. FIGHTER, Assistant Examiner.