US2618556A

US2618556A - Process for preparing photographic emulsions

Info

Publication number: US2618556A
Application number: US787028A
Authority: US
Inventors: Edward H Hewitson; James A Mcclintock
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1947-11-19
Filing date: 1947-11-19
Publication date: 1952-11-18
Anticipated expiration: 1969-11-18
Also published as: FR980385A; BE484329A; GB655082A

Description

Patented Nov. 18, 1952 PROCESS FOR PREPARING PHOTOGRAPHIC EMULSIONS Edward H. Hewitson and James A. MeClintock, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application November 19, 1947, Serial No. 787,028

11 Claims. (Cl. 95-7) 1 2 This invention relates to a process for preparemulsions. Other objects will become apparent ing photographic emulsions and more particuhereinafter.

larly to a process for preparing photographic silver halide emulsions.

In accordance with our invention, we add to a photographic silver halide emulsion which con- For many years, Washed gelatino-silver-halide 5 tains the Water-soluble salts generated during photographic emulsions have been prepared by the formation of the silver halide emulsion from a process consisting of the following general a water-soluble silver salt and a water-soluble steps: halide, a water-soluble salt selected from the (1) Preparation of a silver halide in a watergroup consisting of ammonium salts, salts of the solution of gelatin by mixing a water-solution of alkali metals having an atomic weight less than a water-soluble silver salt (usually silver nitrate) 140, and salts of metals of group II of the periwith a water solution of a water-soluble halide odic arrangement of elements having an atomic (usually an ammonium halide or a halide of an weight of less than 140, in quantity sufficient to alkali metal). The gelatin may be dissolved in coagulate the silver halide, and also add to the either or both of these solutions, or in a separate emulsion a water-soluble acid capable of lowersolu-tion. ing the pH of the emulsion to at least 4, in quan- (2) After the silver halide has been formed, tity sufiicient to lower the pH of the emulsion the emulsion is digested, to a greater or lesser to at least 6, whereupon the silver halide settles extent, in the presence or absence of other chemout rapidly and can be readily separated from the icals, to produce the desired grain size, grain size 1iquid portion of the mixture, washed and redisdistribution, and grain sensitivity. persed to give an emulsion. The process can be (3) The emulsion is then chilled and allowed carried out at any convenient temperature, to gel. More gelatin may be added before chilladvantageously from '75 to 110 F., in the case of ing, if necessary to raise the gelatin concentragelatino-silver-halide emulsions. The salt or the tion so that the emulsion may be set to form acid may be added to the emulsion first or the a jelly. salt and acid can be added simultaneously.

(4) The gel is ground, shredded, or otherwise Our invention can be illustrated in greater subdivided and the water-soluble salts are detail by the following steps: leached out by washing in water, at a temperature sufiiciently low to prevent melting of the gel. (a) A ,Sllver hahde, 15 prepared an s uea (5) w the Washing is completed, the Water solution of a collo d, such as gelatin, by mlxmg is drained 011 an aqueous solution of a water-soluble silver (5) The Washed emulsion is melted, more salt (e. g. silver nitrate) with an aqueous solugelatin and other chemicals may be added to tion watimsoluble ha1,ide (usually condition further the silver halide grains to proammonlum de 01' a halide of an alkali duce the desired characteristics during further metaD- The collold can be d1Ss1Ved 1n elther digestion at more or less elevated temperatures. 109th of these Solutions or in a Separate In preparing photographic emulsions in acsolutlon' ordange ith the above general ro edure t s (b) After the silver halide has been formed, the 3 4 and 5 (the chiming and setting, the 40 emulsion is digested, to a greater or lesser exlashing and the draining Operations) are mtent, in the presence or absence of other chemtively 510W, time consuming Opemfi0ns icals, to produce the desired grain size, grain thermore, as the size of the batches of emulsion Size distribution and grain Sensitivity- ?L mizreased the a ii for these Opem (Thus far, our new process parallels the prior 1on5 mcriases an eme e equlpment an art process discussed above. At this point, howififg Increase t than ever, we proceed differently from the above prior We have now found a process for preparing process) photographic silver halide emulsions which elim- (0) An acid which is capable of lowering the pH inates the above step (3), i. e. the chilling and of the emulsion to at least 4 and acoagulating Sett St p, and reduces a t ally the time or a flocculating agent for the silver halide is eq Or steps (4) and (5), i. e. the washing added to the emulsion, so that the silver halide d draining p grains clump together and settlerapidly to the It is, accordingly, an object of our invention to bottom of the container, leaving a clear or rovide a new process for preparing photog aphic nearly clearsolution above.

At this point acknowledgement is made of the prior art process which discloses the addition to a gelatino-silver-halide emulsion of salts of metals of group IV of the periodic table, e. g. salts of lead, titanium, thorium, zirconium, etc. to coagulate the gelatin of the emulsion. When such salts containing trior tetravalent cations are added to the emulsion the gelatin is coagulated and the coagulated gelatin carries down the silver halide. However, the salts containing the trior tetravalent cation act on the gelatin to harden it permanently, thereby rendering resuspension of the silver halide very diiiicult. Our process differs from the aforesaid process in that it is carried out at a pH of 6 or lower, and the ammonium salts and the salts of the alkali metals and the salts of metals of group 11 of the periodic table having an atomic weight of less than 140, coagulate the silver halide itself rather than the gelatin, although when operating our process at a pH near the isoelectric point of the gelatin in a gelatino-silver-halide emulsion some of the gelatin is coagulated but not permanently hardened.

Following coagulation, in accordance with our invention, we proceed as follows:

(d) Decant the supernatant liquor.

(e) Add water (with or without a coagulating or flocculating agent) and agitate the silver halide grains to allow more of the soluble salts to be leached out, followed by settling, followed by decantation of the supernatant liquid again. (d) and (e) can be repeated as often as necessary to reduce the soluble salts to the desired concentration.

(1) After draining the aqueous liquor from the silver halide grains, water and gelatin are added to the grains, and the grains resuspended by dissolving the gelatin, usually with heating.

(9) The resulting emulsion is digested at more or less elevated temperature, with or without the addition of further gelatin and other chemicals, to produce the desired characteristics.

The following examples will serve to illustrate further the manner of practicing our invention. These examples illustrate our new process as applied to a good positive emulsion formula published in Photographic Emulsions by Wall,

1929 edition, page 86.

Example 1 Potassium bromide 460 grams Potassium iodide 1.04 grams Hydrochloric acid 3.5 cc. Water 3000 cc.

Temperature 50 C. (122 F.)

Silver nitrate 500 grams Water 3000 cc.

"Temperature 50 C. (122 F.)

Gelatine 150 grams Water 1200 cc.

"Temperature 50 C. 122 F.). Run the bromide and the silver into the gelatine solution simultaneously, keeping the bromide in excess. As soon as mixed add:

Ammonia 60 cc.

"Digest for one hour at 43 C. (110 F.)

At this point, we deviated from Walls Formula and cooled the emulsion to approximately 100 F. Then sulfuric acid was added to lower the pH of the emulsion below 3. 400 grams of sodium chloride were then added in the dry state. The resulting mixture was stirred until the sodium chloride dissolved, whereupon the mixture was cooled to about 70 F. The silver halide grains settled rapidly out of suspension. The supernatant liquor was then decanted and cool water (at about 60 F.) was added to make the total volume up to about twice the original volume of the uncoagulated emulsion. The mixture was then stirred briefly, the silver halide grains allowed to settle, and the supernatant liquor was decanted. This washing process was repeated three times, each time adding enough water to make the total volume up to about twice the original volume of uncoagulated emulsion. After the final decantation, enough water was added to make the volume up to a volume equaling the original emulsion volume.

At this point, we returned to Wall's Formula and added the following materials to the emuls1on:

Gelatine, dry 960 grams Ethyl alcohol 650 cc. Chrome alum 4 grams (The two quantities of gelatin -730 grams added before washing and 230 grams added after washing in Wall's Formula are combined to make the 960 grams designated above).

The total bulk was then made up to 12,500 grams with water and the resulting emulsion digested (ripened) in the usual manner.

Example 2 An emulsion was prepared exactly as in Example 1, except that hydrochloric acid instead of sulfuric acid was employed to lower the pH of the emulsion to at least 3.

Example 3 An emulsion was prepared exactly as in Example 1, except that hydrochloric acid was employed instead of sulfuric acid to lower the pH of the emulsion to at least 3 and 650 grams of zinc sulfate was employed instead of sodium chloride to coagulate the silver halide.

Example 4 An emulsion was prepared exactly as in Example 1, except that after the decantation of the supernatant liquor from the first settling operation, 50 grams of dry sodium sulfate (anhydrous) was added. This addition helps to prevent resuspension of the silver halide grains during subsequent washing.

Example 5 An emulsion was prepared exactly as in Example 1, except that 500 grams of ammonium sulfate was employed instead of sodium chloride to coagulate the silver halide.

Example 6 Ammonium bromide Ammonium chloride Ammonium phosphate Ammonium sulfate Ammonium molybdate Ammonium nitrate Barium chloride Barium nitrate Calcium chloride Calcium nitrate Magnesium sulfate" Potassium persulfate Potassium ferrocyanide Potassium ferricyanide Potassium sulfate Potassium chromate Potassium chloride Sodium sulfate Sodium chloride Sodium tetraborate Sodium acetate Trisodium phosphate Zinc chloride Zinc sulfate Zinc nitrate Zinc bromide Saltscontaining monovalent cations are advantageously employed. One or more salts can be employed.

Many modifications of the procedure described above can also-be employed. For example, the salt can'be added to the emulsion before the pH is lowered'by the addition of the acids. More or less digestion time may be given before adding the acid and salt to effect coagulation. Additional gelatin can be added before the coagulation, if desired. Temperatures varied within wide limits and depend more upon the emulsion characteristics desired than upon the requirements of the process. More or less washing of the coagulated silver halide grains may be employed, again depending upon the emulsion characteristics desired. Also the quantitles of acids and salts employed may vary within Wide limits; the optimum quantities being readily determined for each individual emulsion by making simple observations.

It should be observed that our process differs extensively from those which dehydrate the emulsion, thereby producing gelation so that the emulsion can be shredded and washed without resorting to chilling operations. Processes of this nature have been employed for many years and a number of the processes are described in theliteratureinvolving the use of dehydrating salts, such as anhydrous sodium sulfate and dehydrating solvents, such as alcohol, acetone, etc. These older dehydration processes retain most or 'all of the-gelatin which has been added to the emulsion. In. our process, a large fraction of the gelatin remains dissolved in the water and is poured off during the washing operations.

As pointed out above, the quantity of acid used to lower the pH of the emulsion can be varied within relatively wide limits. The actual pH value to be obtained depends not only upon the particular coagulating salt which is employed but also upon the quantity of the coagulating salt. The optimum quantity of acid will correspond to the quantity at which the minimum amount of coagulating salt is required. The kind :of acid employed also has some effect upon the amount required and upon the optimum value of pH. Similarly, the quantity of coagulating salts can be varied within wide limits. Some salts, particularly nitrates and acetates, must be employed in relatively large quantities.

may be Li l Others, notably sulfates, phosphates, and molybdatesv may be used in relatively small quantities. It appears that the anion in any given salt is more important in determining the effectiveness of the salt than is the cation. Some salts, for example, ammonium molybdate, can be used in amounts as low as one part per .500 parts of photographic emulsion, by weight. Others, e. g. calcium nitrate must be employed at the rate of 1 part per 5 parts of emulsion.

In addition to the advantages pointed out above, the following advantages are realized by employing our new process:

1. If desired, the entire making, washing and finishing operations ordinarily employed in emulsion making may be carried out in one container, one operation following immediately after'th'e preceding one without delay forchilling, washing'and draining.

. The amount of equipment required for the production of the emulsion is reduced.

3. More concentratedemulsions can be produced since the coagulated emulsions settle to a more compact form, to which little or much water can be added, as desired. In the usual emulsion-making process, the maximum concentration obtainable is determined by the sensitometric characteristics required. In our new process, this is not the case.

. The time required for the production of any given type of emulsion is reduced as is the labor required.

Any of the customarily employed washed emulsions can be prepared by our new process, e. g. silver bromide, silver bromoiodide, silver chlorobromide, silver chlorobromoiodide, silver chloride, silver chloroiodide, etc. emulsions.

What We claim as our invention and secure by Letters Patent of the United States is:

1. In a process for preparing a photographic gelatino-silver-halide emulsion, th steps which consist in (1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from a water-soluble silver salt and a water-soluble halide, after completion of the first digestion, at least one water-soluble, inorganic salt selected from the group consisting of ammonium salts, salts of the alkali metals having an atomic weight less than 1-10 and salts of metals of group II of the periodic arrangement of elements having an atomic weight less than 140,

in quantity suihcient to coagulate the silver halide, and also adding to the emulsion at least one water-soluble acid having an ionization constant at least that of acetic acid in quantity sufficient to lower the pH to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, l) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) .being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and ('7) redispersing the washed silver halide grains in an aqueous gelatin solution.

2. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consist in (1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from a water-soluble silver salt and a water-soluble halide, after completionof theflrst digestion, sodium chloride, in quantity sufiicient to coagulate the silver halide, and also adding to the emulsion hydrochloric acid in quantity sufficient to lower the pI-I of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixture thus obtained, settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the Watersoluble halides is reduced to the desired level, and (7) redispersing the washed silver halide grains in an aqueous gelation solution.

3. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consist in (1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from a water-soluble silver salt and a water-soluble halide, after completion of the first digestion, ammonium sulfate, in quantity sufficient to coagulate the silver halide, and also adding to the emulsion sulfuric acid and citric acid in quantity sufficient to lower the pH of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and (7) redispersing the washed silver halide grains in an aqueous gelatin solution.

4. In a process for preparing a photographic gelatino-silver-halid emulsion, the steps which consist in (1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from a water-soluble silver salt and a water-soluble halide, after completion of the first digestion, ammonium sulfate, in quantity sufficient to coagulate the silver halide, and also adding to the emulsion sulfuric acid in quantity sufficient to lower the pH of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the watersoluble halides is reduced to the desired level, and ('7) redispersing the washed silver halide grains in an aqueous gelatin solution.

5. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consist in (1) adding to a gelatino-silVer-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from a water-soluble silver salt and a water-soluble halide, after completion of the first digestion, an inorganic salt of an alkali metal having an atomic weight less than 140, in quantity sufficient to coagulate the silver halide, and also adding to the emulsion a water-soluble acid having an ionization constant at least that of acetic acid in quantity sumcient to lower the pH of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide,

s(4) agitatin the mixture thus obtained, 5) settling the mixture, (6) decanting the supernatant 8 liquor, said steps (2'), (3), (4), (5) and (6) being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and (7) redispersing the washed silver halide grains in an aqueous gelatin solution.

6. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consist in (1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from a water-soluble salt and a watersoluble halide, after completion of the first digestion, an inorganic ammonium (NH4) salt in quantity sufficient to coagulate the silver halide, and also adding to the emulsion a water-soluble acid having an ionization constant at least that of acetic acid in quantity sufficient to lower the pH of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixturethus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and (7) redispersing the washed silver halide grains in an aqueous gelatin solution.

7. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consist in (1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from silver nitrate and an alkali metal halide, after completion of the first digestion, at least one Water-soluble, inorganic salt selected from the group consisting of ammonium salts, salts of the alkali metals having an atomic weight less than and salts of metals of group II of the periodic arrangement of elements having an atomic weight less than 140, in quantity sufiicient to coagulate the silver halide, and also adding to the emulsion at least one water-soluble acid having an ionization constant at least that of acetic acid in quantity sufficient to lower the pH to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and (7) redispersing the washed silver halide grains in an aqueous gelatin solution.

8. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consist in 1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from silver nitrate and an alkali metal halide, after completion of the first digestion, an inorganic salt of an alkali metal having an atomic weight less than 140, in quantity sufficient to coagulate the silver halide, and also adding to the emulsion a water-soluble acid having an ionization constant at least that of acetic acid in quantity sufiicient to lower the pH of the emulsion to at least 6, (2) decantin the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the water- 9 soluble halides is reduced to the desired level, and (7) redispersing the washed silver halide grains in an aqueous gelatin solution.

9. In a process for preparing a, photographic gelatino-silver-halide emulsion, the steps which consist in (1) addin to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from silver nitrate and an alkali metal halide, after completion of the first digestion, sodium chloride in quantity sufficient to coagulate the silver halide, and also adding to the emulsion hydrochloric acid in quantity sufficient to lower the pH of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and (7) redispersing the washed silver halide grains in an aqueous gelatin solution.

10. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consists in 1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from silver nitrate and an alkali metal halide, after completion of the first digestion, ammonium sulfate in quantity suflicient to coagulate the silver halide, and also adding to the emulsion sulfuric acid and citric acid in quantity sufficient to lower the pH of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide (4) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), (3), (4), (5) and (6) being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and (7 redispersing the washed silver halide grains in an aqueous gelatin solution.

11. In a process for preparing a photographic gelatino-silver-halide emulsion, the steps which consist in (1) adding to a gelatino-silver-halide emulsion which contains the water-soluble salts generated during the formation of the silver halide from silver nitrate and an alkali metal halide, after completion of the first digestion, ammonium sulfate in quantity sufficient to coagulate the silver halide, and also adding to the emulsion sulfuric acid in quantity sufficient to lower the pH of the emulsion to at least 6, (2) decanting the supernatant liquor, (3) adding water to the coagulated, substantially gelatin-free silver halide, (4) agitating the mixture thus obtained, (5) settling the mixture, (6) decanting the supernatant liquor, said steps (2), .(3), (4), (5) and (6) being performed successively until the concentration of the water-soluble halides is reduced to the desired level, and (7 redispersing the washed silver halide grains in an aqueous gelatin solution.

EDWARD H. HEWITSON.

JAMES A. MCCLINTOCK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,844,716 Lambert Feb. 9, 1932 2,258,755 Glaser Oct. 14, 1941 2,401,051 Grouse et al May 28, 1946 2,489,341 Waller Nov. 29, 1949 FOREIGN PATENTS Number Country Date 585,758 Great Britain Feb. 24, 1947 OTHER REFERENCES The Silver Bromide Grain of Photographic Emulsions, Trivelli and Sheppard monograph, Research Laboratory, Eastman Kodak Co., pp. 33, 36 and 39 cited.

Ausfuhrliches Handbuch der Photographic, Eder, vol. 3, part 1, 6th Complete Edition by Wentzel, 1930, pp. 92, paragraph 11, 134, footnote 2, 141, paragraph 3, and 182-184, published by I-Ialle, William Knapp, 1930.

Claims

1. IN A PROCESS FOR PREPARING A PHOTOGRAPHIC GELATINO-SILVER-HALIDE EMULSION, THE STEPS WHICH CONSIST IN (1) ADDING TO A GELATINO-SILVER-HALIDE EMULSION WHICH CONTAINS THE WATER-SOLUBLE SALTS GENERATED DURING THE FORMATION OF THE SILVER HALIDE FROM A WATER-SOLUBLE SILVER SALT AND A WATER-SOLUBLE HALIDE, AFTER COMPLETION OF THE FIRST DIGESTION, AT LEAST ONE WATER-SOLUBLE, INORGANIC SALT SELECTED FROM THE GROUP CONSISTING OF AMMONIUM SALTS, SALTS OF THE ALKALI METALS HAVING AN ATOMIC WEIGHT LESS THAN 140 AND SALTS OF METALS OF GROUP II OF THE PERIODIC ARRANGEMENT OF ELEMENTS HAVING AN ATOMIC WEIGHT LESS THAN 140, IN QUANTITY SUFFICIENT TO COAGULATE THE SILVER HALIDE, AND ALSO ADDING TO THE EMULSION AT LEAST ONE WATER-SOLUBLE ACID HAVING AN IONIZATION CONSTANT AT LEAST THAT OF ACETIC ACID IN QUANTITY SUFFICIENT TO LOWER THE PH TO AT LEAST 6, (2) DECANTING THE SUPERNATANT LIQUOR, (3) ADDING WATER TO THE COAGULATED, SUBSTANTIALLY GELATIN-FREE SILVER HALIDE, (4) AGITATING THE MIXTURE THUS OBTAINED, (5) SETTLING THE MIXTURE, (6) DECANTING THE SUPERNATANT LIQUOR, SAID STEPS (2), (3), (4), (5) AND (6) BEING PERFORMED SUCCESSIVELY UNTIL THE CONCENTRATION OF THE WATER-SOLUBLE HALIDES IS REDUCED TO THE DESIRED LEVEL, AND (7) REDISPERSING THE WASHED SILVER HALIDE GRAINS IN AN AQUEOUS GELATIN SOLUTION.