The preparation of ultra-fine grain photographic emulsions

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1954 J. Sci. Instrum. 31 333

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The preparation of ultra-fine grain photographic emulsions By B. H. CRAWFORD, D.Sc., National Physical Laboratory, Teddington, Middlesex

[Paper first received 18 January, and in final form 24 May, 19541 Work on the application of the Lippmann process of colour photography to the preparation of interference filters has resulted in a simple method of producing a range of fine-grain emulsions,

some having a resolution exceeding 5000 lines per mm.

The Lippmann process of colour photography and its appli- cation to the production of interference filters necessitates the use of exceedingly fine-grained photographic emulsions, such as are not commercially available. It is also essential, to attain satisfactory quality, to coat the plates by the casting technique developed by Dew and Sayce(’) for use in the production of diffraction grating replicas. Finally, it is almost impossible to prepare a Lippmann emulsion and remelt at a later date for coating because of grain growth leading to loss of the most essential property of the emulsion: a grain structure fine enough to record standing waves of light (which are half a wavelength apart). Hence the experimenter with the Lippmann process is forced to manufacture his own emulsions. A procedure has been developed which is very simple and straightforward, and gives close control over the product, especially with regard to grain size, over a wide range of gelatin and silver concentrations. The ability to produce these emulsions, both ultra-he and not so fine in grain, is likely to be useful for many purposes apart from making interference filters, owing to the fine grain and perfect quality attainable. The present paper is therefore devoted to a detailed description of the N.P.L. emdsion-making technique, while a subsequent paper will deal with the special processing of interference filters and with photometric measurements on representative examples.

found that emulsions made in this way are consistent in having a grain size small enough to produce Lippmann filters. The contrast is also very high, y = 10, or even more, an indication of uniformity of grain size and sensitivity.

The greatest advantage of this method of mixing is that the proportion of gelatin to salts, and the absolute concentration of gelatin (and salts) may be varied widely with very little effect on grain size. Emulsions giving clear Lippmann colours have been made with gelatin concentrations from 4 to 12% (reckoned as weight of,air-dry gelatin in finished emulsion before coating) and silver bromide concentrations from 7 to 110% (reckoned as weight of silver bromide to gelatin in finished dry emulsion). Additional evidence of constancy of average grain size for these widely varied concentrations has been obtained by measuring the ratio of blue to red light scattered by the emulsions. This ratio is a function of grain size, not, unfortunately, linear, but nevertheless a method useful in its quickness and simplicity for keeping a check on grain size in fine-grain emulsions. It must, however, be realized that for very small or very large grain sizes the ratio approaches constancy. A detailed description of the apparatus is given in an Appendix. Using as illuminant a gas-med lamp of colour temperature about 2900” K and the Ilford colour filters, spectrum red (No. 608) and bright spectrum violet (No. 621), to isolate blue and red spectral bands, the

P R E P A R A T I O N O F L I P P M A N N E M U L S I O N S

Primary mixing. In the classical method of preparation, silver nitrate dissolved in gelatin solution is poured while being stirred into a similar solution of gelatin containing potassium bromide in equivalent quantity or slight excess. The physical conditions of precipitation are not well con- trolled or defined, however, and the following method, modified and simplified from that described by Jenny>*) has been found a great improvement, giving consistent results over a wide range of concentrations, both of gelatin and of salts. The components of the emulsion-making recipe are reduced to four essentials; gelatin, potassium bromide, silver nitrate and water, all of high purity. The gelatin of “photo- graphic emulsion” quality is dissolved in water at 37” C, filtered and poured into a clean beaker. (These fine-grain slow emulsions do not appear to be appreciably affected by the gelatin used, so long as no impurities are present which would lead to production of fog. The gelatin must also have the necessary mechanical properties giving a suflicient gel strength, especially when the emulsions are to be coated by the Dew and Sayce casting technique.) The beaker is held in a thermostatic water bath and the gelatin solution stirred mechanically as vigorously as possible without forming bubbles or froth. The potassium bromide and silver nitrate, separately dissolved in water, are run into the beaker drop by drop as nearly as possible at the same rate of about two drops a second. At this stage, of course, operations must be conducted in a dark room with red light.

The slow, simultaneous addition of the reagents to the reaction vessel means that their concentration is small and approximately constant throughout the whole process. This favours a small and constant grain size, and it is, in fact,

VOL. 31. SEPTEMBER 1954 33

range of blue/red ratios for all Lippmann emulsions made between the concentration limits mentioned above is from 3 . 3 to 3.7. It is possible that with still closer control of precipitation conditions, emulsions of still h e r and more uniform grain size may be made.

Secondary additions and finishing. With the precipitation of the silver bromide the most critical stage in Lippmann emulsion making has been passed. I t only remains to give whatever colour sensitivity is necessary, to harden the gelatin and to coat and wash the hished product. Chrome alum and sensitizing dyes are added immediately after precipitation while the emulsion is still being stirred.

The quantity of chrome alum to be added is reckoned on the basis of 0.5% of the dry weight of the gelatin. I t is conveniently added in dilute solution; 2 % is generally suitable.

The optimum quantity of sensitizer is related both to the quantity of silver bromide finally present in the emulsion and also to its grain size. I t is generally believed that the sensitizing dye forms a monomolecular Iayer over the surface of the silver bromide grains, so that the smaller the grain size the greater the quantity of sensitizer needed to cover their surfaces, the total weight of silver bromide remaining the same. Some experiments were carried out to determine approximately the amount of pinaflavol needed to sensitize a Lippmann emulsion fully. Five emulsions were made con- taining 4% gelatin, 4 .4% silver bromide and pinaflavol in the amounts 0, 0.025, 0.075, 0.25 and 0 . 7 5 % of the weight of silver bromide. The emulsion with no sensitizer had a very IOW sensitivity, apparently spread over a wide region of the violet and ultra-violet parts of the spectrum. The sen- sitized emulsions showed an increasingly strong and fairly broad band of sensitivity in the green. The sensitivity of the last emulsion was but little higher than that of the pen-

3

B. H. Crawford

ultimate one, so it is assumed that the optimum concentration of sensitizer has been reached, or nearly so. I t is interesting to note that the last emulsion had a sensitivity equal to that of the Kodak maximum resolution plate with a smaller grain size than is normally attained in this plate.

green light. The plates of glass (top plates) which are finally to carry the emulsion are carefully cleaned, and each is provided with three distance pieces of wire or foil fastened in place with adhesive tape (see Fig. 1). The base plates for casting are treated with commercial chlormethyl-silane and

Table 1. Emulsion forinulae Emulsion

I

I1

I11

lv

V

Gelatin Potassium bromide Silver nitrate 4 g i n 3 g in 4 g i n

80 ml water 10 ml 10 ml aqueous aqueous solution solution

4 g i n 0 .75g in

aqueous solution

8 O m l water 1 O m l

4 g i n 0.094gin 80 ml water 10 ml

aqueous solution

1 2 g i n 3 .75g in

aqueous solution

80 ml water 10 ml

12 g i n 0.281 g in 80 ml water 10 ml

aqueous solution

I . 0 g i n 10 ml

aqueous solution

0.125 g in 10 ml

aqueous solution

5 g in 10 ml

aqueous solution

0.375 g in 10 ml

aqueous solution

Chrome alum 1 ml of

2% aqueous solution

1 ml of

aqueous solution 1 ml of

2% aqueous solution

2%

3 ml of 2 %

aqueous solution 3 ml of

2% aqueous solution

The requisite amounts of chrome alum and sensitizer having been added, the emulsion is coated, either by flowing or casting, allowed to set and washed in running water to remove the potassium nitrate, which is a product of the emulsion-making reaction. An emulsion for coating by flowing may contain about 4 % of gelatin, while an emulsion for casting must contain at least 12%. I t is appropriate to insert here some representative emulsion formulae to make clear the range of compositions which are possible and their relation to the purpose for which the emulsion is required.

The quantities shown in Table 1 are those actually used for experimental batches of emulsion; a 250 ml beaker is suitable for mixing. The concentrations and relative proportions of salts and gelatin may be varied within the limits given without affecting the grain size as estimated by the blue/red ratio of scattered light and by the production of Lippmann inter- ference colours.

Incidentally, the production of Lippmann interference colours gives a lower limit for the resolving power of these emulsions, since the Lippmann laminae are half a wavelength apart and must be sharply defined, with clear spaces between, for the interference colours to be produced. Clear colours are observed down to a wavelength of 0 .4 p, and the laminae remain sharply defined when observed under a microscope in transverse section, so that the resolving power of these emulsions must be at least 5000 lines per mm.

Coating, washing and drying. Emulsion coating by flowing is unsuitable for any but the roughest types of filters. For flatness, uniformity and surface perfection the emulsion layer must be formed by casting, as already mentioned. A few minor modifications of the process as described by Dew and Sayce are necessary, due to working in a red or very dim

Pinujavol Manner of coating Characrcr of emulsion

4 ml of 0.5%

alcoholic solution

1 ml of 0.5%

alcoholic solution 1 ml of 0.1 %

alcoholic solution

5 ml of 0 .5%

alcoholic solution 2ml of 0.1 %

alcoholic solution

Flowing

Flowing

Flowing

Casting

Casting

High density image, suitable for very thin coating (E 2 p) for fine graticules and for Lippmann filters of low resolving power and high reflexion factor. Normal density image, suitable for fine graticules and for Lipp- mann filters of moderate resolving power and high reflexion factor. Lowest silver content tried, suit- able for Lippmann filters of high resolving power, but better in higher concentration as casting emulsion (formula V) giving better optical quality. Highest silver content possible with this type of formula, suit- able for fine graticules, etc., as with formula 11. Lowest silver content tried, suit- able for Lippmann filters of high resolving power.

also carefully cleaned. It is convenient to have a separate base plate for each top plate to be coated, and unless excep- tional optical quality is aimed at, quarter-inch plate glass, selected for flatness and freedom from surface blemishes, is usually quite satisfactory. Top and base plates are prepared and warmed before mixing the emulsion so as to be ready for

adhesive

Fig. 1. Method of fixing spacers

for emulsion casting

coating immediately the mixing is finished. For convenience in pouring out and also to avoid the few bubbles which almost inevitably form in the emulsion, the latter is transferred to a warm separating funnel kept in the thermostat. Having supported a base plate horizontally over the thermostat, a puddle of emulsion is poured on to it and the top plate brought down, one edge first, and gently pressed into contact at the three distance pieces. A little experienced guess-work is a sufficient guide to the size of puddle necessary for any given area and thickness of emulsion layer. The emulsion sandwich is then transferred to a horizontal setting slab cooled by running water or with a little ice until the emulsion is set, this taking half an hour to an hour or more according to the thickness of the emulsion layer, the thickness of the base plate and the temperature of the setting plate. The top plate together with the emulsion may then be carefully slid or prised off the base plate and washed in running water

334 JOURNAL OF SCIENTIFIC INSTRUMENTS

The preparation of ultra-fine grain photographic emulsions

until the soluble salts, chiefly potassium nitrate, a product of the emulsion-making reaction, are removed.

The coated plates are finally dried in a current of air, usually with no heating of the air, partly to avoid danger of melting with the thicker coatings and partly to minimize stresses in the dried gelatin as far as possible.

Control of grain size. Although not of any significance in connexion with interference filters, it may sometimes be useful to produce emulsions of higher speed and larger grain size, and it has been found that a very simple modification of the process as described above enables the grain size to be controlled with some precision. A certain proportion of extra potassium bromide is added to the gelatin solution before precipitating the silver bromide, the amounts of potassium bromide and silver nitrate in the burettes remaining unchanged. The average grain size is thus increased, approxi- mately in proportion to the amount' of extra potassium bromide added. At the same time the speed of the emulsion increases, due to increased opacity, and the contrast somewhat decreases. Emulsions have thus been made with properties ranging between those of the Lippmann plate and the average commercial process plate. As an example, the following series of emulsions may be quoted. The general formula is:

solution A : gelatin 4 g in 80 ml water - x g potassium

solution B : potassium bromide 0.375 g in 10 ml aqueous

solution C: silver nitrate 0.5 g in 10 ml aqueous solution. hardener: 1 ml of 2% chrome alum solution. sensitizer: 2 ml of 0.1 % pinaflavol alcoholic solution.

The relation between x, the number of grams of potassium bromide added to solution A , and the properties of the plates is shown in Table 2 and in Fig. 2.

bromide.

solution.

f-c--+ contro!

Fig. 2. Relation between extra potassium bromide and photographic properties of emulsion

VOL. 31, SEPTEMBER 1954 335

Table 2. Effect of extra potassium bromide on properties of photographic emulsion

Additional potassium Speed (relative to bromide (x) Blueired Contrast emulsion with x = 0)

( %) rafio (U) (loglo speed) 0 3.5 7.3 1.00 0 0.1875 3.5 5.8 1.38 0-14 0.375 3.3 4.8 1.86 0.27 0.5625 2.9 4.4 5.01 0.70 0.75 2.0 3.5 17.0 1.23 1.0 1.15 3.1 120 2.08 1.5 0.95 3.0 182 2.26

A C K N O W L E D G E M E N T

The work described in this paper has been camed out as part of the research programme of the National Physical Laboratory and i s published by permission of the Director of the Laboratory.

A P P E N D I X

Grain size, a simple qualitative measurement for control purposes. As pointed out by E w ~ , ( ~ ) the ratio of the scattering powers of a suspension at two different wavelengths is a single-valued function of particle size. A simple apparatus for determining this ratio is shown in diagrammatic plan in Fig. 3. The image of a small lamp filament is focused upon the surface of a thin layer of the suspension; a plate thinly

sDectrum

qlass plate with 'filter

I ,I

f i l ter

Fig. 3. Plan of apparatus for determination of blueired ratio for photographic emulsion layers

coated in the ordinary way with emulsion and dried is ideal for the photographic case. A thin layer is necessary so that absorption effects do not mask the scattering effect. A fraction of the incident beam is deflected by a plain glass mirror to form a comparison field for the direct beam scattered by the specimen. A neutral wedge and neutral filters are inserted in the comparison beam to give equality with the scattered beam for the specimen, first with a red flter before the lamp, then with a blue filter. In order to compensate approximately for lack of perfect neutrality in the various optical parts, the blue/red ratio is determined for a plate of smoked magnesium oxide, known to be neutral. This ratio will, in general, not be quite unity, and is used to correct the ratios found for test specimens.

REFERENCES

(1) DEW and SAYCE. Proc. Roy. Soc. A , 207, p. 278 (1951). (2) JENNY. Symposium on Fundamental Mechanisms of

Photographic Sensitivity (Ed. J. W. Mitchell), p. 259 (London: Butterworths Scientific Publications, 1950).

(3) EVVA. Z. Wiss. Photogr., 47, p. 39 (1952).