The emulsion, when it has gained the requisite degree of fineness, is allowed to cool down to 90°-100° F.
(32°-38° C), when the albumen is poured into it, and thoroughly mixed. All that now remains is to make the bulk up to 20-24 oz. (according to taste), to filter, and the emulsion is ready for coating.
Emulsion made in this manner sets rather more slowly than usual, owing to the smaller proportion of gelatine it contains, but dries rapidly to a hard, glassy film. The films appear thinner and more transparent, especially when dry, than ordinary ones, but show no deficiency in the matter of density. Should still greater hardness be required, if the plates are to be long kept or subjected to the vicissitudes of travel, they may be passed singly, after drying, through a dish of good methylated alcohol carefully filtered. This will coagulate the albumen, and though it gives a little extra trouble the return is worth the outlay.
Such plates are moderately sensitive, giving, with 1/2-3/4 hour's boiling, 15 on the sensitometer. They develop quickly, owing, no doubt, to the comparatively porous character given to the film under the action of the alkaline developer. The ammonia, indeed, seems to penetrate instantly, by its solvent action on the albumen, to the very glass itself.
Other methods of precipitation, such as Abney's, may be adopted if preferred, or Monckhoven's silver carbonate and hydrobromic acid process may be employed, provided the albumen is not added until the silver bromide has been fully formed. But the plan described in detail seems to be the simplest.
The hardness of such films appears to offer a fair prospect of their possessing good keeping qualities, and if no impediment in the way of reduced sensitiveness should intervene, albumen films will probably be much employed in the future.
(6) The glass plates are well cleaned, and flowed with a preliminary coating of 1 part syrupy solution of potash or soda silicate, 5 of egg-white, and 60 of water. Beat to a froth, and filter. The excess is drained off the plate, which is then placed in a nearly upright position till dry. The emulsion consists of: -
(a) Silver nitrate .. 100 gr.
Nelson's No. 2 gelatine 20 gr.
Distilled water .. 1 1/2 oz.
Water containing 1 per cent. hydrochloric acid...... 50 m.
(c) Potassium iodide .. 8 gr. Distilled water .. 1/2 oz.
(d) Hard gelatine .. 120 gr. Water .. .. several oz.
When the gelatine is thoroughly soaked, let all possible water be poured off (d). (a) and (6) are now heated to about 120° F. (49° C), after which (6) is gradually added to (a) with constant agitation; then (c) is added. Heat in a water bath, and stir in (d). After washing, add 3/4 oz. alcohol. To emulsify, pour the solution (b) into a glass bottle; afterwards add, little by little, the silver nitrate solution (a), both solutions having been raised to a temperature somewhere approaching 120° F. (49° C), and (c) is next added.
(7) Knebel offers the following formula : (a) 20 parts hard gelatine (Winterthur) are soaked in 200 of distilled water (1 in 10 by weight) and afterward dissolved by heating. He then adds 24 parts potassium bromide and 1/4 part potassium iodide in solution, and 3 or 4 drops acetic acid or 0.1 part citric acid. (6) He dissolves 30 parts crystallised silver nitrate in 100 of water. (c) A gelatine solution for subsequent use is made of 14 parts hard gelatine and 6 of soft gelatine, for summer use; but if it is to be used in winter, 10 parts of each are taken. They are softened first, and then dissolved in 250 parts water. The solution (6) is gradually poured into the solution (a) and the vessel is rinsed with half as much water (50 parts), which is also added. The emulsion is now digested for 2 hours on a water bath at 150°-160° F. (65°-70° C). It is quickly cooled to 86° F. (30° C.) by placing it in cold water. Next, 6 or 7. parts ammonia (sp. gr. 0.920) are added to (c), which must be nearly cold and not very fluid. It is well stirred and then poured into the emulsion, which is at 86° F. (30° C), shaken thoroughly, and filtered through flannel and afterward in Braun's apparatus, after having first been pressed through canvas and well washed.
It is now ready to be poured upon the plates to dry.
(8) Another method, by Pizzighelli and Hubl, called the cold method, is as follows: -
(a) 1 part gelatine, 50 of water, 2 of ammonium carbonate, 15 of ammonium bromide, 2 of potassium iodide solution (1 to 10), 140 (by volume) of 92 per cent, alcohol, and 1-5 of ammonia water.
(6) Silver nitrate, 20 parts in 100 of water.
(c) Hard gelatine, 24-30. The constituents of (a) are mixed in the order thus given, except the gelatine, which is softened and dissolved, then added. The more ammonia, the softer and more sensitive the photographic film. The emulsion is formed as usual by adding (6) to (a), under the well-known precautions. They are digested as usual about 5 hours, then the emulsion is poured into a beaker glass and (c) is stirred in, allowed 1/2 hour to soften, and completely dissolved on a water bath. It is now rapidly stirred, and 500 parts (by volume) of strong alcohol is added, which precipitates the emulsion. The lumps that form are melted in small portions and poured into cold alcohol, stirred with a glass tube 2 in. in diameter, closed at the lower end. The emulsion attaches itself to the tube, and is then washed 1/2 hour in flowing water.
(9) Silver nitrate .. .. 1 oz.
Water ...... 10 "
Gelatine, hard .. .. 1 "
Water ...... 10 "
Let the gelatine soak for a short time, and then dissolve by placing the vessel in water at about 110° F. (43° C), and warm the silver to the same temperature. Now take into the dark room, and mix by any of the well-known methods, so as to form a very fine emulsion. It may now be cooled by placing the containing vessel in running water; when cold, wash, remelt, filter, and coat in the usual way. (A. Cowan.)