(5) Distilled water .. 1 oz.
Alcohol...... 4 oz.
Silver nitrate .. .. 240 gr.
(c) In the dark room, by a non-actinic light, such as a faint ruby light, 3/4 oz. ammonia, 880°, is added to 2 1/2 oz. of (6), which converts half of the silver solution into ammoniated silver. This is next mixed with the remaining 2 1/2 oz. of (6), and the whole is poured into (a) and well stirred. Silver bromide is thus formed, and it only remains to raise the temperature and add gelatine to complete the operation.
The emulsion, in a beaker, is next set into a water bath, the temperature of which is 120° F. (49° C), and 240 gr. of dry, hard gelatine (Heinrich's) is added, the liquid being continually stirred until all the gelatine has melted. It is then set away to cool, and in a short time the silver and gelatine coagulate at the bottom of the beaker into the form of a cake. The alcohol, about 8 oz., is next poured off and preserved for future use.
The emulsion cake is then broken up into small pieces, and subjected to a 2-3 hours' washing in constantly changing water; it is then remelted by means of the hot-water bath as before stated, and enough distilled water is added to increase the bulk up to 11 1/2 - 14 1/2 oz.; then Thymol ........ 6 gr. Dissolved in alcohol .. .. 4 dr. is added, and the emulsion is complete; after being filtered it can be flowed upon plates. If it is desired to mix up a small batch, 1/2 or 1/4 of the pellicle cake may be remelted, and the proper proportion of water and thymol added. The pellicle cake will retain its sensitive qualities for any length of time if kept in the dark.
Some of the advantages of the process are that successive batches of emulsions of uniform sensitiveness can be made, with great certainty; emulsions can be economically made; less alcohol is required; the gelatine extracts all the water from the alcohol, leaving it free (or nearly so) from the nitrates, which will be found crystallised at the top of pellicle cake; lastly, the alcohol can be continually used over and over again as a vehicle to promote emulsification, provided it is carefully filtered each time and added to the silver and bromide in the dark room, allowance being made for the ammonia it contains.
(a) Gelatine .. .. 30-50 gr.
Potassium bromide 300 gr.
Water ...... 20 oz.
(6) Silver nitrate .. .. 1 oz.
Nitric acid .. .. 1 m.
Water ...... 11/2 oz.
(c) Potassium iodide .. 15 gr.
Water ...... 1 dr.
The 3 solutions having been made, commence their mixture by cautiously adding, drop by drop, the solution (c) to the solution (6), agitating after each addition. The first effect will, of course, be the formation of a precipitate of silver iodide, but this is rapidly redis-solved by the excess of silver nitrate until the solution becomes saturated. Under favourable conditions, the full quantity of iodide may be thus added - that is to say, if the silver solution be sufficiently concentrated and its temperature not too high. However, the addition of iodide is continued until a permanent milkiness remains, when the remainder of the iodide, if there be any, is set aside until again required.
The solution (a) is now raised to boiling-point in the ordinary digester, and (b) is added to it in the usual manner, and thoroughly mixed by shaking or stirring. When this is done, the remainder of (c) is added, and the cooking is proceeded with by any of the methods in vogue, e. g. the ordinary boiling process, which is carried on to the verge of the blue stage if an emulsion of moderate rapidity only is required, or further in proportion. Or, after allowing the emulsion to cool, ammonia may be added, and the whole digested at a low temperature for an hour or so.
By the above plan of adding the iodide, a very fine state of division is ensured, as well as a higher degree of sensitiveness. The proportions of silver and haloids are so near the point of neutrality that very little trouble will be experienced in securing a rapid deposition of the sensitive precipitate; but if it should exhibit a reluctance to subside, longer digestion with ammonia, or longer boiling, as the case may be, will bring about an improvement. An emulsion made in the evening will have invariably subsided sufficiently by morning to allow of the first change of washing water being made; 3 such changes in all will prove ample.
The precipitated bromide, having been thoroughly washed, is transferred to a suitable flask or bottle; or, preferably, the washing may be performed in one of the conical precipitating or "parting" flasks, obtainable at any chemical glass warehouse. Into this is introduced 200 gr. hard gelatine, such as Heinrich's, previously swelled in distilled water, and sufficient water to make the bulk up to 10-12 oz. Heat is applied, and, as the gelatine dissolves, the flask is vigorously shaken to secure the thorough emulsification of the bromide. If, as will probably be the case, the bromide show a tendency to retain a slight granularity, it will be necessary to raise the temperature to about 200° F. (93° C), or even higher, and to keep it there, with occasional agitation, until the necessary degree of fine ness is attained. It is for this reason - the probable necessity for the application of a high temperature - that the albumen is not added until the last stage of the operation.
While the above operations are proceeding, take the whites of the necessary number of eggs, and beat them into a stiff froth without the help of water, ammonia, acetic acid, or any of the ordinary additions. For the above quantities of the ingredients, 4 oz. liquid albumen will suffice, or, roughly, the whites of 4 eggs. These having been converted into froth of such consistency that the vessel containing it may be reversed without any loss of albumen, the latter is transferred to a clean funnel of sufficient size, placed in a bottle or other vessel to catch the albumen as it gradually reliquefies, which it will do in the course of a few hours.