EMULSIONS FOPS DRY PLATES AND FILMS Whether it is feasible for the photographer, amateur or professional, to prepare his own dry plates is a question hardly worth discussing. The processes are attended with precariousness and intricacy at every stage. Each batch of that very uncertain substance, gelatine, must first undergo special testing. The apparatus is expensive. Provision has to be made against changes of temperature ; space set apart in workshops where scrupulous cleanliness can be observed, and, above all, freedom secured from that ubiquitous enemy, dust. Moreover, the greater part of the work has to be performed in non-actinic light. Even under the best con ditions, it is not unusual to see in any great commercial establishment whole tubs full of emulsion, which has had to be condemned for one reason or another. Still it is expedient that the photographer should have some insight into the manner in which the emulsions are prepared, if only to assist him to unravel any problem arising during exposure and development.
Boiling Process.—There are several different methods of preparing the emulsion ; but we shall here only describe two typical ones—the boiling process with hydrochloric acid, and the Monckhoven, or ammonia process. In the first of these, to take a typical formula, 6o parts Nelson's No. photographic gelatine is dissolved at moderate heat in 900 parts of water, and removed into non-actinic light, when 35o parts nitrate of silver dissolved in 48o parts of water, at about i2o°, is stirred in, and the mixture well shaken up after which the solution Potassium Bromide . . . . . 28o parts Potassium Iodide . . . . . to „ Hydrochloric Acid . . . . . 1 minim to every 2 0; Water . . . . . . . . 1,2oo parts is slowly poured in, with much shaking of the vessel t ensure perfect incorporation. All is then digested in water bath raised to boiling point, the emulsion bein occasionally shaken, for 30 to 43 minutes.
Meanwhile 30o parts of hard gelatine have been soake, and then dissolved at Ho°, or less, in 1,960 parts of water this is added to the emulsion when it has been allowe, to cool down to or 9o°, together with ammonia in io pe cent. solution, to minims to each ounce of emulsion.
Ammonia Process.—As this is the more attractive prc cess for experimental purposes, we give alternative formula Both of these are for slow plates, such as the amateur migh possibly succeed in carrying to successful conclusion.
(1) Ammonium Bromide . . . . . . 15 gr.
Solution of Potassium Iodide (to per cent.) . . 3 min.
Hard Gelatine . . . . . . . . 3o gr.
Nitrate of Silver . . . . . . . 25 „ Distilled Water (2) A. Potassium Bromide (Dr. Eder) . . . . . 120 gr.
Solution of Potassium Iodide . . . . 20 min.
Hard Gelatine . . . . . . . . too gr.
Water . . . . . . . . 2 oz. 5 dr.
B. Nitrate of Silver . . . . . . Iso gr.
Water . . . . . . . . 2 oz. 5 dr.
The gelatine is first dissolved in the water, by applicatioi Df moderate heat ; after soaking for about half an hour th bromide and iodide may be added, at a temperature no xceeding i io°. The nitrate of silver solution (to whirl sufficient ammonia has been added, drop by drop, to cause the precipitate at first formed to redissolve) is usually pourer into the emulsion warm and by degrees, with vigorous shak ng. According to the speed required the emulsion ma; then be poured out into a flat dish to set, or digested over a water bath at ioo° for 45 minutes. The proportion of iodide is variable. For landscape work the iodide is re commended as decreasing the liability of halation, and, according to Dr. Eder, plates are rendered thereby some what more sensitive.
Ripening the Emulsion.—lf an emulsion after mixing is digested in the water-bath at too' to 140', according to circumstances, for a time varying from 3o minutes to 2 hours, and then cooled rapidly in running water and allowed to stand for 48 hours, the sensitiveness is enormously in creased. The time must not exceed 48 hours, or fog will probably occur in the plates. No doubt the gelatine itself plays an important part in the formation and development of the latent image ; but, although some theories exist, none arc capable of proof, in view of the difficulties involved in the chemistry of gelatine. The ripening process may also be set going by the addition of ammonia to the emulsion. One alteration brought about by emulsion is an increase in the size of the halide particles.