The art of the emulsion maker is to precipitate the silver salts in the form of grains having the desired photographic qualities, with a certain regularity and uniformity, in commercial quan tities and over long periods.
Whilst it is relatively easy to obtain the necessary metallic salts with the desired degree of purity (total absence of copper, iron, and lead salts, the causes of desensitization and fog), the scientific testing of gelatines for photographic emulsions still leaves much to be desired, and needs always to be supplemented by an actual manufacturing test on a semi-works scale. The properties of gelatine, even of the best-known brands, vary considerably from one batch to another, and especially from one season to another. These variations can only be compen sated for by more or less successful blending. Gelatines always contain different amounts of substances, formed by their degradation, of which some, soluble in cold water, are eliminated by the washings, while others, which are in soluble, play an important role, sometimes useful and sometimes harmful, as regards the characteristics of the emulsion.
The state of acidity or alkalinity of the medium exerts, at each stage of the emulsion making process, a considerable influence on the photographic properties of the emulsion.
194. In photographic factories most elab orate precautions are taken in order to keep the conditions under which the manufacturing pro cesses are carried out absolutely constant, and to avoid all external causes of contamination.
A photographic factory generally forms an enclosed space, and is only connected with the outside by self-sealing doorways for the passage in and out of the staff of the factory and of the materials used. The air used for ventilation and drying is filtered free from dust and brought to a constant temperature and hygrometric state, generally by washing it in chilled brine, which frees the air from dust and its excess of humidity, and then by passing it over radiators.
The precautions taken to ensure cleanliness and asepsis are at least as thorough as in the best surgical operating theatres. All those in
the factory can enter the part where the emul sions are made only when wearing special over alls and headgear.
A rigorous chemical control is kept of the various raw materials as well as of the emulsion itself during manufacture and afterwards. Lastly, before the final coating takes place, a test coating is carried out to ascertain the photographic qualities of the emulsion. All emulsion that is not satisfactory is treated for recovery of the silver. All those coated and issued under a maker's trade-mark may be regarded as emulsions of perfect quality, with their different characteristics lying between well-defined limits.
195. The Sensitive Emulsion. As an item of interest the approximate mean of the results of recent analyses (1924) of ultra-rapid emulsions is given below '— Emulsion (dried in air) 2 per square foot. 42 to 7o gr.— Percentage Proportions of the Constituents Water - . . so Gelatine . . . 55 Silver chloride . . . (traces) Silver bromide . . 33 to 32 Silver iodide . . . 2 to 3.
A negative emulsion generally has a very feebly alkaline reaction. Its inciting tempera ture when plunged in warm water is about 95° F. (35° C.) in the case of fairly new plates and films, and increases slightly with age.
196. Microscopic examination of suitably diluted emulsions (Fig. 136) shows that the grains of silver halide 3 in an emulsion have very different shapes and sizes. Under a sufficiently large magnification the crystalline structure can be clearly seen, and the forms identified with Tile application of extremely delicate methods has shown that traces of free silver can be detected in ripened emulsions-about 2 milligrammes per square metre. F. Weigert and F. Luhr, 1928.
2 Gelatine is considered to be " dry " when it does not contain more than about 15 per cent of moisture ; in this state it is very flexible, but it becomes very brittle when the water content drops to so per cent.
3 The generic name of haloid, halide, or halogenide salts is given to chlorides, bromides, and iodides.