PREPARATION AND PROPERTIES OF NEGATIVES-GELATINO-BROMIDE EMULSIONS 192. Preparation of Emulsions. Rapid negative emulsions are usually prepared by pouring a neutral or ammoniacal solution of silver nitrate into a slightly warm solution of gelatine con taining potassium bromide with a small propor tion of iodide. The mixing requires to be done during constant agitation, e.g. in a mixer, thus avoiding the possibility of the least excess of silver nitrate. The conditions under which the mixing takes place have a considerable, if not the chief, influence on the final characteristics of the emulsion. Slow, fine-grained emulsions are prepared from weak solutions of metallic salts, mixed in the presence of a large proportion of gelatine ; large-grained crystalline rapid emulsions are prepared from concentrated solutions of silver nitrate and potassium bromide mixed (all at a time or in several separate additions) in the presence of a small proportion of gelatine. 2 The emulsion thus prepared is then subjected to the process of ripening, which consists in cooking it for a given time at an accurately controlled temperature, sometimes after the addi tion of ammonia or ammonium carbonate, unless the silver nitrate was introduced into the emulsion in the form of an ammoniacal solution. In an ammoniacal medium the cooking may be carried out for about 3 hours at 140° F. C); the time is much longer if the emulsion is neutral. During the ripening, the sensitivity increases considerably, and, while the grain size gradually increases, certain of the grains " grow " at the expense of the smallest ones. 3 A sufficient quan tity of a concentrated solution of gelatine is then added, so that, after setting, the emulsion becomes a fairly stiff jelly.
After setting, the emulsion is pressed by an hydraulic press, in a silver cylinder, at the bot tom of which is a perforated plate or a sieve of silver plates arranged on edge, so as to obtain shreds of about in. in diameter. Whilst in this form the emulsion is subjected to long washing in cold water until the excess of bromide and nitrates formed during the reaction, and also the ammonia, are completely removed. The washed shreds are then kept in ice until they are used.
After draining, the shreds of the emulsion are melted in a water bath and the emulsion submitted to a second ripening during which the speed increases, without growing of the grains. To the liquid emulsion different sub stances are then added, in particular chrome alum, which raises the melting point of the gelatine after it has been coated and dried ; traces of potassium bromide (about I part to Ioo parts of silver halide), which gives a cleaner image ; certain dyes for colour sen sitizing (§ 2°8), and, lastly, different sub stances (alcohol, saponin, etc.), which lower
the surface tension of the emulsion, and thus facilitate coating. In this form the emulsion is coated mechanically on any given support, previously given a substratum which varies with the nature of the support and which is used to ensure that the emulsion adheres to the support during the various photographic manipulations.
193. In spite of the considerable progress that has been made in the application of the methods of physical chemistry to the scientific control of the different phases of the manufac turing process, the preparation of photographic emulsions is still largely an empirical art, and will probably still remain so as long as the intimate causes of sensitivity and the exact nature of the latent image are not known, and as long as the manufacture of gelatine, a material of vital importance in the making of photo graphic emulsions, does not itself make the considerable progress which at present it shows no sign of doing.
Silver bromide precipitated in pure aqueous solution is spontaneously developable without previous action of but this reduction can be prevented by the presence of only a very little If silver bromide is precipitated in other colloidal media, such as collodion, the sensitivity is very low and cannot be increased by ripening. Thus the gelatine acts as a sen sitizer, in addition to being the emulsifying agent. The presence of an iodide is necessary for the preparation of the best rapid emulsions ; iodide facilitates the ripening process in that it retards the time of appearance of chemical fog. The sensitivity of the emulsion by no means depends on the sensitivities of the two consti tuent salts, but only on the conditions under which the salts are used and the emulsion made. All the grains in a single emulsion contain both silver iodide and silver bromide (with faint traces of silver chloride due to the chlorides which are always present in commercial bromides and iodides and in the waters used in their preparation and washings), but in variable pro portions. The largest grains, which are the most sensitive, contain the most iodide.