EMULSION (Pr., Emulsion ; Ger., Emulsion) A liquid, usually viscous, containing in suspension an insoluble body in an extremely finely divided state. Plates, films, and bromide, gaslight and printing-out papers are coated with emulsions. To such perfection has the commercial manufacture of these articles attained that it will hardly pay the average worker to prepare his own ; but from an educational point of view the manufacture of emulsions is extremely valuable. In the following notes only tested formula; are given, and it must be clearly under stood that perfection is only attainable after considerable experience, and the tyro must not expect to prepare either papers or plates as excellent or as fast in working as those com mercially obtainable.
The various stages in emulsion making will be outlined, and the reasons for each step explained. Let it be assumed, therefore, that it is wished to make a silver bromide gelatine emulsion. The bromide is formed by double decomposition or chemical interchange between an alkaline bromide, usually potassium or ammonium, and silver nitrate. If aqueous solutions of these two salts were mixed in a haphazard fashion there would certainly be obtained a coarse, granular form of silver bromide which would at once sink to the bottom of the vessel, and there might be an excess of either silver nitrate or bromide. To prevent the immediate deposition of the bromide, and to obtain a fine grain, a vehicle—gelatine is added to the alkaline bromide solution, and the fineness of grain largely depends upon the proportion of gelatine used. If too much gela tine, or too hard a kind, be used during mixing it is difficult to obtain high speed, as the gelatine acts as a mechanical restrainer ; on the other hand, if too little is used, a coarse granular deposit is formed, and the emulsion tends to fog and thinness. When an alkaline bromide and silver nitrate are mixed together they com bine in definite proportions according to their molecular weights. The molecular or combining weight of potassium bromide is 179, and that of silver nitrate is 170. If these quantities were weighed out exactly, whether in grains, ounces, pounds, or tons, or grammes or kilogrammes, exactly 188 parts of silver bromide would be formed, and there would be found in the water neither silver nitrate nor potassium bromide ; but the slightest error in weighing might give an excess of silver nitrate, which would be fatal to the emulsion in development. It is
customary, therefore, to use an excess of bromide or other salt in all emulsions intended for develop ment ; this excess varies in most and is governed by the process used, the quality of the gelatine and the speed required. Some gelatines will give perfectly clean emulsions with a much smaller excess than others. Then, as one of the prime uses of the excess of bromide is to keep the emulsion free from fog, a reasonable excess is useful on this account, and increase may make an otherwise foggy-working formula satisfactory. Of recent years it has been con sidered that an increased excess of bromide tends to give faster emulsions for negative work, but it has at the same time a tendency to pro duce thinness in the high lights. A normal ratio is ioo of silver nitrate to 8o of potassium bromide, though it will be seen that the ratios given in the formula vary from this in some cases.
When first mixed the emulsion is very slow, no matter what formula is used, and would be quite unsuitable for anything but lantern plates. It is therefore subjected to a " ripening " process, either by continued application of heat or the use of ammonia. Exactly what occurs during ripening is a matter of doubt, but it is generally assumed that the silver bromide grain increases in size and that this increase is accompanied by greater sensitiveness to light ; the change is probably more of a physical than chemical nature.
It has been already stated that there is a chemical interchange between the silver nitrate and bromide, and this is represented by the following equation, which may be said to be the chemists shorthand method of explaining what occurs :— + KBr = AgBr + silver nitrate pot. bromide silver bromide pot, nitrate 170 119 101 The figures here are the molecular or com bining weight, and, as has already been explained, 17o parts of silver nitrate combine with 119 of potassium bromide to form 188 parts of silver bromide and 101 parts of potassium nitrate. Alkaline nitrate thus formed must be got rid of, and this is the purpose of the washing, which also removes the excess of alkaline bromide and the ammonia, if this latter has been used for ripening. Were these salts not washed out they would crystallise out on the plate during the process of drying after coating, and either prevent the access of light, or give rise to crystalline markings which would show in the negative.