There are two distinct systems of making emulsions: the acid or boiling process and the ammonio-nitrate process. The former, as a rule, is used for somewhat slow emulsions, and the latter for the faster negative kinds. As to the highest speed obtainable by the acid process, no definite data are available, but certainly zoo H. and D. may be considered the limit, whilst by the ammonia process from 30o to 400 H. and D. can be reached. In the acid process the mixing and the ripening are effected in an acid gelatine solution, whilst in the other process either the whole or part of the silver nitrate is converted into ammonio-nitrate of silver. For amateur work the ammonia process is somewhat easier, and, if excessive speed is not required, nice clean emulsions, giving good density, can be obtained.
Before treating further on the actual mixing of the emulsion, it should be stated that the alkaline bromides and iodides are interchange able, though as a rule that given in ti formula should be adhered to ; still, the accompanying tables, compiled by Ackland, will be found ex ceedingly useful, as they allow of the easy cal culation of the necessary amount of haloid for any quantity of silver, or the substitution of one for another.
Table I. enables the worker to calculate the weight of haloid to convert any given quantity of silver, or, vice versa, the quantity of silver haloid produced from every grain of haloid, or the weight of silver haloid produced from every grain of silver nitrate. If, for instance, the formula is :— Potassium bromide . . 150 Potassium iodide . . To Ammonium chloride . . Gelatine . . . . 200 The quantity of silver required to saturate the above can be calculated by taking the figures in the third column and multiplying by the above. Thus : Potassium bromide 150 x I -427 = 214.05 Potassium iodide io x 1•023 = ro•23 Ammonium chloride 25 x 3.177 = Weight of silver nitrate required 303705 The fourth column enables the worker to cal culate readily what excess of soluble haloid there may be present in the emulsion ; whilst considerable influence on the final result, both as regards speed and density. If the speed required is approximately obtained in the first process of cooking, then pouring out the emul sion into flat pans to the depth of about i in. will quickly arrest the ripening action, especially if the pan or dish is stood in cold running water. In summer-time it is even advisable to pack ice around the pan, which for such small quantities as given in these pages may be a clean (most important) i5-in. by 12-in, dish. On the other hand, if the emulsion is allowed to set in its mixing pot, greater speed is obtained, especially when ammoniacal emulsions are made, because the heat is longer retained, and there is less chance for the escape of the ammonia. It is as
well, however, in this case to cool the bulk of the emulsion by running cold water around the pot and constantly stirring ; ice water, too, may be used, but care must be exercised, as when an emulsion is poured out into flat, ice-cooled pans to set there is danger in the case of negative emul sions of a want of density in the highest lights.
Still more important with regard to the density and speed of the emulsion is the lapse of time between the setting and washing ; and in the case of negative emulsions the speed may be nearly doubled, and consequently the density of the high lights increased, by allowing the emulsion to stand in the solid state in cold for twenty-four to forty-eight hours. Except where otherwise advised, the normal time is about twelve hours ; that is to say, a negative emul columns 5, 6, and 7 enable him to work back when he has determined on a fixed content of silver haloid.
Table II. is extremely useful for finding the weight of one haloid that will replace another. Supposing, for instance, that it is desired to replace 8o grs. of ammonium bromide with the sodium salt ; then in the first vertical column headed " ammonium bromide " there will be found against sodium bromide 1-05 1, which is the weight of this salt that will convert as much silver nitrate as one grain of ammonium bromide. Therefore 1•o51 x 8o = 84•08 would be the quantity of sodium haloid to use.
Before entering into the making of emulsions it may be as well to consider the subsequent operations first, as they play an important part in the quality of the finished emulsion. The method adopted for setting the emulsion has sion made one evening is ready the next morning to be broken up and washed.
The easiest method of breaking up small quantities of emulsion is to use coarse-meshed canvas, the mesh being about in. square. The emulsion should be coarsely cut up by means of a silver knife (do not use a steel one), or scored through with 41 wide-pronged silver or plated fork, and then put into a sheet of the canvas, the ends of the latter being gathered together so as to form a bag. The bag is held under the surface of a dish of clean water and squeezed round and round so as to force the emulsion through the mesh. railing the canvas, a fork can be efficiently used if the emulsion has been set in a dish, as it can be scored longitudin ally and then across at right angles so as to cut it up into little cubes, which should not be too small, as otherwise they pick up too much water.