(1) Burton and other well-known photographers advise the "acid boiling" process for making the emulsion, the operation being conducted in a very slight deep-ruby light. The formulae used by Burton are: -
(c) Autotype gelatine .. .. 450 gr. soaked in water, and afterwards squeezed to get rid of as much of the water as possible.
Iodide is recommended by Capt. Abney, and adds greatly to the quality of the plates. Although ammonium chloride is used in the preparation of this emulsion, it is not intended that there shall be any silver chloride in the emulsion. The bromide and iodide are just estimated to convert the silver, using Wernerke's practical equivalents. The reason for using the chloride at all is that a greater degree of sensitiveness is gained by boiling in presence of an excess of chloride than with an excess of bromide. Probably no advantage arises from the presence of silver chloride in the emulsion.
The amount of gelatine used in emul-sification is rather greater than is sometimes recommended, the quality of the plate being thereby improved. The reason why no definite quantity of acid is given, is because the chemicals themselves are frequently acid. Success in getting a very sensitive, and at the same time clear emulsion, greatly depends on the amount of acidity of the solution. It should be just acid enough to show by litmus paper. If this is the case when the salts are first dissolved, nothing more is wanted. If not, add very dilute hydrobromic acid, till the solution will just turn blue litmus paper red. If it has been neutral at first, about one drop of strong hydrobromic acid will suffice.
Autotype gelatine is the best for the bulk, as frilling never occurs with it. It is very hard setting, but at the same time does not repel the developer as some gelatines do. Somewhat more than the quantity given may be used, if it be desired.
To emulsify, pour the solution (b) into a glass bottle; afterwards add, little by little, the silver nitrate, both solutions having been raised to a temperature somewhere approaching the boiling-point. The whole is then poured into a large beaker -or jelly-can; this is covered with a wooden dish and placed in a saucepan. The lid of the latter is put on, and the whole is allowed to boil. Coat a plate with the emulsion newly made: on looking at a light through the plate, the light appears ruby red. The emulsion is said to be "red by transmitted light." As the process goes on, however, the colour changes, and at last it becomes blue. It is difficult to say exactly when the whole. of the silver bromide has been converted into the blue variety, but it may be discovered by gently drying a plate coated with the emulsion. The blue bromide and the red bromide will separate into patches. When all is converted to the blue, the boiling may cease. The time taken in boiling seems to vary considerably with different manipulators: generally somewhere between 1 and 2 hours; but it may be continued for several hours without producing fog, so long as the emulsion is kept distinctly but slightly acid.
The bulk of the gelatine is then added, and the whole is poured out in a flat dish to set.
When set quite stiff, it is cut up and placed in the " squeezer." It is pushed through wire gauze into a hair sieve held under water. This cuts it into very small particles, and if water be allowed to run through the sieve for 1/2 hour, the soluble salts will all be washed away. After this, the sieve is allowed to stand a short time, for some of the water to drain off. The emulsion is then heated, filtered through 2 folds of a pocket handkerchief, and spread on the glass. Use a small teapot as a* pourer. Pour the emulsion on the plate while the latter is on the levelling shelf. Then take a glass rod in the finger and thumb of each hand; dip this rod into the pool of emulsion on the plate - the emulsion runs by capillary attraction along the rod to the edges of the plate, but no farther. Lift the glass rod about a 1/16 in.; the emulsion rises with it. Pass it rapidly first to one end of the plate then to the other, guiding yourself by keeping your thumb and finger on the levelling shelf, and the plate is absolutely evenly coated. It is never removed from the levelling shelf till it is set. The plates are slightly warmed to begin with.
When set, they are reared on ends in racks; then placed in the drying box to dry.
Plates prepared as described are quite as rapid as the average of the so-called instantaneous plates sold commercially. Capt. Abney first pointed out that an emulsion got more rapid by keeping it cold after it was washed. This only happens when the emulsion is alkaline, or at least not acid. If to the emulsion, made as described above, about 8 drops of strong ammonia be added to the pint, it will be found that, after a week's keeping, the plates made from it will be 2-3 times quicker than before, and quicker than any commercial plates. This adding of a few drops of ammonia, simply to neutralise any acidity which may be in the emulsion, must be by no means confused with the process where digestion is carried on in the presence of 1-2 per cent. of ammonia, before washing.
It is not worth while for an amateur to make his own plates, if he expects to economise by it. It is easy to make a moderately rapid emulsion, and to make a number in succession with uniformity, but it is not so easy to make the plates. It is on their coating and drying that the difficulty comes. Then, if an exceedingly rapid emulsion is required, the difficulties become very great; the amount of light admissible is so small, that manipulation must be performed more by feeling than by seeing.