Commercially, when large quantities of emulsion have to be treated, a power or hand press is used, and the emulsion forced through a perfor ated plate.
After shredding, tht emulsion is washed. According to the quantity to be treated, washing may be effected either in flat trays or by means of a calico or linen bag and a deep jar. In the first case, a sheet of linen should be placed over the dish or tray, and the emulsion placed in it and a stream of water allowed to run in at one corner and out at the diagonally opposite corner ; it is as well to raise slightly that end of the dish at which the water flows in. In the case of the bag method a square of linen should be gathered up into a bag, the emulsion placed therein, and some stout string tied round with a sufficiently long loop to pass over a stick that rests on the mouth of the jar and allows the bag to hang down in the water, By this method the soluble and detrimental salts formed in the emulsion diffuse out into the water. Whichever method is adopted, it is important that the water should be frequently changed, and it is advisable to squeeze the emulsion gently at intervals so as to press out the water.
There is great diversity of opinion as to the best duration of washing, some workers main taining that from eight to twelve hours is not too long, whilst others reduce the time to two hours or even less. The disadvantages of pro longed washing are that the emulsion, par ticularly in hot weather, picks up a large quan tity of unnecessary water which, unless an extra quantity of gelatine is added afterwards, renders the emulsion so sloppy that it is difficult to coat it on glass, and, further—particularly with rapid negative emulsions—the ripening process goes on, and as the soluble bromides in excess are being removed, there is great danger of fog ensuing. From very careful tests the writer has found that all emulsions can be thoroughly washed by repeated changes (about twelve) of water in two hours, and small quantities—par ticularly if the weather is warm—can be thor oughly washed in one hour. The washed emul sion should be allowed to drain well, and is then ready for melting.
The emulsion shreds should be placed in a pot in a water bath at 9o° F. (about 32° C.) and the temperature gradually raised to 120° F. (nearly 49° C.), at which temperature the final quantity of gelatine should be added and the whole well stirred till the gelatine is dissolved. It is just as well to keep the emulsion at this temperature for at least half an hour, and it can then be filtered through dean felt jelly bags, or, failing these, swansdown calico or Canton flannel, previously washed to remove any dressing. If there is any difficulty in getting the emulsion to pass through the material, pressure may be applied by squeezing with the hands, but usually, with the emulsion at the temperature stated, there is no trouble pro viding the filter is first wetted with hot water and well wrung out.
GuLATINo-amoRror OR P.O.P. Emur.,SroN.— The simplest form of gelatine emulsion is that used for P.O.P., and the following formulae will
give excellent results :— Valenta's Formula A. Silver nitrate . . 307 grs. 32 g.
Citric acid . . 77 „ 8 PP Hot distilled water . 3 oz. 160 ccs.
B. Hard emulsion gela tine . . . 922 grs. 96 g.
Distilled water . 14 oz. 7oo ccs.
Allow to soak for half an hour, and then melt in a water bath at 12o° P. and add C. Ammonium chloride. 28 grs. 2.8 g. Tartaric acid . . 28 „ 2.8 „ Sodium bicarbonate 14 „ „ Alum. 18 „ x.8 „ • Distilled water . 3 oz. 140 ccs.
The ingredients in solution C must be dis solved in the order given, and care should be taken to use a sufficiently large vessel, as brisk effervescence ensues. Both B and C should be brought to 12o° F. (nearly 49° C.) and mixed, and then A, at the same temperature, added slowly and with constant agitation. Allow the emulsion to stand for from two to four hours in a water bath at 1 io° P. (about 43° C.) with occa sional stirring, and then add— Alcohol . . . 15 thins. Zoo cc.% and filter thrdugh glass wool or two thicknesses of Canton flannel or one thickness of swansdown. Care must be exercised as to the light used, as the fluid emulsion can readily darken in colour whilst digesting even in strong gaslight. The duration of digestion or ripening is dependent on the speed required. If coated when freshly mixed, the emulsion is slow and gives rich and vigorous prints ; if, on the other hand, it is ripened for four hours it becomes much more rapid, and gives a longer scale of gradation. If it is desired to obtain a paper that will keep for some time without discoloration, an equal quantity of citric acid to that given above should be added after digestion.
Another excellent formula is the following :— Wade's Formula A. Ammonium chloride. 25 grs. 2.6 g, Rochelle salts . . 25 „ 2.6 „ Alum . . . so „ 5.2 „ Distilled water . 21 oz. 125 ccs.
Dissolve, and add B. Gelatine . . . 82o grs. 86 g.
Distilled water . 15 oz. 75o ccs.
previously dissolved at no° F., and finally add C. Silver nitrate . . 284 grs. 3o g.
Citric acid . . 15o „ 15.6 „ Distilled water . 2- OZ. 125 ccs.
The coating temperature of the above emul sions should be about 95° F. (35° C.).
If an emulsion suitable for extremely thin, flat negatives is required, the addition of a small quantity of the chloride of uranium, of nickel or of cobalt, to any of the above formulae will shorten the scale of gradation, but the most satis factory agent is calcium chromate, which must be added with caution, as it is extremely ener getic in its action, o•x per cent, reducing the scale of gradation by about one-third.
For matt surface papers the quantity of gela tine should be reduced so as to make an 8 per cent. solution. More satisfactory matt emul sions can be obtained by incorporating with the emulsion ro per cent. of fine rice starch. The necessary quantity of starch should be rubbed into a cream with a little water and a small quantity of gelatine, taken from the emulsion itself, and the whole should be heated to 160° F. (71° C.) for an hour and then added to the emulsion. (See also " Collodion Emulsion.")