In order to prevent misunderstanding, I repeat, that the method of preparing exceedingly sensitive emulsions by boiling and by treatment with ammonia give, as I have shown in my work, The Theory and Practice of the Gelatin-Emulsion Process, excellent results. For the practical photographer, however, and the manufacturer on a large scale, I strongly recommend the method of long-continued digesting at a gentle heat, and I will accordingly give a short resume of the directions which I have laid down at the end of the above-mentioned work. Dissolve 24 grammes of potassium bromide, and from 30 to 45 grammes of gelatin, in 800 cubic centimetres of water; also 30 grammes of silver nitrate in 300 cubic centimetres of water, and mix the two solutions at a temperature of about 40° C. This emulsion must then be digested over a water-bath, at a temperature of 35° C. If a less sensitive, but clear working, emulsion be required, one which shall be rich in contrasts - as, for example, architectural photographs, reproductions, etc. - the digesting must be extended over twelve hours. This emulsion will be equally sensitive, or even twice as sensitive, as wet collodion plates. It is to be thoroughly recommended in cases where a moderate degree of sensitiveness is sufficient. It should be at hand in every studio, and is often preferred by landscape photographers to a more sensitive emulsion. The same mixture, digested for three days, gives an emulsion specially adapted for portrait photographs. With a suitable developer - particularly pyrogallic - it works very soft and delicate. This emulsion ought to serve the principal wants of the practical photographer, To obtain still greater sensitiveness and softness, the digesting at 35° C. should be extended continuously for five days, and, at nearly the last moment, from one to two per cent, of ammonia should be added, cut it into pieces with an ivory knife, and place these pieces in a canvas bag, as recommended by Messrs. Wrattenand Wainright Strongly press with the fingers, to force the gelatin through the meshes of the time, and allow the email fragments to fall into a dish of cold water. Stir quickly with a glass rod; then poor the whole on a sieve placed in the upper portion of a cylindrical vessel and entering into it to the depth of and the digesting carried on for another half - hour. In this way, it in readily possible to prepare two differently working emulsions from the same mixture. The first portion can be poured off at the end of the first twelve hours' digesting, and put on one side; the remainder being still kept cooking. I recommend the owners of large laboratories, as well as amateurs, to keep both these kinds of emulsion in stock. The sensitive emulsion can be used in the studio; the less sensitive, in the open air. - Dr. J. M. Eder.
If the same quantities of chemicals are weighed up each time, it would be well to have weights made containing the exact number of ounces and grains required of each article used.
Two simple means of producing the bromide of silver in the needful state of fine subdivision are as follows: - The first is a glass funnel, with its end drawn to an exceedingly fine point. This is placed in a retort stand, and the nitrate of silver will be projected into the jar containing the bromide and gelatin solution. The whole can easily be mixed with anything found most convenient, such as a strip of glass or wood, a salad-fork, or one of those wooden mixers used in the cafes for the chocolate - pot. The other plan is the spray producer. This is easily made with a bottle, a good cork, two pieces of glass tubing, a piece of India-rubber tubing, and a valve - ball pear- shaped. (See Fig. 79.)
In cooking the emulsion, a good, trustworthy, well-glazed stoneware jar, with a lid or cover, will be found most useful, and a saucepan or boiler with a well-fitting lid and large enough to hold the jar; also a piece of wood to fit loosely into the saucepan. This keeps the jar from the bottom of the pan, which, when the jar is in it, should only be half filled with hot water. When cooked sufficiently, the emulsion is poured to set in a white, flat-bottomed porcelain dish placed in a zinc box with a deep cover, the whole fitting in a wooden box, which has also a deep cover fitting into a rebate set all around the outside of the wooden box. The zinc dish is made with two supports, so that the porcelain dish is from one and a half to two inches from the bot-tom. This space in hot weather is filled with crushed ice and nitrate of potash. There is also a little tube of zinc passing through the two boxes. To this is attached an India - rubber tube with a brass top to run off the waste water. In cool weather, nitrate of potash and water will suffice. Pieces of ice placed in the cool emulsion will help the setting. A piece of flat ebonite, about five inches by two inches, filed down to a sharp edge at one end and a hole pierced at the other to hang it up by, is most useful to scrape up the set emulsion with.
For breaking up the emulsion a canvas cloth may be used; but any mechanical means about seven centimeters (two and three-quarter inches). The water will fall into the vessel and the gelatin will remain on the sieve. Pour plenty that does the same without so much handling is better. A large zinc tube, silver-coated, with a pierced plate at one end and a wooden plunger to fit the same (Fig. 80), has done good service. It is placed in a large jar partly filled with cold water and the emulsion forced through. There are three knobs or feet to keep the strainer part from touching the bottom of the jar, and this allows the emulsion to come through.