An important question is the temperature of the coating and drying room. As regards coating, with the gelatine he used, Burton prefers to have the room about 60°-65° F.; if colder than that, he warms it; 65° F. is the highest that ought ever to be used for drying, unless the natural heat of the air is greater. By lowering the air below the natural temperature, you produce a certain amount of dampness. The temperature should never be raised above 65° F. If you have a very dry room, you may keep it below that. As regards the method of applying heat, use a very small room, heated with a gas-stove, the products of the combustion of which must be carried outside; if working in a large room, arrange to draw air down the outside of the flue from the draught-box between it and a sliding cylinder fitted outside of it, (Soc, Arts Jl)
(2) Lockett (N. Stafford. Photog. Assoc.) gives the following directions: -
In the first place, all utensils are made as clean as possible. A large saucepan is filled with water, 20 gr. of gelatine put into a bottle with 6 or 7 oz. of water, and 112 gr. AmBr, 6 gr. KI, and 4 gr. AmCl; 2 drops hydrochloric acid; heat the water in the saucepan, place in it the bottle until the gelatine is dissolved, coat, and add by red light 200 gr. AgNO2 in crystals; shake until dissolved, which may be known by the sound. Put this in a dark place until convenient for boiling; will keep in this state for 3 weeks without experiencing any ill effect.
The boiling is done by placing the bottle uncorked in a covered saucepan containing water, which is brought to boiling-point if kept there for 60 min., shaking occasionally; when cold, the full quantity of gelatine is added, viz. 20 gr. per ounce, stirring well up from the bottom, and pouring into a dish to set. When cold, squeegee the emulsion (after being scraped up from the bottom of the dish with a card) through muslin into a solution of potash bichromate, stir it up with a strip of glass, and leave for 1/2 hour or more; then place in a sieve and drain well, afterwards well wash in running water for an hour, drain, melt again, and make up to 10 oz. The addition of alcohol is unnecessary, unless you wish to keep the emulsion, when cover it with methylated spirit.
The plates are cleaned, and a substratum of sodic silicate-solution in water 1/2 per cent. is applied with a sponge, and allowed spontaneously to dry. This is an effectual cure for frilling, and at the same time it renders the operation of coating more easy, allowing the emulsion to flow like collodion without the use of spirit. The emulsion is warmed and passed through 4 thicknesses of flannel into a jug; again pour upon the plates as with collodion, but only slightly drain and then place upon cold glass plates to set, which they do in 10 minutes. Afterwards place upon racks in a well-ventilated drying cupboard, the door of which is not opened until they are judged to be quite dry, or marks will occur in them. These plates give thin images unless plenty of bromide be used. The best proportion is: - 4 gr. ammonia, 3 gr. ammonia bromide, 2 gr. pyrogallic acid (commonly termed simply "pyro") per oz. of water. But if any one gives the common soda carbonate a trial, he will prefer it to ammonia. Use a saturated solution, with which and 1 gr. pyro and 3 ammonia bromide, you will get denser and brighter negatives than with 2 gr. pyro and ammonia.
(3) Although somewhat slower than a bromide emulsion, the chloride possesses greater scope for positive printing than can be attained with the bromide. A. L. Henderson adopts the following formula for a chloride emulsion, which is very practical and useful: -
Hard gelatine .. .. 80 gr.
Water ...... 1 1/2 oz.
Silver nitrate .. .. 75 gr.
Water ...... 1/2 dr.
Dry sodium chloride.. 21 gr. Potassium citrate .. 21 gr. Dissolved in water .. 1/2 dr. which is warmed. The emulsion is poured into a dish and allowed to set. The jelly is now cut into strips, and washed in the usual way; cold water should be used, as the emulsion is very thin. The wash should be carried on under a yellow light. After washing, the emulsion is melted by heat and to it are added:
Dissolved in a small quantity of warm water. After the emulsion is filtered, the plates are coated with it in the usual way. The film is extremely thin, on account of the watery composition of the emulsion. If more contrast is wanted, the emulsion should be made thicker by the addition of gelatine. As it will not keep well, only small batches should be made at a time, enough to cover the plates to be coated.
Opal plates coated with the emulsion are printed behind a negative in a frame in the same manner as with ordinary silver paper; the picture will appear on the surface in the same way. The exposure varies with the density of the negative, and may readily be ascertained by exposing a piece of paper coated with the same emulsion behind the negative. After printing, the plate is first well washed, and is next toned with the ordinary gold chloride and borax toning bath; it is again well washed, and fixed for 10-15 minutes in a hypo bath of 21 per cent. strength; is washed well and soaked again for a few minutes in an alum bath, washed, and dried.
(4) A. L. Henderson has recently made some improvements on what is termed his cold precipitation process of making gelatine emulsions. The formula is as follows: -
(a) Distilled water.. .. 1 oz. Nelson's No. 1 gelatine 5 gr. Potassium bromide .. 180 gr. Potassium iodide .. 2 gr.
The above is heated just enough to melt the gelatine; next is added -
Alcohol...... 4 oz.