Pour a pool of emulsion in the centre of the plate, and smooth over with a glass rod. Or, if the glass is laid on a scrupulously level surface, a smooth film will distribute itself with a little coaxing on the part of the operator. A thin substratum of albumen or gelatine with chrome alum forms a good sizing for the plate, and will prevent frilling. The plate may be warmed, but must not be too hot, and the emulsion must be kept at a high temperature - not less than 1200. It is not easy to decide how thick an emulsion should be. Very thin coatings tend to pinholes; thick coatings will often contain uneven knobs. 1 1/2 drams ought to be the average for a quarter plate, and 2 1/2 drams for a half plate. Some will consider this too liberal.1
Nearly all plate makers have adopted the Hurter and Driffield system of speed-marking. It is a method of sound and accurate character, designed with the utmost thought and care, including a standard light and standard developer. Unfortunately this excellent apparatus is sadly misused by some manufacturers, and the measurements on plate-boxes are often seriously inaccurate. Another ingenious little instrument is the Chapman Jones plate-tester, consisting of a screen-plate with several divisions, which is placed in front of the film to be tested at a certain distance from the standard light. The various divisions provide an index to: (1) The speed of the plate. (2) Its range of gradation. (3) Range of exposure. (4) Colour sensitiveness. (5) Grain. (6) Liability to halation. (7) Safest light for dark room. Every photographic club ought to place this inexpensive and most serviceable aid to correct exposure at the disposal of its members.
A proper drying cupboard, in which the air is kept at a constant moderate temperature, is most important. It should be light-tight and air-tight, with a box underneath the shelves containing lumps of calcium chloride - a moisture absorbent which can always be restored to efficiency after saturation by merely baking in a hot oven. Sudden changes of temperature, or rapid drying under heat, spoil the fibre of the gelatine and cause frilling. The plates should take about 12 hours to dry. If wanted in a hurry, they should be dried by the application of alcohol. When dry they should be packed in the usual boxes, and wrapped up to preserve them from damp and changes of temperature. The best and simplest way of packing is to place each two film to film, with the usual paper spill at each end, and wrap the pair in soft brown paper. A good emulsion, if kept dry and away from light, should be capable of giving a satisfactory negative for at least two years. In practice, dry plates of more than a year old should be rejected. On the other hand we have tested plates five years after the date marked on the box by the maker, and found them all that could be desired.
1 W. K. Burton advises I dram for a quarter plate, 1 1/2 dram for a half plate.
Emulsions on celluloid films are not in essence different from those used on plates; some makers coat the same emulsion on their plates and films of corresponding rapidity. Roll-films first receive a layer of hard gelatine on the reverse side to that which is to take the emulsion. This obviates much of the curling during development, and, still worse, after drying, which was formerly characteristic of these films. Emulsions for bromide or chloro-bromide papers are also prepared in a very similar way, except that these do not undergo any prolonged boiling, digesting, or ripening process.