Slow and Rapid difference between a slow and rapid emulsion is, therefore, not merely in the proportions of the amount of bromide and iodide salts to the nitrate of silver ; although in the latter it is usual to employ a larger quantity of iodide, and also much more bromide than would be required according to Ackland's Tables. A slow emulsion is usually set at once, immediately after mixing. Digestion in the water bath for half an hour at 110° may increase the speed from 4 to 5 times on the Hurter & Driffield Scale.
An excess of bromides and iodides with prolonged digestion at higher temperatures may raise it to 3o times. For in stance, in formula (I), by increasing the bromide to 32 gr. and the iodide solution to 12 minims, stewing for an hour after addition of silver nitrate at 12o°, then cooling rapidly and allowing to stand for 32 hours, the emulsion would possibly show a speed of 200 H. & D.
Chloride Emulsions.—Most silver chloride emulsions are slow and suitable only for contact lantern places, gaslight papers, etc., etc. Liippo-Cramer's formula represents an endeavour to produce a sensitivity in the chloride comparable in some degree with that of bromide : Gelatine . . . . . . . . . io grams Common Salt . . . . . . . . 7 „ Hydrochloric Acid (sp. gr. 1.9) . . . . io c.c.
Water . . . . . . . . . Too c.c.
Heat to 176° Fahr. and add: Silver Nitrate . . . . . . . . To grams Water . . . . . . . . . ioo c.c.
at same temperature. Digest in water bath for a quarter of an hour, and then treat as usual. The ammonia method tends to produce fogged emulsion.
Washing the Emulsion.—The hard, stiff jelly must now be forced, either by machinery through a sieve, or squeezed through canvas netting in cold water, and then collected once more into the canvas bag for from half an hour under running water to a soaking of several hours in still water ; the squeezing process is then repeated, exposing fresh surfaces of gelatine to the action of the water. By this time the excess of nitrates and bromides will have been removed, and the emulsion is very carefully drained over a hair sieve, then collected and melted once more at a temperature of about 120°, when it is filtered, and is finally ready for coating the glass plates. If the gelatine shows signs of degeneration small quantities of formalise or chrome alum are introduced ; but the latter will tend to lessen the sensitiveness of the plate. Special machinery is of course employed in coating plates for commerce. For our trial plates we may take the glass from which old negatives have been stripped, and immerse them, first in some acid bath, and then in caustic potash, after which they are rinsed in warm water and rubbed thoroughly dry with a clean dishcloth. Beware of dust at all stages, but at this moment every speck of dust is a source of destruction and ruin.
Coating the Plates.—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 120 . 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. 1i drams ought to be the average for a quarter plate, and 21 drams for a half plate. Some will consider this too liberal 1 Testing Plates.—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 in accurate. 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 : (I) 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.
Drying.—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.
Emulsions on celluloid films are not in essence different from those used on plates ; some makers coat the same emul sion 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.