The drying of negatives on glass is a comparatively simple matter, owing to the rigid nature of the emulsion support. A large number of plates may be placed in a compact mass in the ordinary plate racks of commerce with the wet sides accessible to a draft of air. Two dozen plates separated from each other by a quarter of an inch and left to dry spontaneously in a room of ordinary humidity and living temperature will dry in two hours and a half. If the surface be wiped with soft cheese-cloth or chamois, so as to absorb all the surface moisture before the plates are placed on the rack, this time may be appreciably reduced. By placing the plates in a forced draft of air, from an electric fan, this time may be reduced to an hour.
Extra rapid drying of plates may be accomplished by placing them in a bath of alcohol before putting them in the racks. The alcohol displaces all the water in the film, and is itself very quickly dissipated into the atmosphere when the plate is taken from the tray. The plate must be left in the alcohol tray long enough for the substitution of the alcohol for the water in the film to take place. Five minutes is long enough. The alcohol before use must be as nearly free from water as possible. The best way to make sure of this is to place in the bottle of alcohol some lumps of calcium oxide, which will take up the water and form calcium hydroxide, which settles at the bottom of the bottle.
Another method of quick plate drying takes advantage of the extraordinary greediness of potassium carbonate for water. The wet plates are placed in a saturated solution of potassium carbonate and left for a minute. If a plate be now taken from the solution and its surface wiped with a soft cloth, it will be found that the film has a greasy, slippery feeling, but that it contains no water and can be printed from at once. Plates so treated should be washed, however, at some time in the succeeding four months, or the traces of potassium carbonate left in the film cause deterioration.