DESICCATED DRY PLATES Dried, or heated, dry plates. It is well known that dry plates, as ordinarily used, retain a con siderable percentage of water, mainly in mole cular combination with the gelatine. Howard Farmer, to whom the idea of desiccated plates is due, found that the water has a large influence on the image at the time of exposure, definition, detail, density, speed, etc., being affected. He found, moreover, that very small differences in the percentages materially affect the result, so that negatives vary with the atmospheric conditions, as to temperature and humidity, at the time of exposure. Accord ing to Farmer, " The drier the film, the better the definition, and the greater the power of rendering fine detail ; in lesser degree, the greater the speed and facility of developing density. This property of the film can be utilised by desiccating dry plates for work where definition, detail, brilliancy, or maximum speed are desired, and in exposing plates wet where softness of image or the destruction of small textures and details are sought for. Extra
rapid and orthochromatic plates, in which the former of these qualities is usually found lacking, gain them when desiccated to an extent hitherto only found in wet collodion or other specially prepared films." A perfectly flat-topped kettle containing boil ing water is a convenient appliance for desiccating plates ; these are simply laid film side upwards on the kettle top with a piece of bibulous paper between to equalise the heating, and kept there ford few minutes at a temperature of 200° P. (about 93° C.) before being placed in the dark slides. Or a thick copper slab with an asbestos cover can be used. Too great or too prolonged beating will crack the dry plate or induce fog.
Desiccation must be carried out in a dark-room, and the plates should be exposed while hot or as soon after the treatment as possible.
In process work, desiccated dry plates have been recommended for direct colour work.