DETERIORATION Lenses, chemicals, and sensitive plates and papers all deteriorate more or less according to the length of time and manner in which they are kept. The commonest form of lens deterior ation is due to the gradual depolishing of the glass surface by dust, the action being assisted by the careless or rough use of the dusting cloth. It is better to remove dust by blowing it off, but if wiping is necessary it must be done carefully (see " Lenses, Cleaning "). Sometimes when a lens is put on one side for a time an iridescence spreads over its surface, due to dampness. Lenses stored in a strong light often become slower in action, because of the yellowing action of light upon the balsam with which the lens glasses are cemented together.
Dry plates and sensitive papers deteriorate very quickly if not properly stored in a dry, airy place, the worst place being a high shelf where gas fumes can get to them. The effect of age on a plate much resembles that of a very slight exposure to light ; but dry plates keep remarkably well if stored carefully, and skilled workers can frequently get good results on plates as old as twenty years.
J. B. B. Wellington in igo5 pointed out that the popular sulphide toning bath has a deteriorating effect upon plates and papers ; everyone knows that the minute quantities of sulphur contained in a London fog will tarnish silver articles, producing upon them a thin film of silver sulphide, and that the sulphur in coal gas has the same effect when the gas is burnt. It is thus easy to understand how much
more deleterious must be the large quantities of sulphur given off from the sulphurising bath of sodium sulphide on the still more delicate silver bromide and silver chloride which go to make sip the emulsions coated upon plates, films, papers, etc. When these are kept in a room in which sodium sulphide is employed, they will become unusable in the course of a few weeks.
Plates affected by sulphur will develop with an iridescent stain, with general deterioration and fog. Bromide and gaslight papers are affected in practically the same way, producing a flat and dirty print. With P.O.P. the surface will assume a metallic lustre, and when printed will be difficult to tone. Self-toning papers appear to discolour more quickly than other papers, but frequently this defect disappears in fixing.