Читать книгу Photography in the Studio and in the Field - Edward M. Estabrooke - Страница 6

Before proceeding to wash the glass, each square should have its edges roughened, by means of a file or a coarse stone, or the edges of two pieces of glass may be abraded against each other in such a manner as to remove the sharpness, which is so liable to injure the fingers in the various manipulations.

In the process of cleaning the glass it is not sufficient p11 to wash it with water. Other means are necessary to remove grease, rust and dirt, which would not yield to the influence of water alone, and for this purpose a solution of caustic potash is most generally used.

The glass is immersed in the potash solution, each piece separately, and when the dish is full, it should be left not longer than is necessary for the potash to have its proper action, for when left for a considerable time the solution of potash (if strong) will attack the surface of the glass, to its injury.

When a suitable time has elapsed (which in the case of new glass should not be more than one day or night, and in the case of old negatives not longer than to cause the film to slip off), the glass should be removed and washed with water, after which it should be immersed in the same manner, in a solution composed of four parts of water to one of commercial nitric or sulphuric acid, or the two mixed, as may be the most convenient.

This latter solution removes rust or other metallic blemishes, that have not been affected by the potash, and also effectively neutralizes all traces of the potash that have remained on the surfaces of the glass.

The glass may be permitted to remain in the acid an indefinite time, as it can have no injurious effect on it, as might the potash, and when required for use, it should be carefully washed and immediately albumenized and set up in racks to dry.