GLASS PAPER or Cloth is made by powdering glass more or less finely, and sprinkling it over paper or calico still wet with a coating of thin glue. When it is dry the powdered glass adheres firmly, and can then be used for polishing woodwork, etc.
GLASS PLATES.óGlass plays such an important part in photographic operations that a few words with regards to its choice must be given. The different methods of its preparation have been given under Glass. It is usually used in sheets or plates. Patent plate is often recom mended. It is, however, very expensive, and it must be remembered that this kind of glass, although perfectly flat and having a good polish, does not possess its proper hard surface. This has been ground off in the process of manufacture, and an artificial surface put on. The plate having been denuded of its hard outer skin, will not easily resist the action of acids and chemicals. It has also been found that this glass absorbs impurities. If it be packed with pieces of printed paper between the sheets the glass will often absorb the ink from the type, and a permanent impression, in some cases irremovable by acids, is fixed on the glass, and will reproduce itself on the photographic positives printed from the negative made on the glass.
The difficulty to be experienced with other kinds of glass is their want of flatness, due to the manner in which they are manufactured. Sheet glass, for instance, is true one way, but slightly curved another. For small sized plates this curvature is so slight that it does not make any
difference, and the glass has the advantage of a hard surface.
Crown glass usually possesses a double curvature, and its employment for large-sized nega tives is to be avoided, because of its liability to crack in the pressure frame. Flatted crown glass, if perfectly flat, is the best kind to use, although a good quality is nearly as costly as patent plate. It has a hard surface, and is, for this reason, superior to plate. It must be noted, however, that only one surface is perfectly flat and smooth, as it is heated and one side flattened on a plain surface, the other becoming by this process slightly irregular.
A good method of testing the evenness of the glass is to hold it in a slanting direction from the eye, and examine the reflection of the sides of the room or any straight lines. If these do not appear quite straight the glass is not perfectly true.
The most important sizes of photographic plates of glass are the following: Air and light both act upon glass, probably by their oxidizing properties. Flint glass, which contains lead, is acted upon by sulphuretted hydrogen, the surface becoming opaque and iridescent. Lenses should therefore, be carefully preserved from the action of sulphur.