An essential condition for the success of the operation is the absolute cleanliness of the support, which must therefore be thoroughly washed in plenty of water whenever seen to be dirty. The cleaned and dried sheets of plate glass are polished with talc powder, the excess of powder being removed. Ferrotype plates are polished with a flannel on which a few drops of a thin polish have been poured, and then finished off with a dry pad of flannel or chamois leather.
Of late it has become usual to employ a glazing or stripping solution in which prints are soaked for a minute or two immediately before being laid on the glasses. Such a solution con sists usually of purified ox-gall. This evil smelling product can be replaced by its active constituents sold under the name of industrial biliary salts (a mixture of about 75 per cent of glycocholate and 25 per cent of taurocholate of sodium) to be used in aqueous solutions at con centrations of from o.5 to I per cent, or (A. Seyewetz, 1936) by all other substances reducing the surface tension of the water (wetting agents of the textile industry), and especially by solu tions of about i per cent strength of various sulphonated fatty alcohols (Ocenol, Lorol, Igepon, etc.).
Immediately they are Avashed,' the prints are applied face downwards on the polished surface of the plate-glass or the ferrotype plate, which is placed flat on a table. After covering with a piece of cloth or parchment paper which has been previously wetted, bubbles of air and the excess of water are expelled with a soft rubber roller or a squeegee, working from the centre towards the edges. Any excess of moisture is then removed from the back of the print with blotting paper, and the whole allowed to dry by itself. Trays, linen sheets, or sheets of clean paper should be placed so as to receive the prints should they detach themselves spontaneously.
When large size prints are being glazed it happens that the corners, being the first to dry, loosen spontaneously, so that the image has iridescences at the edge of the parts prematurely freed. As glazing plates are placed horizontally
each corner can be weighted with a small piece of metal, or the drying of the corners can be retarded by laying on them pieces of wet blotting paper.
In establishments doing developing and print ing work for amateurs, the glazing machines resemble drying machines in their arrangements. The prints are placed face upwards on an endless belt which applies them against a chromium plated cylinder, heated within and kept rotating slowly.
The prints must not be too wet if they are to be glazed by heat, for the water evaporating between the hot metal and the gelatine coating can leave matt spots. The best results are obtained with machines fitted with a device for polishing the cylinder at each revolution, so as to remove traces of gelatine remaining from previously treated prints, and the deposits of lime left by the evaporation of the Prints glazed at an excessive temperature often lose contrast and undergo a change in colour tending towards yellow. The liner the reduced silver the greater these changes are. These phenomena are sometimes accompanied by the beginning of reticulation.
615. Facing the Prints with Cellophane. The picture surface of a photographic print can be protected by causing to adhere to it a trans parent sheet of cellophane (regenerated cellu lose) of the kind used for packing some food products. A sheet of cellophane is distended by immersion in cold water for at least 5 minutes and is then applied to a glazing plate. A print that has not been hardened is applied to it by its gelatine surface. (In the case of a hardened print the gelatine is softened by immersion for about io minutes in tepid water, of which the warmth must be in proportion to the degree of hardening). The whole is kept under pressure beneath some sheets of blotting paper until the next morning, when it is lifted by a corner and trimmed U. L. Sheldon, 1930. Before removing the print from the glazing plate a cloth may be pasted on the back of the paper.