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BOOK ILLUSTRATIONS 279
to the highest lights, and must be by no means hard ; also all negatives must be reversed.
In the preparation of the prepared glass plate an oven must be used in which the film can be dried while the plate is kept in a level position, and also kept absolutely free from dust particles. It is the practice to give the plate glass a preliminary coating of albumen or some similar substance in order to hold the sensitive film down on the plate during the printing operations. Before putting on the sensitive coating of bichromated gelatine the plate is placed in a horizontal position, warmed to a temperature just over 100° F. After coating with the albumen it is again dried, artificial heat being used, the temperature for this operation being varied to suit the particular kind of gelatine employed
The plates are ready for use as soon as the gelatine is dry, but, on the other hand, with care they may be kept well over a week without undergoing any deterioration. Of course the drying must be done in the dark room, and if kept the plates must be left in the dark.
The collotype plate itself is to be used in printing, hence great care has to be taken in order that only that part of the gelatine which is necessary to form the picture remains on the plate. For this reason, previous to the exposure the negative must be masked in such a way that no light will penetrate to the gelatine through the negative except it goes through the picture. The printing-frame for collotype plates must be of the strong heavy kind already described. A paper mask is first laid on the thick plate glass, then the negative carefully placed on the mask, and lastly, the collotype plate placed upon the negative and the frame firmly closed.
It is generally best to allow the exposure to take place in diffused light. When the exposure is complete, the image will appear on the plate as slightly brown upon a yellow ground.