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BOOK ILLUSTRATIONS 271
hydrate has been added drop by drop until the whole turns a bright yellow colour. When this prepared albumen is poured over the plate, the plate must be whirled so as to get an even layer, and it is usual to coat and whirl the plate twice. The coating is dried by whirling it over a not too hot flame or stove. It may be taken for granted that it is not being heated too much if it can be held in the hand. The film on the plate is now sensitive to light, hence the operations must be performed in a yellow light.
The printing-frames used for exposing these prepared zinc and copper plates to light under the negative have to be of an especially strong construction, for it is absolutely essential that the prepared plate and the film of the negative should be in intimate contact. A printing-frame of this description is shown in fig. 110.
A thick piece of plate glass is first placed in such a frame, then the negative from which it is desired to obtain a print on the prepared metal, then the metal sheet, and lastly a backing of paper, the whole being finally firmly screwed together. The time required for an exposure will of course depend upon the light, the negative, and the nature of the subject, i.e. whether line or half-tone ; and it may vary from a minute or two up to half an hour.
When the print on albumenized metal comes out of the frame, it is just possible to distinguish a very faint outline of the image. It is at once rolled up with some special transfer ink which has been thinned down with turpentine, and when a uniform coating is obtained it is dried.
The next operation is the development of the image ; this is done by placing the plate into a dish containing cold water and working upon the prepared surface with a small mop of cotton wool. By this means all the parts which have been unacted upon by light will be