PREPARING THE PLATE.
Sheets of plate-glass, from in. to -c; in. thick, and several inches larger each way than the required picture, are necessary. These must be ground with fine emery on one side. A simple way of doing this is to place two plates to gether, with moistened emery powder between, and work them over each other, renewing the emery from time to time, until the glass is sufficiently ground. If the grinding is excessively fine, the film will not adhere well, but the grinding Must on no account be coarse. The plates are next scrubbed with a brush and clean water to remove the emery, and stood up to dry. If metal plates are to be used, zinc is perhaps to be preferred. Level sheets about in. thick may be obtained from any photomechanical dealer. They should be thoroughly cleaned from every trace of grease by means of pumice pow der and water, and then roughened or grained in a nitric acid bath as follows : Nitric acid Alum (sat. soh.) ozs.
Water 40 ozs.
The dish should be rocked till the entire surface of the metal is of an even grey lint. The plate is then washed under the tap, and any sediment or scum removed with a tuft of cotton-wool. Boiling water is next poured over the plate, which, when the surplus moisture has been re moved with a thoroughly clean wash leather, is stood up to dry. Copper plates are ground with emery powder similarly to glass ; for aluminium, a weak solution of sulphuric acid or a strong alkali is used. Although, .as already men tioned, metal plates require no substra tum, and are not easily broken, it is a disadvantage that the printing cannot be examined through the back, as with glass.