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270 THE ADVANCE OF PHOTOGRAPHY
As the collodion film is so extremely delicate when dry, it is often found advisable to protect it by giving it a coat of varnish.
The next process is the preparation of the zinc or copper plate from the negative so obtained.
Zinc plates suitable for this work can be obtained either in large sheets from which plates can be cut as required, or plates the exact size may be purchased if desired.
Both the copper plates and the zinc plates must be quite clean before they are coated. It is usual to use pumice powder for this purpose with the zinc plates, and charcoal or very fine whiting for the copper plates.
Nearly all the line work is done on zinc plates, but much of the half-tone work is done on copper ; again, the coating used for the plates is usualfy one of albumen for line work, while fish glue is substituted for this when the plate is for half-tone work.
Before the plate can be successfully coated it must be given a slight grain in order that the coating solution may flow evenly over its surface. For this purpose the plate is placed in a solution composed of about a pint of water, a half ounce of saturated solution of alum, and about a dram of nitric acid. When removed from this solution, in which it should have been kept about five minutes, the large wooden tray in which the operation is performed being rocked to and fro, the cleaned surface of the plate should have a fine frosted appearance.
After removal from this bath the plate should be carefully rinsed, any impurities sticking to the cleaned surface being removed by means of a little mop of cotton wool.
Suppose now the plate is to be coated with albumen, then for this purpose the whites of eggs are used, and this is made sensitive to light by the addition of a solution in water of ammonium bichromate, to which ammonium