Zincographic processes differ essentially in the preparation of the plates. In the one, the plate is given a coating which has a similar composition to a litho stone; in the other, the plate is roughened and given a fine grain by a mechanical method, such as the sandblast, in order that it may be then oxidized by a chemical process, with acids or alkalies. All these processes are more or less advantageous. The zinc plates cost about one-tenth to one-twentieth of the cost of the stones, and this may well be sufficient grounds for the proprietors of large establishments being interested in zinc plates, as the stones are rather costly.
Excellent zinc plates, coated with a film or merely oxidized, can be procured commercially in Germany and Austria. For those {6} who wish to prepare their own plates the following tested formulæ are given:—
A zinc plate from 1 to 1·5 mm. thick is roughened by means of the sandblast till it has a fine, even grain and appears smooth, but clean. The following solution is prepared:—
| Concentrated solution of alum | 20 | parts |
| Phosphoric acid | 2 | parts |
| Water | 20 | parts |
or
| Alum solution | 10 | parts |
| Gallic acid | 15 | parts |
| Nitric acid | 2 | parts |
| Water | 30 | parts |
The roughened plate is first flowed over with water, and immediately afterwards, with one of the above solutions, in the same way as a plate, is covered with a developer. The plate is oxidized by this, and there is formed on the surface a white precipitate. It is then washed thoroughly under a rose tap and dried, and the plate may be either coated with a light sensitive substance for printing or a transfer be made on to it.
After the development or the transfer, the plate is gummed and then etched with an etching solution consisting of—
| Gallic acid | 10 | parts |
| Phosphoric acid | 2 | parts |
| Gum solution | 10 | parts |
| Water | 30 | parts |
this being allowed to act for 30 or 40 seconds.