This kind of etching is more of a preparation of the plate for printing than engraving or biting, the object being merely to fill up the pores of the metal with gum, and prevent it receiving printers’ ink from the roller elsewhere than on the lines of the drawing.
The solution most commonly employed for this purpose is a mixture of gum and decoction of nut-galls, in use at the Ordnance Survey Office, Southampton, and given by Sir Henry James in his work on Photo-zincography. It is prepared as follows: Four ounces of Aleppo galls are bruised and steeped in three quarts of cold water for twenty-four hours; the water and galls are then boiled up together, and the decoction strained. The gum-water should be about the consistency of cream. One quart of the decoction of galls is added to three quarts of the gum-water and to the mixture are added about three ounces of phosphoric acid, which is prepared by placing sticks of phosphorus in a loosely corked bottle of water, so that the ends of the sticks may be uncovered. The oxidation of the phosphorus produces phosphoric acid, which dissolves as fast as it is formed. {116}
The etching solution should only just mark a piece of plain zinc.
In Richmond’s Grammar of Lithography the following modifications of this formula are given:
| Decoction of Nutgalls | 3⁄4 | pint. |
| Gum Water as thick as Cream | 1⁄4 | pint. |
| Phosphoric Acid Solution | 3 | drachms. |
Boil one and a quarter ounces of bruised nutgalls in one and a quarter pounds of water till reduced to one-third, strain, and add two drachms of nitric acid and four drops of acetic acid.
Richmond recommends, however, the use of a simple decoction of galls without acid, and gumming in after etching.
Scamoni has the following, by Garnier: Boil about one and a half ounces of bruised gall-nuts in a pint of water till reduced to one-third, filter, and add two drops of nitric acid, and three or four drops of muriatic acid. For very fine work this may be weakened with water. It is applied for about a minute, then washed off, and the plate gummed.
Zinco-typographic Etching.
In biting zinc plates in relief, the acid generally used is nitric of different degrees of strength, according to the nature and state of the work.