Phosphoric acid may be used for etching as well as for preparing the stone; it exerts a very faint etching action and is seldom used for this purpose, but principally for making corrections.
Acetic acid plays a very important part in lithography. It possesses the property, even in very dilute condition, of stopping the action of the gum and oxalic acid, and, indeed, even that of the stronger nitric, sulphuric, and hydrochloric acids. It finds, therefore, considerable use in negative printing, in making corrections, etc. It converts the stone into its original condition and makes it suitable again for taking ink.
Citric acid fulfils the same purpose, but is, however, much higher in price.
Oxalic acid is not used for etching, but only for preparation of the stone. By rubbing the stone with a solution of oxalic acid with a pad of cloth it takes a mirror-like surface or polish, into which the fatty ink cannot penetrate. It is used for preparing the stone for lithogravure, negative drawing, and negative transfer, etc.
Tincture of galls, or instead of this, what is simpler, gallic acid, is used in lithography. It contains plenty of tannin, and is one of the most efficacious means of preventing the combination of the stone or zinc plate with fatty substances.
Gum arabic or some other vegetable gum is indispensable for litho and zincography. A 10 per cent. solution of gum is generally used. It penetrates all the pores of the stone and the plate, and gives the same a thin but very firm film, and prevents the ink taking on these places. If it has become slightly acid it acts as a weak etcher, and as an addition to nitric acid it makes the same of thicker consistence and more easily distributable. The action of gum can be removed with acetic or citric acid; with water alone this cannot be perfectly effected.
7.—Etching the stone.
The ordinary etching chiefly causes the fatty substances to assume a new chemical condition, since in combination with the stone they form insoluble fatty salts of lime, which represent the printing surfaces. It has, however, also the property of cleaning the blank stone surfaces from any adherent grease, and makes the blank parts of the stone incapable of taking ink; also it makes the drawing clearer and more precise.
The strength of the first etching may differ, according to the following circumstances:—
(a.) The stone; for hard stones stand a stronger, and soft stones a weaker etching. {11}