Having referred in the preceding chapter to the principal Lithographic agents—stones, ink, and water, as materials required in lithographic drawing and writing, we now proceed to notice the chief materials used in printing.

11. By Varnish, in Lithography, is understood the vehicle in which pigments are ground to form the printer's ink. It is made by subjecting the best linseed-oil to the continued influence of heat, until it becomes more or less thick and viscid. The heat must be raised until the oil will take fire, and must be kept at that heat until the varnish is brought to the proper consistency. The operation is very dangerous, inasmuch as the flame from the burning oil will sometimes reach a great height, even though the quantity of oil be only a quart or two, and, for that reason, every precaution should be taken in its manufacture.

One reason for making varnish for one's self would be to obtain an article known to be pure, for comparison with that which may be bought, as it is possible to thicken the varnish with resin (as is done with varnish for letter-press ink), instead of producing the viscidity by burning only.

As varnish is an article manufactured on a large scale, there is no difficulty in purchasing it of a quality to answer the lithographer's purpose. It is made of several degrees of strength, known in the trade by the terms thin, tinting, medium, and thick.