17. Red and other coloured Printing Inks, are made from linseed oil, boiled into a varnish, as described in the black ink, with the addition of some dry pigment of the required colour, which is ground up with the varnish, with a stone and muller in the manner of oil paint. Thus, for preparing the bright red printing ink, vermilion is ground up with the varnish, in such quantity as will give the required depth of tint. In like manner for blues, yellows, oranges, greens, &c. the Prussian blue, indigo, orpiment, chrome, red and orange lead, verdigris, and in general the pigments used by house painters, are similarly combined with the varnish.
18. Copperplate Printing Ink is of a somewhat similar kind to that used for type printing, but the oil is less boiled, and the. varnish is in consequence much more fluid ; and instead of lamp-black, they either use the black from Frankfort, (which is said to be the charcoal of vine twigs, prep in a peculiar manner,) or when this cannot be procured, or is too costly, the finest Ivory black they can obtain.
19. Lithographic Printing Ink differs from that used in typography, only in being a much thicker varnish ; and the lamp-black which is used as the colour ing matter is not mixed with it in the mass, but small portions of the varnish are taken from time to time, as it is required, and the lamp-black then only ground up with it for immediate or very early use. See LITHOGRAPHY.
20. Removing Stains of Ink. The stains of ink on cloth, paper, or wood, may be removed by almost all acids ; but those acids are to be preferred which are least likely to injure the texture of the stained substance. The muriatic acid, diluted with five or six times its weight of water, may be applied to the spot, and after a minute or two may be washed off, repeating the application as often as may be found necessary. But the vegetable acids are attended with less risk, and are equally effectual. A solution of the oxalic, citric (acid of lemons), or tartaric acids in water, may be applied to the most delicate fabrics, without any danger of injuring them ; and the same solutions will discharge writing, but not printing ink. Hence they may be employed in cleaning books that have been defaced by writing on the margin, without impairing the text. Lemon-juice, and the juice of sorrel, will also remove ink stains, but not so easily as the concrete acid of lemons, or citric acid.