Lithographic crayon is a composition of soap, wax, oil and lampblack. A No. o crayon is very soft, No. r soft, No. 2 medium, No. 3 hard, No. 4 very hard and No. 5 copal. In warm weather a No. 3 or No. 4 crayon should be used; in cold weather any may be used but it is advisable never to use a No. 5.
Great care should be taken to keep the drawing surface ab solutely clean, free from dust, crumbs, etc. In all cases, the artist should be careful not to touch it; perspiration, which contains grease, will reproduce in smudges, marring the finished print. A piece of paper or light felt should be kept between the drawing surface and the artist's hand and arm.
Generally, drawings transferred soon after be ing finished yield the best results, but drawings protected by a film of gum may be preserved indefinitely. Some printers prefer to transfer to a zinc or aluminum plate rather than to stone, be lieving that the plate reproduces fine tints better. Transferring a drawing on paper to either involves the same method, but a stronger acid solution is used for stone. The drawing is first placed in a "damp book," which consists of damp blotters with a rubber cover, and if the paper tends to wrinkle it is stretched with thumb tacks. It is left between the blotters from 20 to 6o minutes, until the printer believes it ready to transfer. It is then dropped on the stone or plate, already on the press, and left in the position in which it lands ; to move it is to smudge. It is covered with "backing"—a sheet of clean paper and a sheet of compressed fibre-board whose upper surface is greased to further the passage of the scraper over it. In running the stone or plate through the press care should be taken that the scraper runs entirely across the drawing but not to the edge of the stone or plate, which the scraper slipping off under pressure would damage. To facili tate this a margin of at least one inch should be allowed for every stone or plate; and this holds whether the drawing is made direct on these surfaces or transferred to them. The bed of the press is slid forward, normal pressure is applied, the bed is turned through the press, the pressure is released and the bed is returned to its original position. If special transfer paper has been used
it should be soaked off ; ordinary tracing paper should be pulled off without dampening. The plate or stone is then fanned dry and allowed to stand for about 3o minutes.
Etch.—What has happened is that some of the greasy litho graphic crayon has been forced off the paper onto the plate or stone. If the printer is satisfied with the transfer he gives it what is known as an "etch," i.e., a solution of, for a stone, gum arabic and nitric acid, or, for a zinc plate, gum arabic, chromic acid and phosphoric acid, or, for an aluminum plate, gum arabic and phosphoric acid, flowed over the surface with a brush or a piece of absorbent cotton. The gum arabic desensitizes the sur face not drawn on, decreasing any affinity for grease that it may have, and also is so absorbed by the pores of the stone or plate that it serves to "hold the drawing in place"; the acid eats away any invisible particles of grease that may be on the surface, but when in the correct solution (see formulas appended), does not affect the drawing itself, which is protected by the wax in the lithographic crayon. If the printer thinks the transfer poor, he coats the surface with pure gum arabic, washes it off with water, inks it up until the drawing appears, dusts the inked drawing with rosin and then gives it an "etch." For a stone, after the "etch" has dried the surface is washed with clean water, sponged with a pure solution of gum arabic, rubbed down as smooth as possible with a clean rag and fanned dry. For a plate, the acid solution is washed off without being allowed to dry. For both, the crayon is next washed off with tur pentine, leaving the drawing invisible, apparently destroyed; but it is preserved, of course, in the grease that the stone or plate has absorbed. A little asphaltum is rubbed in and fanned dry. The stone or plate is then again washed with water so that when the ink, which contains grease, is applied it will take only on the grease and be repelled by all parts of the surface not drawn on.