It is then damped over with a sponge and inked up with a roller. It is advisable, finally, to dust the stone or plate with rosin dust, to clean it up, polishing out all dirt, and to repeat the process, after which it is ready for printing.
Drawings made direct on a stone or plate are given an "etch," and then prepared for printing in the same way. Lithographic engraving, or intaglio printing, is done with a needle on stone desensitized by a solution of gum arabic and nitric acid. Such engravings are inked with a dabber that pushes the ink into the engraved lines; otherwise they are treated like a lithogtaph drawn direct on stone.
Lithographic ink is made from the best dry colour and linseed oil. It is generally sold stiff so that the printer can soften it with varnish to the consistency demanded by his work ing conditions and the results that he wants. It ig available in various colours, but, no matter what colour or combination of colours is wanted in the print, all drawings are made in black, one stone being prepared for each colour desired. It is applied by means of a roller made of wood covered with grain leather; during the operation, difficult because the ink must be applied evenly until judged strong enough, the stone or the plate rests on the bed of the press in order that an impression can be made at once.
The pressure used in printing a lithograph is sliding pressure. The scraper, which is made of box or apple wood sharpened to a blunt edge and covered with well greased leather, is set by a screw. Hand-made paper is generally used for lithographic prints and the best grades come from Italy, Sweden, France and Japan; machine-made paper, however, gives good results in practice. The paper is generally moistened to give it a greater af finity for ink and to enable it to print very fine tints in full value. It is then placed on top of the plate or stone as it rests in the centre of the press bed, and is covered with backing. The pressure is applied and, the same precautions being taken as in transferring, the stone is cranked through the press; the pressure is released and the bed of the press is returned to its original position. The first prints pulled have a tendency to be light ; the proper strength is built up gradually. If, after a few prints have been pulled, the drawing has a tendency to grow too dark it is advisable to repeat the "etching" process; if the stone is in colour, before being given an "etch" it is washed with tur pentine, rolled up with black ink and dusted with rosin. As many
prints as are wanted can be had, but, when taken from the press, the stone or plate, to keep the drawing in place, should be rolled up with ink, gummed down with gum arabic and rubbed down very smoothly with a clean rag.
The proportions of gum arabic and acid used in the solution known as the "etch" vary with the quality of the stone or plate, the type and age of the drawing, and the temperature of the room. Only the following approximate proportions can be given: For a stone, one half glass of gum arabic to one tea spoonful of nitric acid. For a zinc plate, one half glass of gum arabic to one teaspoonful of chromic acid to from 12 to 15 drops of phosphoric acid. For an aluminum plate, one half glass of gum arabic to from 15 to 20 drops of phosphoric acid. (See also The history of the invention of lithography leaves no chance for ingenious theory and learned speculation. Art authorities may argue over the origin of etching and wood-engraving, but the in ventor of lithography, Alois Senef elder
himself gave the facts of his invention in A Complete Course of Lithography— Der Steindruck (I8i8)—a book which is half autobiographical, half technical, and wholly absorbing. He was a Bavarian actor and dramatist. Not over successful, he endeavoured to be his own printer and publisher as well and, to that end, experimented with metal plates and Solenhof en stone, acids and inks. According to his often-quoted story, his mother asked him, one day when he was at work, to write her week's washing list and, paper not being handy, he wrote it with his greasy ink on a smooth stone upon which he practised writing in reverse. Later, instead of cleaning off the writing, he was curious to see what would happen if he etched the stone and pulled a proof. What happened was his discovery of relief engraving on stone (1796). But only after "thousands of experiments" did he make the further, the all important discovery (1798) that without etching the writing or design in relief, he could prepare the stone so as to get a print from the surface. This is lithography-chemical surface printing.