After the stone is etched, it is washed suc cessively with water and turpentine; the de sign is scarcely perceptible, but after repeated moistening and rolling in with ink it reappears and accepts the ink. It is then covered with a weak solution of gum arabic and is ready for printing.
The stone-engraving manner—used exten sively for bank notes and stationery — is based on the same principle, although the mode of procedure differs in every respect. A polished stone is used and its face is prepared or etched with gum-water acid and covered with a dark ground consisting of lampblack in solution with gum-water or albumen. Into this ground the design is engraved or scratched with an en graving needle or a diamond. Linseed oil is then poured over the whole and is absorbed by the stone where the needle has laid it bare. The ground is washed off, the surface moistened and printing ink rubbed in with a tampon or dauber,. the ink adhering only to. the lines of the design, by reason of the oil which they have absorbed.
The greatest achievement of lithography undoubtedly centres in reproduction of oil paintings and aquarelles, commonly known as chromo-lithography. Either the crayon or pen stipple manner alone, or both together, in con nection with other manners, such as °rub tints,* tints,* etc., are brought into the work. Its most simple form is the tint used for crayon drawings, its highest, the reproduction in colors of an oil or water color painting, re quiring a series of drawings on separate stones, one for each of the colors necessary to pro duce the facsimile. To obtain perfect register of the various printings, a key plate is neces sary. A detailed tracing is made of the original by means of an engraving needle, scratching the surface of a sheet of transparent gelatine or celluloid, indicating the boundaries of even the most minute patches of color. These engraved lines are charged with litho graphic ink and then transmitted to stone by pressure. Register marks (usually crossed lines) are then put in the margins. The stone is etched, rolled up with printing ink, and im pressions corresponding to the number of colors or printings required, are pulled. These impressions 'are then powdered with finely ground powder (usually Venetian red) and are called sets,o which are then transmitted to the surface of the stones to be drawn on and furnish a guide for the artist in drawing his color plates. In cases where no key of the
original is made, offsets from the drawing of the black supplied with the necessary register marks answer the same purpose. The number of colors necessary to produce a given result varies largely according to the nature of the original to be reproduced, ranging for com mercial purposes from 3 to 15, although es pecially fine results may require 20 or more.
Photography.— Since its invention pho tography has been more or less allied with the lithographic art. Its early application was the process called photo-lithography, by means of which pen or line work or reprints from type could be inexpensively transferred to stone or plate. The half tone or Meisenbach process was also used to advantage in furnishing key plates for chromo-lithography, but the introduction and development of the offset press, which was made possible through development of rotary printing from plates instead of stone, has greatly enlarged the use of photography, opened entirely new fields and possibilities in the art of color reproduction, and has caused an immense expansion of the lithographic in dustry. One of the earliest photo-lithographic processes is that of Lemercier, patented in 1852, which is based on the sensitive property of asphaltum, discovered by Niepce in 1833. A lithographic stone is coated with a solution of asphaltum in oil of lavender and exposed to light under a half-tone screen negative, the film of which has been turned on the glass to secure the necessary reversed position, effect ing a slow change in the asphaltum. The parts thus affected by the light become insoluble in turpentine, leaving the other parts soluble. Thus a (reversed) positive is rendered on stone, which, owing to the unctuous nature of its composition, can be prepared for printing purposes in the usual manner. The method most commonly used is the albumen process, invented by Poitevin in 1855. In this case the stone is coated with albumen in solution with bichromate of potassium which is much more sensitive to light action than asphaltum; after exposure the stone is immediately covered with printing ink to prevent further action, and washed with water which removes the parts that remain soluble, while the ink adheres only to the parts that have become insoluble.