Page:The New International Encyclopædia 1st ed. v. 12.djvu/375

* LITHOGBAPHY. 333 LITHOGRAPHT. shellac, tallow, mastic, turijcntine, soap, and lampblack. Lithographic 'tusche' or ink, which is used for drawing with the pen, contains the "same ingredients with the grease a trille more predominant, in a liquid state. When a drawing, which necessarily is made re- versed, is completed on stone, its surface is bathed with a solution of acid and gum arable. The object of this treatment, called "etching,' is to prepare the surface of the stone having no design for the better retention of water; to clear the surface from any imperceptible fatty parti- cles arising from contact with fingers, etc. ; and, finally, to render the drawing insoluble in water, by decomposing the alkali contained in the soa]) which is one of the bases of its composition. After etching, the stone is thoroughly washed, first with water, then with turpentine, which removes every visible trace of the drawing, leav- ing only the fatty substance of the crayon or ink on the surface. After being repeatedly moist- ened and rolled in with printing-ink it is ready for printing. Of the various processes employed in drawing on stone, the crayon process is without question the most important. For this purpose the stone is grained. This is done with sand and water between two slabs of stone, which are rubbed together with a circular motion until the lower one has the grain desired — either fine or coarse, according to the nature of the work to be per- formed. Next in importance is the penstipple process. For this purpose the stone is polished with pumice. Stippling consists of drawing with small dots, the values in shading being obtained by the relative density of these dots. A mode of procedure which diff'ers widely from both crayon and pen-stipple manner is engraving, though based on the same principle. In this case the stone is finely polished and etched and then covered with a dark ground — lampblack and gum arable dissolved in water. The drawing or writing is scraped into the ground with an en- graving-needle or a scraper, laying bare the sur- face of the stone. When the design is finished, the stone is covered with linseed oil, the ground washed off. the surface moistened, and printing- ink is applied by means of a dauber, which then adheres only to the lines of the design, by reason of the oil they contain. Cheomolithogbaphy. In this either the crayon process, the pen-stipple, or a combination of both, is employed. The important feature of this branch of lithography is to obtain a perfect register of the colors to be superposed. To ac- complish this, for higher class color work, a key is made by tracing with an engraving-needle on gelatin a complete outline of the original, fol- lowing even the most minute patches of color. The lines thus engraved on gelatin are filled in with lithographic ink and transmitted to stone by pressure in a hand press. Register marks of thin crossed lines are then added in the margin of top, bottom, and sides, which become the guides for accurate register in printing. The design is then 'etched' and rolled up with printing-ink and impressions to the number of colors required are made therefrom. These so-called 'offset' or 'key' impressions are in turn transmitted to stone by pressure, having first been powdered with a finely ground dry color, which, owing to its fatty quali- ties, adheres only to the line work. Wliere a drawing-stone is used in place of the key, offset impressions of the same, supplied with the regig- ter-marks, accomplish the same purpose. The number of colors used varies according to the quality of the work desired. For commercial purjjoses the number ranges from three to fifteen, and in some cases, where especial results arc de- sired, as many as twenty and thirty colors are used. For monochromes four tinUs are usually employed : one for the main drawing, a light ground tint, a medium tint for modeling, and finally a dark tint for accents. PHOTOLiinoGB.^Puy. In later years photog- raphy has been made a valuable adjunct to lithography. One of its applications is photo- lithograpiiy, by means of which pen drawings are faithfully reproduced at comparatively small ex- pense. Another result is the application of the half-tone or Jleisenbacli process to lithography, for monochromes, and chromolithographs, in which ease a photographic reproduction from the original on stone forms the basis for the several color plates, taking the place of the key, subject to the manipulation of the expert chromolithog- raphcr. Of the several methods in use to achieve a photographic reproduction on stone, the oldest one, invented by Xiepce in 183.3, is based on the property of asplialtimi dissolved in eil of laven- der to become insoluble when exposed to actinic action. Thus when a half-tone negative is su|)cr- imposcd upon a stone coated with an asphalt so- lution, in perfect contact, and exposed to the sun's rays, the asphalt solution will only be affected by such light as can pass through the negative. Dissolving the parts not affected in this manner, by means of turpentine, will pro- duce a positive on stone, which, having in its com- position the necessary ingredient, grease, is pre- pared for printing purposes in very much the same manner as drawing on stone. The method most generally employed, however, is the albumen process. Albumen in solution with bichromate of potassium has the same properties as the asphaltum solution. It is more sensitive to the chemical action of light, achiev- ing the results desired in a much shorter space of time. After exposure the .stone is immediately covered with printing-ink to prevent further ac- tion. The parts that remain soluble are washed away with water, the ink adhering only to the parts that have become insoluble, thus creating the conditions essential for lithographic purposes. The three-color process is also applied to some extent in lithography, reproducing on stone the three negatives necessary for this process by cither of the methods described,. other photo- lithographic process, which, though still in its ex- perimental .stage, deserves mention, is the grain I)rocess. The object in view is to substitute for the geometrical pattern characteristic of half- tone work, a somewhat irregular grain finish which more closely resembles the results of a crayon drawing on "stone. Some very fair results have been achieved in this direction. Tr. "sfeb Process. The rapid development of the lithograpliic industry led to the universal adoption of the transfer process, by which the same dcsisu can be reproduced as many times at one impression as the size of the sheet to be printed will admit. Impressions from the orig- inal stone are made with transfer ink (contain- ing practically the same ingredients, as litho- graphic ink) on paper coated with a sizing of