PROCESSES AND MATERIALS. When a drawing is made on stone or metal with either litho graphic crayon or ink and all other parts of the surface are moistened with water, both the fatty substance and the moisture will penetrate the porous surface. A roller covered with fatty or resinous printing-ink passed over this stone or metal will cause the ink to adhere to the fatty or resinous parts constituting the design, and will be repelled by the moist parts; in consequence, the former only will appear in an impression.
The stone used in lithography is a variety of calcium carbonate of porous texture, known as 'lithographic stone.' The best variety is found at Solenhofen, Bavaria, and almost the entire industry receives its supply from that source. Stone of an inferior quality is quarried in Eng land, France, Italy, Prussia, and Russia, and in later years in Canada and some of the Western States. Lithographic stone varies in color from a blue gray to a yellowish gray, the hest quality being found among the light gray, and sometimes among the dark yellow varietis. The dark blue gray or French stone is very fine in texture, but its color does not contrast sufficiently with that of the design to enable the artist or transferer to judge his work as well as on the light gray stone, It is sawn at the quarries into slabs from three to four inches in thickness, varying in size from 6 X S inches to 44 X 64 inches, and sold at prices ranging from 114 cents to 30 cents per pound, according to size and quality. The larger sizes are extremely scarce when without flaws, such as open veins, streaks of glass, or soft lime spots.
it is partly due to this fact, and partly to the greater expense attached to the handling of stone, that almost since the invention of lithog raphy a substitute has been sought for it. Zinc has been used during the last seventy years or more, in its natural state as well as, later on, with a coating embodying the components of the lithographic stone, under the name of `cale sinter' plates. The use of zinc plates as a sub stitute for stone is, however. confined to the more ordinary grades of work, owing to the inferior and uncertain results obtained. About 1890 John Mullaly experimented with aluminum as a substitute for lithographic stone. His experi ments were finally crowned with success, but it was some time before lithographers could be per suaded to make use of it. In the year 1894 Dr. Otto Strecker, of Mainz, Germany, invented a method now called the 'Strecker process,' for treating aluminum plates with a coating of alu minum salt. This method has come into very general use in Germany, and similar processes are now very considerably used in England, Ger many, France, Russia. the United States, and other countries. Aluminum plates are said to have advantages over stone as to quality of work, and, apart from that, to offer all the advantages re sulting from their uniform quality, their econ omy, flexibility, exemption from breaking, and ease of handling and storage. Designs can be
removed from either stokes or plates by chemical or mechanical means and their surfaces prepared for the reception of new designs about 200 times in the case of a stone four inches thick and of an aluminum plate 29-1000 of an inch thick. Lithographic crayon is composed of beeswax, shellac, tallow, mastic, turpentine, soap, and lampblack. Lithographic 'tusehe' or ink, which is used for drawing with the pen, contains the 'same ingredients with the grease a trifle 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 arabic. 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 soap 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 pen-stipple 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 differs 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—lamphlack and gum arabic 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.