Page:De Vinne, Invention of Printing (1876).djvu/30

20 on copper a sharpness of line, a brilliancy of color, a delicacy of tone, and a receding in perspective, which have always won for this branch of printing the preference of artists. Yet it is a slow and expensive process. A steel-plate engraver may be engaged for many months upon a large plate, from which but forty perfect impressions can be taken in a day. On ordinary work on a large plate, three hundred impressions per day is the average performance of a copper-plate press.

Steel and copper-plate printing is largely used for bank-notes, portraits, fine book illustrations, revenue and postage stamps, and sometimes for commercial formularies, but it is in every way unfitted for the printing of books. It has not been much improved since its invention. Steel plates may be duplicated by means of electrotyping, or by the process of transfer to soft steel, but these duplicates cannot be made so cheaply as typographic stereotype plates, nor so promptly as transfers by lithography. The inking and cleansing of the plate, always dirty and disagreeable work, has hitherto been done only by hand. All the manipulations of copper-plate work are slow and difficult: they present many obstacles to the use of labor-saving machinery.

In lithography the design to be printed, which may be engraved on stone or copper, or written with pen on paper, is transferred by a greasy ink upon the smooth surface of a stone of peculiar fineness and firmness. This stone, which is found in its best state only in Bavaria, where the art was invented, is a variety of slate, which faithfully responds in printing to the slightest touch of a graver or a crayon, and permits the use of fine shades and tints which cannot be produced on wood or on copper. The transferred lines of the design cling to and dry upon the surface of the stone, which is then subjected to the action of a weak acid, which hardens the ink in the transferred lines, while it slightly etches and lowers the surface where it is unprotected. The process of printing begins by dampening the stone with a moist sponge, the water in which is absorbed by the