The same general principles apply where plate instead of stone is used, with variations of solutions and methods for the preparation of the plate, the developing and reversing of the design.
The perfection reached in printing half-tone work on the offset press has made possible the faithful reproduction of almost any color sub ject, through the use of color sensitive or col lodion emulsion photographic plates, usually three plates, one for each of the primary colors, yellow, red and blue, which when prop erly developed give color-separated negatives which are then printed on the plate in the usual manner. For variations of the primary colors the plates are either over or under printed and after development are carefully worked over by the artist adding to or taking from the work as desired.
Transferring.— Unless limited editions are required, designs that have been drawn or en graved on stone are very rarely printed from directly. By the process of transferring, one design may be reproduced as many times as the relative size of the work and sheet to be printed will allow — ensuring greater economy in print ing and keeping intact the original drawing stone. Transfer impressions are taken on a paper, coated with a sizing of starch and gly cerine with a specially prepared ink. These impressions are laid in their proper positions on a sheet of paper of the required size and fastened to it by means of pricking with the dull point of an engraving needle. The sheet to which the transfers adhere is laid face down on a polished stone or plate and pulled through a hand press until the transfers adhere firmly to the stone, the paper being frequently dam pened during this process. This done, the i sheet is carefully raised from the stone, leaving the thin paper transfers still adhering to its surface and these in turn are dampened and removed. A weak solution of gum and water is applied and the stone rolled up with printing ink. It is then etched in the same manner as an original drawing and is ready for the press. In chromo-lithography the first transfer made is that of the key plate, supplied with register marks. An impression from this transfer is mounted on a sheet of aluminum or zinc and coated with shellac to prevent shrinking or stretching.. All succeeding transfers are °stuck up" or fixed on this key sheet, which ensures accurate register of the colors.
Lithographic Printing.— The lithographic hand-press consists of a frame provided with rollers on which the bed runs to and fro, the scraper or impression-bar and a tympan of leather, fitted to an iron frame hinged to that end of the bed nearest the scraper. The scraper consists of a flat piece of box wood beveled on both sides and covered with leather. The manner of printing is as follows: The stone with the design upon it is placed face up upon the movable bed, then moistened with a sponge, rolled up with ink and the sheet which is to take the impression laid thereon. Several sheets of paper are placed over it to secure the necessary backing, the tympan lowered upon it to cover the entire bed and stone, and the carriage brought forward under the scraper. Pressure is applied by means of a hand-lever at the side of the press, and the entire length of the bed passed under the im pression bar. The pressure is then released, the bed brought back to its original position, the tympan raised and the printed sheet taken off. Until the introduction of the power-press
in 1867 this method of printing was exclusively used for lithographic printing. At the present time its only utility is for the purpose ing artist-proofs and transfers for printing on power-presses. The lithographic flat bed power-press differs but slightly from that of the typographic power-press. The principle of operation remains the same. taking in place of the letter-press form a lithographic stone in its bed and being supplied with a contrivance for distributing the moisture with which the stone must be covered before the ink is ap plied. The use of rotary printing presses, with an increased speed of 50 per cent over the flat-bed presses, has been made possible by the substitution of the flexible aluminum plate for lithographic stone, and has enlarged the field of multi-color printing by the lithographic process, whereby two or more colors are printed in rapid succession before the sheet is delivered. The rotary press consists prin cipally of two cylinders — one to hold the plate and the other to furnish the pressure— and is supplied with a mechanism similar to that of the flat-bed press for distribution of moisture and ink Rotary presses having two, three and four plate cylinders and printing re spectively two, three and four colors each time a sheet is fed are in general use. The presses described above all print by direct con tact from the stone or plate to the paper, whereas the offset press, a very important de velopment in modern lithography, prints from the plate to a rubber blanketed cylinder, which in turn prints on the sheet. These presses are built on the same general principle as the rotary press but having three cylinders, one to carry the printing plate, another to carry the ruhher blanket and a third to carry the sheet which takes its impression from the rubber blanket. Provided with automatic feeding ap paratus, a production double the rotary presses is possible. They produce wonderfully soft effects on rough or uncoated papers being in fact the only method whereby half-tone color subjects can be printed on other than the highly glazed or coated papers.
Bibliography.— Bergman,
Revised by FREDERICK RAUSCH, Of Sackett and Withelms Corporation, New York.