(or Quadricolor) Since some color values are lost in three-color reproduction by photography, it has been cus tomary with many to produce a fourth plate, called a key-plate, in which the main design is emphasized. This key-plate is printed either in gray or photo-brown ink, and is usually printed the last of the series. Otherwise the process is similar to tricolor printing.
Presswork in is a great dearth of literature on this branch of color printing. Experience has demonstrated that ordinary printers handling black work do not secure satisfactory results when handling the printing of three and four-color plates, hence this work is done mainly in printeries devoted wholly to color work, developing into a separate branch of the printing art. One of the great difficulties of the color printer arises from the uncertainties of the paper surface on which he prints. Rough paper will not yield good results, and to secure the smoothest surface highly coated papers have come into use for color printing. If the coating is a little too heavy or too brittle, it tends to flake off, and minute specks are pulled off the paper in printing, mix ing with the ink, clogging the inking rollers, and producing no end of annoyance. The great variations in humidity in American atmospheres invite stretching and shrinking of paper, and when a sheet either stretches or shrinks between the times of printing any of the colors, of course the colors will not strike exactly as they should, and exact superposition of tints becomes impossible. To avoid danger of shrinkage it is
common to print the most particular color work in sheets or forms of small size; but in the ef fort to keep down the cost of printing large forms are desired, hence a good deal of three and four-color printing has been done on large sheets with more or less uncertainties of regis ter. By improved methods of heating and producing uniform high heat in the pressroom these irregularities have been reduced. The modern color printer seeks to avoid loss from shrinkage by maintaining a high and uniform heat in the pressroom— 75 to 80 degrees, and by keeping the piles of paper protected from dampness by coverings. It is found hest to print the colors in this order: yellow, red, blue and lastly the key-plate. In making ready for the yellow plate, a flat printing was at first employed, but it was learned that color values were thus lost, and the best practice now is to start the printing of the yellow in a dark ink, so that the pressman can see the design clearly, and bring up all parts of it sharply, before going ahead with the regulation yellow, which is hard to see when alone on a white sheet. The second color should not follow until the first is dry, and it is best to run the sheets through the press in the same order as on the first printing, rather than to reverse the order. See COLOR Punroo StAPHY ; ENGRAVING ; PHOTOGRAPHY.