Page:Popular Science Monthly Volume 46.djvu/626

608 be printed. And not only does an impression from plate excel in delicacy but also in force and depth of color.

There never was and never will be a woodcut line having the power of a deep line in a plate, for, in an impression from wood, the print is only a blackened surface of paper, whereas that from plate is a cast with an additional thickness of ink, for the damp paper is forced into the line and brings out the ink upon its embossed surface.

Plate ink is soft and thin as compared with that used for surface prints, and the body of it is ground carbon mixed with oil for black; and colored ink is made from white lead mixed with dry colors and very finely ground in oil. Some inks are much more wearing on the plates than others, green being about the worst in this respect.

Retouching, or "re-entering," as it is called in the trade, is done by re-entering the roll upon the lines of the transferred work and putting the pressure on, as in the original transfer. This sharpens and restores the line, making it print as good as when new.

The very best linen fiber paper is used in printing bank notes, bonds, stock certificates, etc., and it is dampened before printing to make it more pliable, and it also takes the ink better in this condition. After printing the sheets are placed on racks in a drying room heated by steam. When they are thoroughly dry they are found wrinkled and curled, so they are placed between Bristol boards and put under a hydraulic pressure of several hundred tons and kept there for a few hours, then taken out ready for trimming, numbering, and shipping. Each time the impressions are handled they are counted and kept track of, good or bad, so there shall be no possibility of loss or theft.

There are some very delicate machines used in bank-note work, known as ruling machines and cycloidal and geometric lathes. The straight and curved line ruling machine is used in making the background of portraits and vignettes, shade and ruled faces of letters, background of panels, and is capable of ruling three thousand six hundred lines to the inch with great perfection and regularity, but is seldom set as fine as this.

The cycloidal ruling machine is more complicated than the plain line ruler, having from one to four "eccentrics," or "cams," in connection with the forward and back movement of the bed, and is used for producing fine tints to print over other colors and work. The principle of its operation is readily understood. A diamond point is arranged in the machine and given a circular motion by the action of the "cams." Now, while the point is revolving, let a forward movement be given to the plate and the line traced by the point will assume a form like this (Fig. 7),