The photo-printing department is equipped with a mechanically operated whirler and vacuum printing frame, as well as a heating apparatus for the burning-in of the enamel image with which the polished copper has been coated.
The etching department is equipped with machines in which the plates are etched by hand, by mechanical rockers, by acid, spray, blast, or by electrolysis.
The routing department is equipped with routing machines, circular and jig-saws, planing, squaring, and mounting machines. Plates to be mounted on metal are usually sent to the electrotyper for that purpose.
The engraving room has heating and cooling devices, also small trays containing acid for re-etching small surfaces. Proofing presses range from the old-fashioned hand-press and special en graver's small power presses to large cylinder machines.
The negative is next placed with its film side to the coated zinc plate and put into a screw or vacuum printing frame with a plate glass front, in close contact. The printing frame is then exposed to light (usually electric arc), the time of exposure varying ac cording to the density of the negative and the intensity of the light. The plate, after being slightly warmed, is rolled up with a greasy ink.
It is then placed in a tray of clean water and the surface gently rubbed over with absorbent cotton. Where the albumen coating has not been hardened by the action of light, it washes away, leaving those parts which have become insoluble by exposure to light on the plate.
The plate is dried by heat and defective lines retouched. The plate then powdered with a finely ground topping powder which sticks to the ink providing an effective resist to the acid. The surplus resin is removed and the plate heated to melt the resin.
When the plate is cool, it is ready for etching, by placing in a diluted bath of nitric acid for the first etch; but before doing so, the margins and back of the plate are coated with a varnish which prevents the acid from etching away those parts which are to be left unetched.
The acid is agitated, either by rocking the bath by hand or by machines now generally used for this purpose, in which the acid is sprayed by means of air or mechanical action on to the sur face of the plate. When the plate has been a short time in the
bath, it is removed, washed and dried by heat, then powdered with "dragon's blood," applied in four directions so that all sides of the lines will be protected from the acid which is apt to "undercut" the lines. The plate is again heated, causing the resinous powder to melt and run down the sides of the etched lines, until the necessary depth is obtained.
A more recent practice which saves much time is flowing a zinc plate with a so-called "cold enamel" solution, eliminating the rolling up with ink. The plate is developed in alcohol and is then ready for etching.
The line process is occasionally used for colour work. The colours are generally made up of a number of "solid tints" or "stipple," the usual method being to take "key prints" from the etched "key plate," "stopping out" or painting in those parts re quired for the particular colour in which it will be printed. Where stipple or lines are to be introduced, shading mediums ("benday") are usually employed. These consist of gelatin or celluloid sheets with the design of the particular grain, dot, line or stipple in relief.
The film is rolled up with transfer ink and placed face down wards on the plate, which has been gummed over on those parts which are not required, pressure being applied by a roller or squeegee to those parts of the key plate which are to be printed in the particular colour.
Each colour requires a separate plate, which is rolled up, pow dered and etched in the usual way.
Halftone Plates.—Halftone engraving is the process by which full-tone subjects in monochrome and colour are reproduced. The negative is made in the usual way, but to provide the necessary reticulated surface the exposure is made through a screen. This may be composed of grains (mezzograph), or what is most gen eral, a series of lines running across each other at right angles. The cross-line screen consists of two sheets of thin plate glass, ruled with diagonal lines of mechanical exactitude. These are engraved into the glass, each piece of glass being ruled at right angles to the other. The lines are filled with a black pigment, leaving transparent spaces between. The two sheets of glass are placed face to face and sealed, the black lines crossing each other and leaving square transparent openings through which the light passes forming a halftone or dot image on the photo graphic plate or film.