Half Tone

From Encyclopediak

Half Tone, a process by which engravings, wash drawings and photographs are reproduced on metal plates and made ready for printing. The fundamental principle of the half tone is the diamond-ruled screen introduced into this country by Max Levy. The process consists of taking a photographic negative of the drawing to be reproduced, made through this screen, which has been prepared as follows. A plate of the finest sheet glass is coated with a varnish of asphalt and wax, which is placed on a bed of an automatic ruling machine capable of making lines to a fineness of 500 to an inch. The cutter is a diamond-pointed graver and is gauged to cut any desired width. The lines are cut through the coating into the glass diagonally at an angle of 45, the number depending upon the character of the plate desired, whether for fine book work or for coarse printing. The plate is then treated with hydrofluoric acid, which eats or bites into the exposed lines. Afterwards, these lines are filled with an opaque pigment, and the plate is baked in an oven, which hardens the pigment. The plate is then cleaned and polished smooth, leaving the clear glass transparent. Two of these plates identically ruled and prepared are now cemented together with Canada balsam, being so placed that the lines of each cross at right angles, forming diamond shapes. This screen is placed in the camera in front of the sensitive plate, and the image is broken into dots, whereby the light and shade of the original is reproduced, the dots being so fine as to be unrecognizable. A sensitized plate of copper is exposed under the half-tone negative and is given a cold-water bath, and the portions not printed on are washed out. The picture reproduced on this plate is hardened and then etched afterwards, mounted on wood type-high for printing. The sensitized solution con sists of a compound of fish glue, albumen, chromic acid, bichromate of ammonia and water, and this is flowed over the plate a number of times or spread by having the plate rapidly revolved in a machine made for the purpose, so that the sensitized solution may be evenly distributed. When it is required to reproduce drawings, they should be made two or three times larger than the print desired, because in reducing their size in the camera the lines come out with more distinctness in the half tone than if made originally the same size. A process known as stagmatophy has recently been devised that does away with the ruled screen and produces a picture free from the dotted or lined appearance common to most half tones. In this process a plate of copper or zinc is coated with a gum-gelatin emulsion, made sensitive to light by the addition of potassium bichromate. The emulsion is made by mixing solutions of gum arabic and gelatin. The gum arable does not dissolve in the gelatin, but is distributed through the solution in minute globules, which come to rest at approximate equal distances from each other. The dry plate is exposed under an ordinary negative and etched at once. The globules of gum arabic form the necessary granulation for the half tone, and by this process it is claimed that a much more satisfactory picture can be obtained than by the ordinary method. See PHOTOGRAVURE ETCHING.