918. Collotype. In the collotype process the printing surface is usually prepared on a thick glass plate which has received first a substratum of silicate and then a coating of bichromated gelatine. The plate is then baked in an oven, the coating becoming grained or reticulated. The various points of each grain thus acquire different degrees of sensitiveness, allowing of the tones of an original being rendered as a more or less continuous grain, as may readily be seen by examining a collotype print with a magnify ing lens.
Printing on the sensitive plate is done from a reversed film negative, usually prepared by stripping the film of the original negative from its glass, transferring it to a flexible support, and trimming and retouching.
After exposure to light, the plate is washed out with water to remove excess of bichromate and soaked in diluted glycerine (the so-called etching bath), which keeps the parts not hardened by the action of light in a moist and swollen condition ; the hardened parts remain dry, and thus retain the greasy ink, which is transferred to the paper in the subsequent printing.
A collotype plate made by a highly-skilled operator will allow of 1,000 impressions being printed ; the printing of a larger edition requires several plates.
919. Whenever possible, the collotype printer should have the original negative of the subject to work from, or, failing that, a positive trims parency of first-rate quality, preferably made on a stripping plate. With each such plate is sent a print marked to show the part of the subject to be included ; if the collotypes are to have a white margin this print should be mounted on paper of the size of the reproduc tion so as to show the width, etc., of the margins. The print should also bear the wording for the titles or other lettering, which, as a rule, is printed from type ; and it is also advisable to settle at the same time the quality of paper and colour of ink to be used by the collotype printer.
920. Photo-lithography. In this process of reproduction, printing from an inked metal 1 plate is done either directly or indirectly. In the former process the image on a zinc or aluminium plate is inked for printing directly on to the paper. In the indirect or offset process, now increasingly used for large editions, the ink is taken from the metal plate on to a cylinder provided with a rubber covering, from which it is in turn transferred to the paper, the inked rubber surface adapting itself to the irregularities in the surface of the paper.
The printing on the metal, in the case of a line original, is done as for photo-zinco minus the etching. The parts of the metal correspond
ing with the white ground of the original are treated with a preparation of gum arabic and an acid (e.g. phosphoric, chromic, or fluosilicic), by which a porous film of insoluble salt is formed oil the surface of the metal, after which they are moistened with clean water as a protection against the greasy ink.
The printing may also be done directly under a positive, such as a tracing, or type-matter proofed on transparent film, the image being reversed, as regards black and white, after printing.
With full-tone originals, a screen negative is first made as for a half-tone block (§ 929) but of greater contrast, since the contrasts are not enhanced as they are in the etching of a metal plate. This negative is then used in the same way as that from a line original.
In offset printing, the impressions, as a rule, are not as vigorous as those from a typographic block, the black being usually a dark grey ; by a very slight biting of the metal in the image of the blacks, more vigorous images are obtained and a much larger number of copies can be printed from the same plate.
The instructions to be given as regards scale of reproduction, screen ruling, and amount of subject are the same as those specified above for photo-zinco (§ 928) and half-tone (§ 932), and need to be supplemented only by those for the actual printing.
921. Rotary Photogravure. The negative is retouched as may be necessary, and a positive transparency of suitable contrast made from it, preferably on film or a stripping plate. This is retouched and mounted with the films of letter press or other illustrations to be arranged on the same cylinder. From this composite positive, a negative is printed on special gravure carbon tissue. Before or after exposure under the positive, the carbon tissue is exposed under a special screen (an original or a duplicate) of cross-line pattern of fine transparent lines on an opaque ground. The screen is of a ruling of 150 to 175 lines per inch, the transparent lines being about one-quarter the width of the opaque squares. After printing, the carbon negative is transferred to the copper-surface cylinder, viz, a cylinder of cast iron coated with copper by electro-deposition or provided with a copper sleeve attached by hydraulic pressure. The negative pigment image is " developed " as in the carbon process, and, after drying, the various parts of the subject-matter are marked off, blocking out with varnish parts, such as borders, interspaces, etc., which are to print white.