GB279186A - Improvements in the printing of posters and other large sheets by photo-mechanical processes - Google Patents

Abstract

279,186. Entwistle, H., and Entwistle, W. Aug. 4, 1926. Photo-mechanical printing-surfaces. - Monochrome or multicolour prints in tone, or line, or both, are produced from a gelatin printing layer prepared by making a photographic transparency of the original, and producing therefrom by projection a print which when placed in contact with a composition of gelatin and ferrous salts will produce therein a differentially-hardened gelatin image. The process is particularly applicable to the production of large posters &c. without the use of large printing-blocks. A negative of small size, in grain or half-tone, may first be made from the object to be reproduced if this is, for example, a photograph or wash drawing, and from this negative a positive enlargement is made by projection upon a sensitized surface on a transparent or translucent support. Alternatively, a screen enlargement may be produced from an ordinary negative. From this positive, prints are made on paper sensitized with ferro-prussiate or other ferric, chromic or like salts, and these are placed, without developing or fixing, each in contact with a layer containing gelatin and ferrous or like salts, as in the known " true to scale " process. By such contact the gelatin layer is hardened at the parts in contact with the unreduced portions of the ferro-prussiate or like print, but unaltered at the parts in contact with the reduced portions. When the differentiallyhardened layer is rolled up with ink, the hardened parts accept the ink while the unhardened parts reject it, and prints may be taken therefrom upon paper by transfer under pressure. Where many prints are required transfer ink may be used in the known manner. If prints in two or more colours are required, separate printing-layers are made for each colour from a set of colour-record negatives produced in known manner, and prints are made therefrom in registered superimposition on one sheet of paper &c. Prints exhibiting composite features, such as flat tones and half tones may similarly be produced by separate superimposed printings. The negative employed in making the enlargement may be produced by first making from the original a small photoengraved block in the ordinary way on zinc, copper, &c. by the use of a ruled screen. The interstices in the grain of the block are filled with a white pigment such as magnesium carbonate, and the negative is made from the block as by the wet collodion process. In the production of posters &c. from letterpress or from drawings &c. in line, dot or stipple or in flat tones, an ordinary negative is used instead of a negative in half-tone or grain.