Photo-Lithography

In order to convert a positive printing image into a negative various methods may be used; the most convenient for the lithographer is the following:—A good litho stone is first ground in the ordinary way and then the surface rubbed with a pad and oxalic acid solution till it has a high glaze or polish. On to this stone is then transferred a sharp impression in a non-greasy ink from the positive printing image. According to the destination of the stone for large or small editions one of two processes may be adopted. If a thousand or more pulls are required from the stone the transfer may be dusted with resin powder, and this melted with the burning, heating, or ether process and the drawing etched in relief with 8 to 12° nitric acid and gum. Then the stone may be well washed with water and the acid removed with one to two per cent. acetic acid.

For small runs the first etching is omitted, and the stone treated with acetic acid. In both cases the stone should be well washed, dried, and then coated with dissolved lithographic tusch or autographic ink. The greasy substances of these materials penetrate into the surfaces of the stone that are laid bare, and firmly adhere. When the tusch or ink is completely dry the stone is washed with turpentine, inked up, and one now has a negative image from the subject in question, which shows all the original printing places in white and all white places black.

The process of preparing such negative prints by the use of bichromated gelatine papers is much simpler.

A sheet of gelatinized paper is sensitized for this purpose on a 5 per cent. bath of potassium bichromate, squeegeed to a sheet of plate-glass and well dried, and on this is made with black ink, which is very opaque, in the dark or in gaslight, a sharp clean print. This print is then laid on a flat cable, under a sheet of plate-glass, so that it cannot roll up, and exposed to the light for from five to thirty minutes. A long exposure rarely does harm in such cases, as the good opaque black ink allows no rays of light to pass through to those places which later should print white.

The next manipulation is the development and inking up, which is done as in ordinary photo-lithography. In this operation all {84} those places which were not covered up by the printing will take the developing ink; on the other hand, those places where the impression was will wash out clean and white. The print has assumed the appearance of a photographic positive. The print is now treated like any other photo-lithographic print, and finally transferred to a stone or zinc plate. This method is distinguished by its simplicity and safety; it has also the advantage that any desired printing subject can be directly transferred in negative form to a zinc plate for relief etching, which otherwise could only be done by preliminary transfer to stone and subsequent transfer to zinc.