COMPARATIVE PROCESSES.
Processes, at first chiefly of the heliogravure or photogravure variety—processes, that is to say, of the intaglio or plate-printing description, printed in the same way as etchings and mezzotints, from dots and lines sunken in a metal plate instead of standing out in relief—date back almost to the invention of photography in 1834; and all modern processes of reproducing drawings have a photographic basis. Even at that time it was demonstrated that a glass negative could be used to reproduce the photographic image as an etched plate that would print in the manner of a mezzotint. Mr. H. Fox-Talbot, to whom belongs, equally with Daguerre, the invention of photography, was the first to show this. He devised an etched silver plate that reproduced a photograph direct.
Photo-relief, or type-printing, blocks date from such comparatively recent times as 1860, when the Photographic Journal showed an illustration printed from a block by the Pretsch process.
At this present time there are three methods of primary importance for the reproduction of line drawings—
- The swelled gelatine process,
- The albumen process,
- The bitumen process.
The first of these three processes is the most expensive, and it has not so great a vogue as the less costly methods, which are employed for the illustration of journals or publications that do not rely chiefly upon the excellence of their work. It is employed almost exclusively by Messrs. A. and C. Dawson in this country, and it is in all essentials identical with the old Pretsch process that first saw the light thirty-three years ago.
Acids do not enter into the practice of it at all. The procedure is briefly thus: A good dense negative is taken of the drawing to be reproduced to the size required. The glass plate is then placed in perfect contact with gelatine sensitized by an admixture of bichromate of potassium to the action of light. Placed in water, the gelatine thus printed upon from the negative, swells, excepting those portions that have received the image of the reduced drawing. These are now become sunken, and form a suitable matrix for electrotyping into. Copper is then deposited by electro-deposition. The copper skin receives a backing of type-metal, and is mounted on wood to the height of type, and the block, ready for printing, is completed.
This process gives peculiar advantages in the reproduction of pen-drawings made with greyed or diluted inks. The photographic negative reproduces, of course, the varying intensities of such work with the most absolute accuracy, and they are repeated, with scarcely less fidelity, by the gelatine matrix. Pencil marks and pen-drawings with a slight admixture of pencil come excellently well by this method.