So far this manual has treated of the production of negatives by photography, rather for the purpose of amusement than for any large commercial project, but it must not be supposed that the province of photography ends there. It is almost universal in its applications and the valuable aid which it renders to science, literature, and art. The illustrations for many of our serial illustrated magazines are in many cases effected almost entirely through the agency of the camera, and the processes by which these re-productions are made are termed Photo-mechanical, because photography in conjunction with a mechanical printing process is used. It would be impossible to enter at any length into minutiae and working details of the different processes, but the following short resume may give a general idea of the whole subject. Every process is founded upon the chemical action which light sets up in a mixture of gelatine and an alkaline salt of chromium. The precise chemical action is practically immaterial, but its results most important; its effect is to render the gelatine acted upon by light insoluble and incapable of absorbing water. The various processes may be divided into four classes:-
1. - Typographic Blocks, which are blocks, the groundwork of which is eaten away by some acid liquid, leaving the image in relief or raised up like any ordinary type; these blocks are chiefly used for illustrating serial papers.
2. - Plates in which the image is bitten, by the use of an acid liquid, leaving the groundwork untouched; the image is said to be etched in intaglio.
3. - Woodbury type, in which the image is on a very thick gelatine film which is used to obtain a mould or impression on metal
4. - Collotype or Heliotype, in which the film itself is printed from.
To prepare these the subject to be re-produced is copied by the collodion process, and after development the resulting negative is strongly intensified till the image shows as bare glass upon an absolutely opaque background. A print is taken from this negative in the printing frame in the ordinary way, upon paper coated with chromated gelatine, and after exposure taken from the frame and given a thin coating of printing ink, and soaked in cold water, when it is found that the printing ink will leave the gelatine in those parts protected from the action of light and only adhere to the image. This gelatine print in greasy ink, is now placed face downwards upon a sheet of zinc and passed through a press, when the ink leaves the paper and adheres to the zinc. The image on the plate is then further charged with ink and then etched, special precautions being taken to prevent the lines of the image from being eaten away by the etching fluid; when etched deep enough, the plate is printed from in the ordinary way in an ordinary steam-press.
By this process some of the most beautiful pictures of the day are produced. A film of chromated gelatine is exposed under a positive in the printing frame and developed. As in the carbon process the result is a film of gelatine bearing a picture in which the blacks are represented by little elevations and the whites by depressions; this film may be attached to a copper-plate and etchings begun at once, or it may be covered with powdered graphite and a mould taken from it by electrolysis. The plate when finished has to have the ink rubbed into the depressions representing the image, and the surface of the plate thoroughly cleaned between each impression taken from it.
A film of chromated gelatine is exposed under a negative as usual, and cemented face downwards on to a sheet of glass, and washed for some hours under hot water; allowed to dry and stripped. It has at this period the apperance of an extremely thin transparent piece of silk, with the picture slightly in relief. It is then placed on a sheet of hard rolled lead and a plate of steel placed above it, and a pressure varying from one to five hundred tons brought to bear on it. The gelatine film is forced into the lead and makes an impression the same as a seal on hot sealing wax, the film itself being unharmed and ready to make any number of such moulds. The lead with the impression on it is now put into a press and special hot liquid gelatine ink is poured on to it, and a sheet of paper laid on top; pressure is brought to bear upon it, and the ink leaves the parts where there is no impression, collecting in the depression. The ink is allowed to get cold and the paper stripped, bearing the image with it; it is then washed in alum and dried.
The most simple of all the processes A film of chromated gelatine fastened to a glass or metal plate, is exposed under a reversed negative, washed and dried; only a very faint image can be seen at this point. The plate is now put in a press and damped with water and printer's ink applied with a roller, when it is found that the ink will adhere to those parts acted upon by light; the shadows in the picture will take most ink, the whites none. Paper is placed on the inked film and both passed through the lithographic press, and the result is the finished print.
For further instructions in these photo-mechanical processes, the amateur is referred to Wilkinson's "Photo-Mechanical Printing."
E. J. W.
Third Edition Revised and Enlarged. 24th Thousand.