The advance of photography : its history and modern applications (1911)

PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 89 is black ; hence the light passes unimpeded through those places. The half-tones are formed by the varying height of the gelatine film. (Compare fig. 22 e.) If this gelatine relief is suffered to dry, it becomes wonderfully hard and firm. It can then be placed with a plate of lead under a strong press, and a cliche of the relief can be thus obtained in lead. The prominent parts of the gelatine relief appear, of course, depressed in the lead, and the depressions prominent, as represented in fig. 22 /. Woodbury used this lead relief as a printing plate. But he did not print it off with oily printer's ink, which is too opaque, but with a semi-transparent gelatine ink. This was poured warm on the plate in a horizontal position, it penetrated into the depressions, and then a piece of paper was placed upon it and pressed gently down, the gelatine consolidated quickly, and an impression in relief was obtained on the paper. As the ink used was transparent, it appeared in thin sheets much less black than in the thick, and in places where its thickness gradually diminished, there occurred a transition from black to white — a perfectly homogeneous half-tone. As soon as the coating dried, the relief contracted considerably, but the semi-transparency remained, and thus it became possible to reproduce the most beautiful half-tones of photography by printing. Any colour could be used in this process. This relief-printing of Woodbury attained a high importance. It made the multiplication of photographic negatives from a single printing form possible without the help of light. It was therefore of importance where a great number of pictures were required — for example, in the production of copies of oil-paintings and drawings. Photographers did not use it much in portraiture, because the production of a faultless gelatine relief and its impression on lead require great practice and an expensive