PART VI.COLLOGRAPHIC PRINTING.CHAPTER I.HALF-TONE PHOTOGRAPHIC NEGATIVES.
In all the processes and methods treated of up to Part III., the photographic negative has been what may properly be termed, a black and white negative, the lines being rendered clear, and the portions representing the whites of the original being as near black as possible. The extremes of clearness and opacity in these negatives are obtained by having the nitrate of silver bath kept and worked in a very acid state; then the opacity is obtained by intensifying with a solution of mercury followed by ammonia.
Now for the methods to be treated of in this part. An entirely different class of negative is required in which the half-tones are fully preserved, both in the shadows, and in the lights. These half-tone negatives may be made either by the wet collodion process, or upon ordinary gelatine dry plates. For producing them by the wet collodion process, directions are given on page11.
In the processes treated in Part II. the aim has been to get the photographic half-tone broken up in such a manner that the picture could be reproduced by the same means as an ordinary wood block, but in collographic printing the half-tones are not (visibly) broken up, the gelatine holding the ink in exact proportion to its exposure to light, thereby giving a print with the smooth gradations of an ordinary photograph.
The collographic methods are called “heliotype” and collotype. They are analogous in principle, but differ in detail; the heliotype is printed from a film of gelatine, which, after being dried upon a plate of glass, is stripped off, exposed to light under a reversed negative, then mounted upon a plate of pewter to support it during the operation of printing from. The collotype printing surface is the film of gelatine upon the glass or metal plate upon which it was dried. Note also the different printing methods.