These are melted together in a crucible over a small gas-jet, and to them are added four ounces of chalk printing ink, and the mixture reduced to the consistency of cream with spirits of turpentine. A soft sponge is saturated with this mixture and rubbed gently over the exposed paper (in this stage the nature of the grain can be best seen). An ordinary letter-press roller, charged with a little ink from the inking slab, is then passed over the transfer, causing the ink to adhere firmly to the parts affected by the light, and removing it from the parts unacted upon. It will be found that with practice, rolling slowly and carefully as a letter-press printer would his form, the ink will be removed by the roller according to the action that has taken place by light, leaving the shadows fully charged with ink, and the high lights almost clear, the result being a grained transfer in greasy ink. The transfer is next put into a weak bath of tannin and bichromate of potash for a few minutes, and when taken out the surplus solution should be carefully dried off between clean sheets of blotting paper. The transfer is hung up to dry, and when thoroughly dry, the whole of the still sensitive surface should be exposed to light for about two minutes. A weak solution of oxalic acid should be used for damping the {149} transfer (about 1 in 100), and this should be applied to the back of the transfer with a soft sponge. After it has been damped about four times, it should be carefully put between clean sheets of blotting paper, and the surplus moisture removed. A cold polished stone is then set on the press, and when everything is ready the transfer is placed on the stone and pulled through twice. The stone or scraper is then reversed, and the transfer is again twice pulled through. A moderate pressure and a hard backing sheet should be used, care being taken not to increase the pressure after the first pull through. The transfer is taken from the stone without damping, when it will be found that the ink has left the paper clean. Gum up the stone in the usual way, but, if possible, let the transfer remain a few hours before rolling up. Do not wash it out with turpentine, and use middle varnish to thin down the ink.
It should have been mentioned that varying degrees of fineness of grain can be given to the transfer by adding a little more ferridcyanide of potassium in the sensitizing solution, and drying the transfer paper at a higher temperature, or by heating the paper a little before exposure, or by adding a little hot water to the cold water bath, after the transfer has been fully exposed; the higher the temperature of the water, the coarser the grain will be. The finer grain is best suited to negatives from Nature, when a considerable amount of detail has to be shown.
The coarse grain is best for subjects in monochrome, or large negatives from Nature, of architecture, etc., where the detail is not so small. Even from the finer grain, several hundred copies can be pulled, as many as 1200 having been pulled from a single transfer. It would have produced a great many more if required.
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 page [11.]
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.