Prepare the mixture three or four hours before using, and filter it carefully. The subsequent operations are the same as with glass, except that the sunning of the back of the film must be dispensed with, and the second formula for the bichromated gelatine mixture must be used, as the first one gives too high a relief for copper.
Plates that have been used are cleaned again for further trial, by immersing them in a pickle of American potash or concentrated lye, then well washing, regrinding, washing, and finally coating with the beer and water-glass substratum, exactly as before mentioned.
CHAPTER IV. PRINTING FROM THE COLLOTYPE PLATE.
Collotype plates are usually printed from, upon a typographic press, but the best press is one with a cylinder, like the lithographic machines. Such presses give a more even pressure, and at the same time will enable more impressions to be made in a given time.
In the chapter on heliotype printing, a description is given for preparing the tympan for printing from the heliotype skin, and that description holds good for collotype plates; but we shall require a slate slab, the full size of the bed of the press, and a little under half an inch in thickness, upon which to fix the collotype plate by means of plaster of Paris.
The press being ready, take the dried plate, and soak it for half an hour in clean cold water. After that wipe it with a clean soft rag, and scrape any gelatine from the back that may have found its way there; spread as evenly as possible some fine plaster of Paris over the slate slab on the bed of the press, then damp with sufficient water to make the plaster the consistency of cream, and lay the damp collotype plate in the centre, and press it well down, working it a little so as to get it level and well bedded in; allow the plaster to form a ridge all round the edges outside the plate, levelling these ridges by running the finger over the moist plaster, so that they do not project above the face of the plate. Now cover the collotype plate with a mixture of
| Glycerine | 5 | ounces. |
| Water | 10 | ounces. |
| Ox-gall | a little. | |
Allow this fifteen minutes to permeate the film, taking care that it covers the whole of the surface evenly.
Whilst the plate is thus soaking up, get the inking slab clean, and the two rollers ready, viz.: An ordinary lithographic roller, for inking up the heavier portion of the image, and a composition roller for inking the half-tones and clearing the image; then mix the ink to the desired shade by taking some stiff ink from the tin, and mixing to the proper consistency with a little olive oil. The ink must not be made thin, but just sufficiently softened to work up with {172} the palette knife. The ink used must be very finely ground by machinery with a minimum of olive oil. This, in hand grinding, is difficult to do, and the presence of too much varnish in the ink is prejudicial to the production of bright collotype prints.
The ink being mixed, a little is taken up on the point of a palette knife and smeared along the face of the leather roller and then worked vigorously upon the inking slab, until the ink is thoroughly distributed upon both roller and slab. Do not attempt to thin the ink so as to lessen the labor of distributing the ink, as that will only render the collotype print flat and weak. The ink must be stiff and spare. Now remove the soaking fluid with a very soft sponge, and blot off with fine plate paper, then proceed to roll up with the leather roller, until the image just shows nicely on the plate. Do not attempt to get it to take sufficient ink to look black. Next take the composition roller, and a little ink thinned with lard to about half the consistency of that used with the leather roller, and touch it here and there with the palette knife holding this thinner ink, and roll up on another slab until the ink on the slab and roller are quite even; now apply the composition roller to the image inked up by the leather roller, and roll with a light pressure and quick motion. This will complete the inking up of the image. Now take a piece of the mask paper mentioned for heliotype printing, and having cut out a suitable aperture, place it in position upon the plate, then put a piece of paper in position upon the mask; upon this paper fix a large sheet of smooth waste paper, then loose the blanket, lower the tympan, run the bed of the press under the platen, and after adjusting the pressure, pull over the lever, run out the bed, and examine the proof. From that deduce whether the ink is too thin, or too thick, or not enough, or too much. Damp with a sponge dipped in water, then blot off or remove the surface water with a damp soft rag, then ink up again, and repeat until a suitable print is the result.