This process, roughly described, consists in carefully grinding with fine emery, a plate of thick glass, then coating it with a mixture of white of egg, or of stale beer and silicate of soda. After the plate is dried it is rinsed in water, and dried again, then warmed, and coated with a thin film of bichromated gelatine. This coating is then dried in a suitable oven, after which the plate is exposed to light under a reversed negative, followed by a short exposure of the back of the film to light, then washed in cold water until the whole of the bichromate is removed from the film, when the plate is allowed to dry. It is next soaked in cold water, then in a mixture of glycerine and water, after which it is placed upon the press, and the printing proceeded with.
As in lithographic printing, the size of the paper upon which the picture is printed determines the size of the stone upon which the transfer is made, so as to give a good margin, not only large enough to prevent the paper being marked by the edge of the stone, but also to allow plenty of room for working the roller; so in collographic printing, the glass plate upon which the picture is to be made must be sufficiently large to allow a margin of at least four inches all round the outside of the picture. This is not only requisite for the reasons that hold good in litho. printing, but for the additional one of giving plenty of room for the manipulation of the mask used to protect the margin of the print.
Collographic plates should also be sufficiently large to prevent their edges from cutting the composition rollers used for inking the image; for instance, a roller nine inches long should never be used upon a plate less than twelve inches wide.
Collographic films are made upon good patent plate-glass half an inch thick, free from scratches, with the edges nicely bevelled to avoid cutting the rollers; this bevelling the plates is absolutely necessary for use upon the machine, but is not so important for the hand press.
To prepare plates for use they are first ground with fine emery powder. To do this place one plate upon a smooth board or a couple of bearers across the sink, sprinkle with fine emery powder (sifted through book muslin), then with {164} water. Now, with another plate on the top, proceed to grind with a circular motion, continuing the grinding until the emery is worn smooth; then renew the supply of emery, sprinkle with water and again grind until the emery is smooth; now work well with water, squeegee dry, and examine the surface; if it is evenly ground the plates are ready for the next stage. If they are not ground all over, cover them again with emery and water, and continue to grind until the surfaces are of an even matt all over.
The plates being ground, they are well rinsed under the tap, and both sides are well rubbed with a rag to free them from all particles of grit, etc., then flood them with a mixture of
| Liquor Ammonia | 1 | ounce. |
| Alcohol | 1 | ounce. |
| Water | 5 | ounces. |
rub this well in with a clean rag, and again well wash with clean water, then allow to drain and flood with
THE PRELIMINARY COATING OF | ||
|---|---|---|
| Stale Beer | 5 | ounces. |
| Water | 5 | ounces. |
| Silicate of Soda (Waterglass) | 2 | ounces. |
made up an hour or two before using, and filtered.