and dried.
Now the sensitizing process differs according as whether the cliché is positive or negative. In the former case the preparation is sensitized with the solution employed in the black process, proceeding afterwards as usual; in the latter, that is, when the cliché is negative, the best process is the cuprotype.[12]
For printing, special frames are employed to permit one to examine the progress of the impression from time to time without the possibility of either the wood block or the cliché moving. These frames open in two. The upper frame is provided with screws on the four sides to hold firmly the block when it is placed into contact with the cliché by means of the screws fixed on the cross bars. As to the cliché, if it is made on a glass plate, it is secured on the thick glass plate of the lower frame by two wooden bars against it pushed by screws.
When the block is ready for printing, the prepared side is usually concave. It is straightened by slightly wetting the back and resting it on one end, prepared side against the wall.
Printing on Canvas.—The canvas should be first brushed with a solution of aqueous ammonia in alcohol, 1:3, to remove greasiness until the thread just commences to show, then, when rinsed and dry, rubbed with fine sand to give a tooth, dusted, washed with a sponge and then coated with the following solution, proceeding afterwards as in the cuprotype process:
| Isinglass | 8 parts |
| Uranic nitrate | 5 parts |
| Copper nitrate | 2 parts |
| Water | 200 parts |
Printing on Opal, Celluloid, etc., is quite simple; it suffices to coat the material with the following gelatine solution, and, when the film is dry, to proceed in operating by any one of the processes before described.
The sensitizing compound may be incorporated to the gelatine solution, but we prefer not to do it and to sensitize the plates as they are wanted for use.
| A. | Gelatine | 4 parts |
| Water | 70 parts in volume |
Dissolve and mix little by little in order: