For repeated printing one to two per cent. of gallic acid is added to the damping water.
If the plate has been correctly treated the print or the photo-litho transfer will furnish thousands of good impressions, just like a stone. Further details as to the printing on the zinc plates, or the transfer of the chromated gelatine prints on to the same, will be found in Chapter IV.
4.—The greasy drawing materials
The two first are used in the liquid state, the chalk, however, in solid form. The ink can be obtained in a liquid form and the two others in solid state. All three materials consist principally of soap, tallow, wax, resin, and soot. Soap and tallow give the necessary grease, wax and resin give hardness and consistency, and soot the colour. The proportion of grease in the drawing materials must be so great that even the finest lines or points of a drawing can be well transferred to the stone, so that they may not be attacked by the proper etching solution, and do not break away from the stone by continuous printing. As much black as possible in the drawing materials is pleasant for the draughtsman, but is not actually necessary for this particular purpose. {7}
The tusch must dissolve well in distilled water and flow fine and clean from the pen. It should be tolerably brittle, and the fractured surfaces should be shiny. The photo-lithographer will frequently require the lithographic tusch for additions or for corrections. The best is the so-called Lemercier’s tusch, which is used in nearly all works. It consists of—
| Yellow wax | 2 | parts |
| Mutton tallow | 2 | parts |
| Marseilles soap | 6 | parts |
| Shellac | 3 | parts |
| Lamp black | 1–2 | parts |
These ingredients should be melted together by boiling.
The autographic ink is, as a rule, used for drawing or writing on paper, and the result is then transferred to the stone. Instead of this, any litho tusch can be used, but for some work the autographic ink is more advantageous. This ink must be thin, run easily, but not patchily, from the pen, and must keep for a long time equal in quality and action. It is very advantageous that specially-prepared paper is not required, since this always makes the work more difficult; firm, hard writing-paper is quite good enough for this work. The commercial inks generally possess these good qualities, and if much is not required one will hardly care to prepare it oneself. A good successful autographic ink which draws in brown may be obtained from the following formula:—
| Marseilles soap | 10 | parts |
| Tallow | 10 | parts |
| Shellac | 12 | parts |
| Yellow wax | 12 | parts |
| Mastic | 5 | parts |
| Asphalt | 4 | parts |
| Vine soot | 3 | parts |
| Distilled water | 125 | parts |
Originals prepared with this ink transfer as well immediately as after several months, and ordinary well-sized writing paper can be used for drawing or writing on.