The following kinds of negatives are used for the various photo-lithographic processes:—
For the transfer process with chromated gelatine paper or direct printing on stone or zinc direct, a reversed line or autotype negative.
For Orell and Füssli’s process, a reversed ordinary half-tone negative.
For Bartös’ process and for photogravure on stone, a direct ordinary half-tone negative.
3. PREPARATION OF THE DIRECT GLASS NEGATIVE.
The preparation and cleaning of the plate glass is as follows:—First of all the plates are roughened with a piece of sharp pumice stone on the extreme edges to the breadth of about 0·5 cm. in order to prevent the frilling or floating off of the collodion film. This is very little trouble considering the advantage it offers. After being edged the plates are placed in a solution of
| Nitric acid chemically pure | 1 | part |
| Water | 10 | parts |
Instead of nitric acid, chromic acid may be used, but this is not absolutely necessary.
After one or two hours the plates are taken out of the acid bath, well washed under a strong water rose, and dried in a place free from dust. Then the plates are well rubbed with 1 : 10 tincture of iodine solution with a pad of clean filter paper and then polished with 50 per cent. alcohol. This preparation must be done with absolute accuracy, and thus they make all substrata of albumen, india-rubber solution, etc., for direct negatives unnecessary.
Plates which have been previously used are placed in pure water till the collodion film has become loose, and are then scrubbed with a stiff brush, treated for some hours to a bath of caustic soda, and then treated as above, when they may be used again. {37}