| Glycerine | 35 | g. |
| Water | 25 | g. |
| Alum | 2 | g. |
and this should be allowed to act for about five minutes and then removed with blotting-paper. After this operation the pigment image can be easily destroyed. The pigment image is now exposed to the action of a weak sandblast. The sand acts on the relief image and gradually destroys it, and so that the film of varnish underneath is gradually grained.
After the pigment film has been removed the picture shows in all its details in grain on the varnish of the stone or plate. The image is now visible on the film of varnish, according to the strength of the tones in partial graining, and can be etched into the stone or plate.
This is effected by etching with phosphoric acid—
| Water | 100 | g. |
| Phosphoric acid | 4 | g. |
When the etching solution has acted for about two minutes, the stone is washed and coated with a solution of gum—
| Water | 100 | g. |
| Gum arabic | 5 | g. |
and allowed to dry.
Then the film of varnish is removed with a pad of cotton wool damped with turpentine, the dry stone rolled up with litho ink, damped, and the excess of ink rolled off. The picture now shows with all its finest details on the stone, and can be printed.
In preparing a relief printing plate the film of varnish forms the resist for the first etching. It is then rolled up and etched in the usual way.