COLAS’S BLACK PROCESS.
This process will often be found useful to reproduce a drawing on yellow tracing-paper, so as to give a more suitable copy to photograph from.
Coat with a fine sponge a hard, well-sized paper, with
| Water | 300 | parts. |
| Gelatine | 10 | parts. |
| Perchloride of Iron (dry) | 20 | parts. |
| Tartaric Acid | 10 | parts. |
| Persulphate of Zinc | 10 | parts. |
and dry in a dark-room.
When dry, expose under the tracing until the greenish-yellow tint of the paper has disappeared, except where covered by the opaque lines.
Development is effected in a bath made by dissolving 20 parts of gallic acid in 200 parts of alcohol and 1000 parts of water; then wash in plenty of clean water.
MORDANTS FOR ETCHING.
I. MORDANTS FOR STEEL.
Plain Acid Mordant.—(Kruger.) | |||
|---|---|---|---|
| First biting: | Muriatic or Nitric Acid | 1 | part. |
| Water | 8 | parts. | |
| Stronger: | Acid | 1 | part. |
| Water | 4 | parts. | |
| Deepest: | Equal parts acid and water. | ||