The following, used by Callot and Piranesi, is similar and prepared in the same way:
| Strong Vinegar | 8 | parts. |
| Verdigris | 4 | parts. |
| Sal Ammoniac | 4 | parts. |
| Salt | 4 | parts. |
| Alum | 1 | part. |
| Water | 16 | parts. |
For relief etching use only 10 parts of water.
Relief Etching.
For etching copper in relief, the following glyphogen is recommended by Deleschamps:
| Nitrous Acid, at 30° | 2 | ounces. |
| Acetate of Silver | 6 | drachms. |
| Hydrated Nitric Ether | 16 | ounces. |
The nitric ether is prepared by mixing two ounces each of nitric acid and alcohol, and, when the reaction commences, stopping it by adding eight ounces of distilled water.
Tint Etching.
Flour of sulphur mixed with oil forms a good composition for etching tints. It can be applied with a brush. For a flat tint, Hamerton says “Oil the plate liberally with olive oil, and blow flour of sulphur upon this.” The sulphur, if allowed to remain on the plate, will produce a flat tint, more or less deep in proportion to the time it remains.
Roret gives the following formula for tint etching: