| A. | Gum arabic | 15 parts |
| Water | 100 parts | |
| B. | Ammonia ferric citrate | 45 parts |
| Water | 100 parts | |
| C. | Ferric chloride | 45 parts |
| Water | 100 parts |
For sensitizing mix in order:
| Solution A | 100 parts |
| Solution B | 40 parts |
| Solution C | 20 parts |
The mixture very much thickens at first, but becomes sufficiently fluid for use in a few hours. It keeps well for two or three days. Leaving out B and replacing it by rain water, this makes also a good solution for the cyanotype.
THE BLACK OR INK PROCESS. (Ferro-tannate Process.)
This process gives black positive impressions on white ground from positive clichés, and negative impressions from negative clichés. It has been attributed to Mr. Colas, but in reality it was invented by Mr. Poitevin, who describes it as follows in his communication of May, 1860, to the Société Francaise de Photographie:
“I make a solution containing—”
| Iron perchloride, cryst | 10 parts |
| Tartaric acid | 3 parts |
| Water | 100 parts |