In the Appendix some of the most important processes, with or without silver salts as reagents, will be described.
THE ANILINE PROCESS.
The aniline process was published in 1865, by Mr. Willis, the inventor of the platinotype.[11] It is based on the oxidation of aniline by chromic acid, thus: A sheet of paper brushed with a solution of potassium bichromate and sulphuric acid, dried, and after insolation under a cliché exposed to the fumes of aniline which, in reacting with the chromic compound not reduced by light, forms a blue-black image. The process gives, consequently, a positive impression from a positive cliché.
There are various methods of operating; we will briefly describe them.
| SENSITISING SOLUTION. | ||
|---|---|---|
| 1. | Potassium bichromate | 6 parts |
| Sulphuric acid | 6 parts | |
| Magnesium chloride | 10 parts | |
| Water | 150 parts | |
Willis recommended 10 parts of solid phosphoric acid instead of sulphuric acid; the latter forms a preparation about twice more rapidly reduced.
| 2. | Potassium bichromate | 10 parts |
| Manganous sulphate | 4 parts | |
| Potassium bisulphate | 20 parts | |
| Water | 300 parts | |
| 3. | Ammonium bichromate | 5 parts |
| Ammonium chloride | 5 parts | |
| Cupric sulphate | 1 part | |
| Sulphuric acid | 8 parts | |
| Water | 150 parts |
Good well-sized paper should be employed. Rives is too tender and absorbs too much. Steinbach is better. For small sizes, whatever be the paper selected, it is well to size it with starch and, if possible, to calender it on a hot steel plate, or, in lieu, to iron it. This is not, however, a sine qua non. [pg 48] The paper is sensitized by brushing or by floating. To sensitize by floating, it should be left but for a few seconds on the solution and removed by dragging it on a glass rod in order to remove the superfluous liquid. Only the surface of the paper should be impregnated, otherwise the whites would be more or less tinted and the image imbedded not as sharp.