Waterhouse gives the following formula:—
| — | Solution 1. | Solution 2. | Solution 3. | Solution 4. |
| Water | 650 | 150 | — | 100 |
| Gum arabic | 170 | — | — | — |
| Tartaric acid | — | 40 | — | — |
| Ferric chloride solution 45° Baumé | — | — | 150 | — |
| Ferrocyanide of potassium | — | — | — | 20 |
Solutions 1 and 2 are mixed and No. 3 added gradually with constant stirring. The mixture is left twenty-four hours, and diluted with water to a specific gravity of 1·100.
The paper is coated with the solution and used as already directed, being developed in ferrocyanide of potassium solution and washed with water, treated with weak hydrochloric acid, and then finally cleaned from all traces of acid.
Black Lines on a White Ground.—This modification of the ordinary blue print is arrived at with the following formula:—
| Water | 96·0 | parts. |
| Gelatine | 1·5 | „ |
| Perchloride of iron (in syrupy condition) | 6·0 | „ |
| Tartaric acid | 6·0 | „ |
| Sulphate of iron | 1·5 | „ |
The paper is coated with the solution. After printing, the image is developed with a solution containing
| Gallic acid | 1 | part. |
| Alcohol | 10 | parts. |
| Water | 50 | „ |
A final washing of the print with water completes the operation.