The process to turn the blue color into a green was devised by Mr. Paul Roy. It is as follows: Dissolve 7 parts of borax in 100 parts of water, and acidify the solution with sulphuric acid added drop by drop until the litmus paper becomes red; then, in the same manner, neutralize with aqueous ammonia not in excess, but just enough to show an alkaline reaction; this done dissolve 1 part of powdered catechu and filter. In this the proof is immersed after development until the desired effect is attained. Wash, etc.
To clear the lines, or to make additions, or to write on the blue margin of the proof a solution of potassium oxalate is employed. It dissolves the blue without leaving scarcely any trace of it. The solution can be prepared by mixing the two solutions whose formula is given below:[9]
| A. | Oxalic acid | 10 parts |
| Water | 100 parts | |
| B. | Caustic potassa | 12½ parts |
| Water | 100 parts |
The blue prints are permanent. When drying they darken a little from oxidation; exposed to sunshine for some hours, they bleach considerably; but in the shade the faded pictures progressively absorb oxygen from the air and assume their original intensity and color in a period so much the longer as the insulation has been more prolonged; it may take weeks if the picture were much bleached.
THE CYANOFER. (Pellet's Process.)
This process gives blue impressions on a white ground from positive clichés, and white impressions on a blue ground from negative clichés. It is termed “positive ferrotype process.”
The cyanofer is an application of one of the numerous and useful inventions for which photography is indebted to A. Poitevin. In 1863 he discovered that certain organic substances were rendered insoluble by ferric chloride, and that they again became soluble; when under the influence of light the ferric chloride has been reduced to a ferrous salt. This curious phenomenon is the base of the process now to be described. As usual the process has been modified by compounding the sensitive solution in various ways and by minor details in the manner operating. But although these modifications have rendered the process easier to work with, there is not a great difference in the results obtained. We give two formulas. Aside from the addition of gum arabic, which was suggested by Mr. Pellet, and which constitutes the capital improvement of the process, the formula is substantially that devised by Mr. Poitevin.
Prepare three solutions as follows: