If one object to the trouble of crystallizing, the solution can be prepared by dissolving the ferric oxide in a hot solution of 30 parts of ammonium oxalate and 25 parts of oxalic acid in 180 parts of water observing that the oxide must be in excess.[42]
The following sensitizing solution gives also excellent results:
| Ammonio-ferric oxalate | 10 parts |
| Ammonio-ferric lactate | 4 parts |
| Water | 100 parts |
After exposure, which varies from five to ten minutes, according to the intensity of the light and the printing quality of the negatives, the picture appears negative from formation of ferrous oxalate. It may be developed in a great many ways: by a solution of silver nitrate at 2 or 3 per cent. of water acidified slightly by an organic acid—citric acid, for example—or a diluted solution of ammonio-nitrate of silver, which most likely constitutes the best developer; the image is black and consists of metallic silver and ferric oxide, with formation of silver oxalate, which dissolve in the ammonia. If the print be treated by a weak solution of aqueous ammonia, the image turns green, then brown, and if, before the latter coloration is obtained, gallic acid or pyrogallol be added, the image becomes bluish-black or brown-black. In the same circumstances tannin (gallo-tannin) produces a blue-black image; catechu-tannin[43] and quino-tannin give green, etc. Employed as [pg 117] a developer, potassium ferricyanate develops an image in prussian blue, and auric chloride one in the characteristic violet metallic gold. To fix the images obtained by the latter reactions, it suffices to wash them in a few changes of water, and, if developed with silver, they can be toned by any of the alkaline solutions of auric chloride used in the printing out silver process, etc.
The photographs obtained by all these processes are permanent.
DR. J.B. OBERNETTER'S PROCESS (1863).
| Copper chloride | 100 parts |
| Ferric chloride, sol. sp. gr. 1.5 | 13 parts |
| Hydrochloric acid, conc. C. P. | 12 parts |
| Water | 1,000 parts |
Float the paper on this solution for about two minutes and hang it up to dry. The keeping quality of the prepared paper is remarkable; it has been kept for two years without apparent change; its sensitiveness is at least one-third greater than that of silver albumen paper. Unless developed within an hour or two, the vigor of the proof is much impaired; after twenty-four hours a print can be taken over on the same.
When exposed, only a faint image is visible. It should be fixed in the following solution:
| Potassium sulphocyanate | 12 parts |
| Sulphuric acid, conc. | 1 part |
| Sensitizing solution | 10 to 12 parts |
| Water | 1,000 parts |