So called from the introduction of the salts of fluoric acid, consists of the following process of manipulation :—
J Bromide of potassium, 20 grains.
( Distilled water ... 1 fluid ounce.
Fluoride of sodium . 5 grains.
Distilled water ... 1 fluid ounce.
Mix a small quantity of these solutions together when the papers are to be prepared, and wash them once over with the mixture, and, when dry, apply a solution of nitrate of silver, sixty grains to the ounce of water. These papers keep for some weeks without injury, and become impressed with good images in half a minute in the camera. The impression is not sufficiently strong when removed from the camera for producing positive pictures, but may be rendered so by a secondary process.
The photograph should first be soaked in water for a few minutes, and then placed upon a slab of porcelain, and a weak solution of the proto-sulphate of iron brushed over it; the picture almost immediately acquires an intense colour, which should then be stopped directly by plunging it into water slightly acidulated with muriatic acid, or the blackening will extend all over the paper. It may be fixed by being soaked in water, and then dipped into a solution of hypo-sulphite of soda, and again soaked in water as in the other processes.
Mr. Bingham has the following remarks on this process, and he gives a modified form, into which a new photographic element is introduced:—
" We find it is better to add to the proto-sulphate of iron a little acetic or sulphuric acid : this will be found to prevent the darkening of the lights of the picture to a great extent, and it will be found better not to prepare the paper long before it is required for use, this being one reason why the picture often becomes dusky on application of the proto-sulphate.
" Reasoning upon the principle that the action of light is to reduce the salts of silver in the paper to the metallic state, and that any substance which would reduce silver would also quicken the action of light, we were led to the following experiment :— The protochloride of tin possesses the property of reducing the salts both of silver and of gold : a paper was prepared with the bromide of silver, and previously to exposing it to light it was washed over with a very weak solution of the chloride of tin ; the action of light upon the paper was exceedingly energetic ; it was almost instantaneously blackened, and a copy of a print was obtained in a few seconds".
The use of fluorides has been recently introduced as a novelty by some French photographers, but reference to the author's Researches on Light, published in 1844, will distinctly show that I was the first to employ these salts, as photographic agents.