A weak solution of nitrate of silver also yields very fine pictures, but the exposure should be very short, and the proofs must be fixed in water containing a small quantity of oxalate of ammonia.
In order to impart to the proofs the gloss of silver photographs, they should be albumenized in the ordinary manner, and the albumen insolubilized by well known means.
The chemical actions in this process I explain in the following manner: On the paper there are Fe2Cl3 and CuCl, the latter in excess. By the action of light, and according to the transparency of the negative, Fe2Cl3 is reduced to FeCl, while CuCl suffers no alteration.
If the paper be immediately placed in an _absolutely_ dry room after exposure, the picture remains unchanged. In a moist atmosphere FeCl attracts moisture and, with a part of CuCl, is so decomposed that Fe2Cl3 is formed together with Cu2Cl.
After this action has commenced, if the proof be not immediately immersed in a solution of sulphocyanate of potassium, Cu2Cl passes over to a higher combination of chlorine, and the paper is again fit to be impressed anew by the action of light.
As long as FeCl or even Cu2Cl is present, if the print is immersed in the sulphocyanate solution, sulphocyanate of copper is immediately formed on the reduced parts, while on the others the sulphocyanide of copper, formed and dissolved by the sulphocyanide of potassium in excess, becomes decom- posed with water in soluble sulphocyanide of copper and deposited as such on the parts already covered with the salt.
Frequently the prints appear yellow from formation of the double sulphocyanide of copper, but the color disappears by washing in water.
Red coloration is due to decomposition into ferrocyanide of copper.
L. LIESEGANG'S PROCESS (1865).
Pour ammonia into a nitrate of uranium solution, wash the precipitate of uranate of ammonia in distilled water, then dissolve in citric acid.
Mix this solution of citrate of uranium and a little of a solution of chloride of gold with a paste prepared by dissolving tapioca in hot water.
The quantity of chloride of gold must be small and the heat not too great, otherwise the gold would be reduced.
Spread the mixture with a sponge on the paper, which takes a brilliant yellow color, and expose when quite dry; the proofs have the delicacy and vigor of albumen prints.
The proofs come from the frame with a bluish-black color; they should not be toned, but merely fixed by washing until the yellow color of the paper has disappeared.
The color of the picture can be changed to a purple by a solution of chloride of tin.
GUARBASSI'S PROCESS (1867).
The paper is floated in the dark for four or five minutes on a saturated solution of bichromate of potash. When dry, it is printed a little longer than for silver prints and afterwards floated, face upwards, on a water bath until all the unaltered bichromate is dissolved. It is then immersed in the following solution, which improve by use and tones the pictures to a reddish color:
Saturated solution 4 parts nitrate of mercury, as free from acid as possible Saturated solution 1 part bichromate of potash Distilled water 28 parts
This solution should be prepared, filtered and allowed to stand for some time before use. The print is left in the bath until it has assumed an intense red color, the whites remaining perfectly pure. It is then washed and put in another bath to obtain a brownish tint. This bath is thus composed:
Conc. aqueous ammonia 2 parts Distilled water 100 parts
The print must be immersed at once, and when, in a short time, it has assumed the proper color, it should be washed immediately.
The picture is toned in a very diluted solution of chloride of gold, 1:7,000, in which the color passes from a light brown to a deep black or a violet black tone, when it is washed in two changes of water.
A. POITEVIN'S PROCESS (1870).
"I use a paper prepared with iron sesquioxide rendered sensitive to light by tartaric or, better, citric acid in concentrated solution. This paper, after desiccation and exposure to light, possesses the property of reducing the solution of silver nitrate and that of chloride of gold, and of turning blue with a solution of potassium ferncyanate in the parts where light has reduced the iron sesquichloride into the oxide at the minimum."
"To coat the paper with an equal layer of iron sesqnioxide, I brush it with a tuft of fine linen dipped in a solution of iron perchloride at 10 or 12 per cent. of water, and dry the sheets in the dark. I immerse afterwards these sheets, one after the other, in a tray containing aqueous ammonia, in such a manner as to well wet each sheet successively. A sufficient number of sheets being immersed, I pour off the ammonia in a vial, and, in the tray, I wash them several times, and remove them one by one to hang them up to dry, even in full light, the iron sesquioxide not being sensitive to light."
"The paper can be prepared in quantities beforehand. To use it I apply upon each sheet a solution of citric acid at 30 or 35 per cent. of water(44)-which may be done by daylight-and let them dry in the dark."
"Exposed under a negative of the ordinary intensity, the paper is impressed in sunshine in a few minutes; in the shade it requires about the same time as chloride of silver paper."
"After exposure the image is not visible, and without being obliged to shelter it from light, I immerse the print in a solution containing about 1 per cent. of silver nitrate. This solution can be used over and over again, by adding to it a little of the silver salt. It does not become turpid by use; it simply turns slightly green from formation of iron nitrate. The image appears soon and rapidly becomes vigorous; in half an hour it will be completely developed. When the exposure is sufficient the color is deep sepia, but not so intense if the quantity of citric acid is feeble. No fixing is necessary; it suffices to wash in several changes of waters."
"The image can be toned with great facility by a weak solution of gold or of platinum chloride, or, better, by a mixture of these two salts. If the impressed paper be treated by a very diluted solution of potassium ferrocyanate, one obtains very pretty blue proofs."
"A weak solution of gold chloride develops a violet image. A solution of platinum chloride has no effect."
"All the various phases of this printing method can be followed in full (diffused) light; there is only the desiccation of the paper when sensitized with citric acid, which requires to be done in the dark."