A print is floated on this solution, face downward, for three or four minutes, taking care to agitate the liquid as little as possible; the print is afterwards immersed and another one floated in its place, thus proceeding until all the prints are immersed or the solution can hold no more. A fresh solution is then added to strengthen it: the older the solution the more rapidly and better it works. In this developer copper cyanide is precipitated on the parts acted on by light, and this exactly in the proportion to the luminous action. The time of immersion depends on the method selected to finish the proofs; it its [pg 118] from five minutes to half an hour. If the proof is immersed for, say, twenty-four hours, the image comes out in a relief which may bring the shadows to two lines in depth. When well developed and thoroughly washed, the proof can be dried and the subsequent operations made at any convenient time.
Various processes may be employed to give to these proofs the tone required; thus: the prints well washed are placed in a solution of ferricyanate of potassium at 6 to 12 per 100 of water, where they take a red color increasing in intensity. If left over night the color becomes a splendid velvet deep red with perfect clear whites. To obtain the color of silver photographs one hour's immersion is sufficient. After this operation the proofs are washed until the water is no more tinged yellow.
By immersion in
| Ferrous sulphate | 100 parts |
| Iron sesquichloride | 40 parts |
| Hydrochloric acid | 80 parts |
| Water | 200 to 300 parts |
the proofs undergo the following gradation of colors: red, reddish violet, blue-violet, black and greenish black. As soon as the desired color is obtained, the proofs are washed in acidified water and dried.
The most beautiful purple violet is obtained by leaving the proofs in the iron solution until green-black, and then washing for a moment in a dilute solution of sub-acetate of lead.
A brown-black may be produced by treatment, after washing, with an ammoniacal solution of hypermanganate of potash.
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 [pg 119] of the negative, Fe2Cl3 is reduced to FeCl, while CuCl suffers no alteration.