All this being ready, the margin of the proof is turned upwards—so as to form a disk of which the outside is the impressed surface—in order that the ferrocyanate solution does not find its way on the back of the proof, which would produce stains. Now the proof is laid, the lower edge first, on the developer, and gradually lowered upon it, when, taking immediately hold of it by the two corners nearest to the body, [pg 40] it is lifted out and held upright to allow one following the development of the image; and, presently, if any air-bubbles are seen on the proof, they should at once be touched up with a brush wetted with the ferrocyanate solution; the reason explains itself.
The image appears at once. As soon as the fine lines are well defined, the blue intense, and, especially, when the ground has a tendency to be tinged blue, the proof is placed in the tray filled with water and in this turned over two or three times, when it is immersed in the diluted sulphuric acid. In this bath the print acquires a deep blue coloration, consisting of Prussian blue, and the ground becomes tinted with a blue precipitate without adherence, which is easily washed off by throwing the liquid on the proof with a wooden spatula, or, better, by rubbing with a rag tied to a stick. When the ground is cleared, and after three or four minutes immersion to dissolve the iron salts acted on, the proof is rinsed in water several times renewed to free it from acid, and hung to dry.
There are two causes of failures in this process, viz., over and under-exposure. In the former case the fine lines are broken or washed out in clearing the proof (which may also arise from the drawing made with an ink not opaque enough); in the latter the ground is more or less stained.
The blue stains, the lines for corrections, etc., are erased with the the potassic oxalate (blue salving, as it is termed) whose formula has been given.
The additions, corrections and writing are made with a Prussian blue ink prepared by mixing the two following solutions:
| A. | Ferric chloride, dry | 4 parts |
| Water | 350 parts | |
| B. | Potassium ferrocyanate | 15 parts |
| Water | 250 parts |
The precipitate being collected on a filter and washed until the water commences to be tinged blue, is dissolved to the proper consistency in about 400 parts of water. This ink does not corrode steel pens.
It has been stated that the cyanofer process keeps for years [pg 41] if preserved from the combined action of dampness and the air. The writer found in his practice that the ferric salts in presence of the organic matters (the sizes) acts as does potassium bichromate and renders, in a certain period, the cyanofer film insoluble even after a prolonged insulation. Paper freshly prepared is always more sensitive and gives better whites and generally finer results.[10]
The prints can be toned black in operating as in the cyonotype, but the results are seldom good.
Captain Pizzighelli's formula is as follows: Prepare