Take hold of the paper by the two opposite corners and fold it into a loop, lay it on the iron solution, the center of the sheet first placed in contact with the liquid, and then gradually spread it by lowering the corners with a little pressure. No solution should run over on the back of the paper; it would be [pg 31] a cause of stain. This done, and without allowing the liquid to penetrate in the paper, immediately take hold of the two corners near the body and withdraw the paper by dragging it over on a glass rod for this purpose fixed on the edge of the tray. Now pin up the paper to dry, which should be done rapidly, and sensitize a second time in proceeding in the same manner. If this second sensitizing be found objectionable, let float the paper for no more than ten seconds; of course this method of sensitizing is not applicable to prepare larger sheets of paper. In this case the paper is pinned by the four corners on a drawing board or any other support, lined with blotting paper and quickly brushed over with a sponge sparingly imbued with the sensitizing mixture, so as to wet the paper with a very small excess of liquid.
The rationale of this manner of sensitizing is to impregnate only the very surface of the paper with the ferric salts, and thereby to obtain an intense blue with very good whites, which latter it would be impossible of obtaining should the sensitizing solution be allowed to reach in the fibers of the paper, for, in this condition, it is impossible, owing to the exigencies of the process, to wash out thoroughly the iron salts to prevent the chemical changes which cause the whites to be tinted blue. It is for this reason that better results are also obtained with well sized papers.
The sensitizing should be done by a very diffused daylight, and the drying, of course, in a dark room. When sensitized the paper is yellowish green. It should be well dried for keeping, and rolled or wrapped in orange or brown paper and preserved from the action of dampness and of the air. It does not keep well, however, no more than two or three months, perhaps, in good condition; but the sooner it is employed the finer the proofs, the better the whites and more rapidly is the paper impressed.
There is in the market a paper which keeps for a long time. It is prepared by adding a small quantity of gum arabic or of dextrine to the sensitizing solution. Good for the reproduction of line work, it does not give very satisfactory results for pictures in half tones.
The following compound gives a paper much more sensitive, but not keeping so long, than that prepared according to the formula previously given:
| Tartaric acid | 25 parts |
| Ferric chloride, solution at 45 deg. Baumé | 80 parts (in volume) |
| Water | 100 parts |
When the acid is dissolved, add gradually concentrated aqueous ammonia, just enough to neutralize the solution—170 volumes, about. The chemical change consists in the formation of ferric tartrate. Let cool the solution, then, after adding the following, keep it in the dark:
| Potassium ferricyanate | 21½ parts |
| Water | 100 parts |
Another and very sensitive preparation is the following:
| A. | Iron perchloride, cryst | 40 parts |
| Oxalic acid | 10 parts | |
| Water | 100 parts | |
| B. | Potassium ferricyanate | 20 parts |
| Water | 100 parts | |
| Mix |