Solution of gallic acid for developing—a saturated solution.
Solution of hyposulphite of soda, of the strength of 1 part of the salt to from 6 to 8 parts of water.
For both the wet and dry process I iodize my paper as follows :—In a tray containing the above solution I plunge, one by one, as many sheets of paper (twenty, thirty, fifty, etc.) as are likely to be required for some time. This is done in two or three minutes. I then roll up loosely the whole bundle of sheets while in the bath; and picking up the roll by the ends, drop it into a cylindrical glass vessel with a foot to it; and pour the solution therein (enough to cover the roll completely); in case it should float up above the surface of the solution, a little piece of glass may be pushed down to rest across the roll of paper and prevent its rising. The vessel with the roll of paper is placed under the receiver of an air-pump and the air exhausted; this is accomplished in a very few minutes, and the paper may be left five or six minutes in the vacuum. Should the glass be too high (the paper being in large sheets) to be inserted under the pneumatic pump-receiver, a stiff lid lined with India-rubber, with a valve in the centre communicating by a tube with a common direct-action air-pump may be employed with equal success. After the paper is thus soaked in vacuo it is removed, and the roll dropped back into the tray with the solution, and thus sheet by sheet picked off and hung up to dry. when, as with all other iodized paper, it will keep for an indefinite time.
"I cannot say that I fully understand the rationale of the action of the air-pump, but several valuable advantages are obtained by its use:—1st, The paper is thoroughly iodized, and with an equality throughout that no amount of soaking procures, for no two sheets of paper are alike, or even one perfect throughout in texture, and air-bulbs are impossible. 2d, The operation is accomplished in a quarter of an hour, which generally occupies one, two, or more hours. 3d, To this do I chiefly attribute the fact that my paper is never solarized even in the brightest sun ; and that it will bear whatever amount of exposure is necessary for the deepest and most impenetrable shadows in the view without injury to the bright light".
To begin with the wet process. Having prepared the above solution of aceto-nitrate of silver, float a sheet of the iodized paper upon the surface of this sensitive bath, leaving it there for about ten minutes. During this interval, having placed the glass or slate of your slider quite level, dip a sheet of thick clean white printing (unsized) paper in water, and lay it on the glass or slate as a wet lining to receive the sensitive sheet. An expert manipulator may then, removing the sensitive sheet from the bath, extend its sensitive side uppermost on this wet paper lining, without allowing any air-bubbles to intervene ; but it is difficult ; and a very simple and most effectual mode of avoiding air-globules, particularly in handling very large sheets, is as follows:—Pour a thin layer of water (just sufficient not to flow over the sides) upon the lining paper after you have extended it on your glass or slate, and then lay down your sensitive paper gently, and by degrees, and floating, as it were, on this layer of water, and when extended take the glass and papers between the finger and thumb by an upper corner, to prevent their slipping: tilt it gently to allow the interposed water to flow off by the bottom, which will leave the two sheets adhering closely and perfectly, without the slightest chance of air-bubbles; it may then be left for a minute or two standing upright in the same position, to allow every drop of water to escape; so that when laid flat again, or placed in the slider, none may return back again and stain the paper. Of course the sensitive side of the sheet is thus left exposed to the uninterrupted action of the lens, no protecting plate of glass being interposed ; and even in this dry and warm climate, I find the humidity and the attendant sensitiveness preserved for a couple of hours.
In preparing sheets for use when dry for travelling, etc., I have discarded the use of previously waxed paper, thus getting rid of a troublesome operation, and proceed as follows :—Taking a sheet of my iodized paper, in place of floating it (as for the wet process) on the sensitive bath, I plunge it fairly into the bath, where it is left to soak for five or six minutes; then removing it, wash it for about twenty minutes in a bath, or even two of distilled water, to remove the excess of nitrate of silver, and then hang it up to dry (in lieu of drying it with blotting-paper.) Paper thus prepared possesses a greater degree of sensitiveness than waxed paper, and preserves its sensitiveness, not so long as waxed paper, but sufficiently long for all practical purposes, say thirty hours, and even more. The English manufactured paper is far superior for this purpose to the French. To develope these views a few drops of nitrate of silver are required in the gallic acid bath, and they are finely fixed and waxed as usual".
It will be apparent to the careful reader, that all the processes given, are in all essential particulars the same. To a few simple alterations in the manipulatory details are due all the variations in sensibility, and in the general effect of the resulting picture.
The main principles are :—
To iodize the paper—that is, to secure a uniform coating of iodide of silver over every part of the surface of the paper, and an entire absence of either the alkaline or the metallic nitrate.
To have an excess of nitrate of silver spread over the iodide a short time previously to using the paper; and if a high degree of sensibility is required, the combination of such an organic decomposing agent as gallic acid.
In all the photographs obtained by this process, the impressions from green leaves are very imperfect. This is only to be obviated by adopting the advice of Herschel, and substituting the bromide for the iodide of silver. The following remarks are so much to the point, that I have transferred them from the Journal of the Photographic Society :—