The use of organic matter in facilitating the change of the silver salts very early engaged the attention of Sir John Herschel; and from time to time, following his suggestions, others have employed various organic matters, albumen and gelatine being the favourite substances. These have been principally used for the purpose of spreading photographic preparations on glass, which we shall have particularly to describe : at the same time, they are stated to have been employed with much advantage on paper by some photographists. For the negative pictures, Gustave Le Gray gives us the following directions and particular information :—
Dissolve three hundred grains of isinglass in one pint and three quarters of distilled water (for this purpose use a water bath).
Take one-half of this preparation while warm, and add to it as under :—
Iodide of potassium..... 200 grains.
Bromide of ditto......60 „
Chloride of sodium .....34 „
Let these salts be well dissolved, then filter the solution through a piece of linen, put it, still warm, in a large dish, and plunge in your paper completely, leaf by leaf, one on the other, taking care to prevent the air-bubbles from adhering to the paper.
Put about twenty leaves at a time into the dish, then turn the whole, those at the top to the bottom, then take them out one by one, and bang them by one corner with a pin bent like the letter S, to dry spontaneously.
When hung up, attach to the opposite corner a piece of bibulous paper, which will facilitate the drying.
When the paper is dry cut it the size required, and preserve it in a folio for use; this paper may be made in the day-time, as it is not sensitive to light in this state.
The bromide does not, in this case, act as an accelerator, as it does on the silver plates of the daguerreotype, because, instead of quickening, it retards the operation a little ; its action is to preserve from the gallic acid the white of the paper, which would blacken more rapidly if you employed the iodide of potassium alone.
Prepare, by the light of a taper, the following solution in a stoppered bottle :—Distilled water, 6 fluid ounces; crystallized nitrate of silver, 250 grains.
When the nitrate is dissolved, add 1 ounce of crystallizable acetic acid : be careful to exclude this bottle from the light, by covering it with black paper. This solution will keep good until the whole is used.
When you wish to operate, pour the solution upon a porcelain or glass slab, surrounded with a glass or paper border to keep the liquid from running off. I usually take the solution out of the bottle by means of a pipette, so as to prevent the distribution of any pellicle of dust or other impurity over the glass slab.
Take a sheet of the iodized paper by two of the corners, holding them perpendicularly, and gently lower the middle of the paper upon the centre of the slab; gradually depress until the sheet is equally spread: repeat this operation several times until the air-bubbles disappear; take also the precaution to keep the upper side of the paper dry.
In order to prevent the fingers from spotting the paper, pass a bone paper knife under the comer of the sheet, to lift it from the slab between that and the thumb.
Let the sheet remain upon the slab until the formation of the chloro-bromo-iodide of silver is perfect.
This may be known by the disappearance of the violet colour which the back of the paper at first presented; it must not be left longer, otherwise it would lose its sensitiveness.
The time required to effect this chemical change is from one to five minutes, depending upon the quality of the paper.
Spread upon a glass, fitted to the frame of the camera, a piece of white paper well soaked in water; upon this place the prepared sheet, the sensitive side upwards.
The paper which you place underneath must be free from spots of iron and other impurities.
It is also necessary to mark the side of the glass which ought to be at the bottom of the camera, and to keep it always inclined in that direction when the papers are applied ; if this precaution is neglected, the liquid collected at the bottom, in falling over the prepared paper, would not fail to produce spots. The paper thus applied to the glass will remain there for an hour without falling off, and can be placed within that time in the camera.
When I am going to take a proof at a distance, I moisten the sheet of lining paper with a thick solution of gum arabic, and can thus preserve for a longer time its humidity and adhesion. I can also in this case make use of two glasses between which the paper is placed, according to the direction of M. Blanquart Ever-ard; but it is necessary to take great care that the plates of glass are perfectly clean, and to have them re-polished if scratched.
I employ for this purpose blotting-paper to clean them, as well as my plates ; it is much superior to linen, and absorbs liquids and impurities that adhere to it. I never spare the blotting-paper, for I would rather use a leaf too much than be uncertain about the cleanness of my glass.
When the sheet of lining paper adheres well to the glass, it should not be removed, but only moistened afresh with water, after which you may apply another sheet of the sensitive paper.
In preparing several sheets of the sensitive paper at a time, it is not necessary to wash the slab for each sheet; you need only draw over it a piece of white paper to remove any dust or pellicle formed.
When your operations are finished, you may pour back the aceto-nitrate of silver into a bottle, and reserve it for another time.
The necessity of employing M. Le Gray's papers in a wet state is their most objectionable quality, but certainly the results obtained by strict attention to his directions are often exceedingly beautiful. For developing the image the following is recommended, which does not, however, differ essentially from the developing processes already described.