Make about a pint bottle of saturated solution of gallic acid, having acid in excess, and using distilled water ; decant a portion into a smaller bottle for general use, and fill up the other bottle ; you will thus always have a clear saturated solution.
Pour upon a slab of glass, kept horizontal, a little of this liquid, spreading it equally with a slip of paper, then apply the paper which has been exposed in the same manner as described for the negative paper, being careful to keep the back dry. Watch its development, which is easily observed through the back of the paper; you may leave it thus as long as the back of the image does not begin to spot.
When it is rendered very vigorous, remove it quickly to another clean slab, and well wash it in several waters, occasionally turning it, and gently passing the finger over the back ; by this means you remove any crystals of gallic acid which might spot the picture.
The appearance of the image at the end of this process will enable you to judge if it was exposed in the camera the proper time.
If it becomes a blueish grey all over, the paper has been exposed too long ; if the strongest lights in the object, which should be very black in the negative, are not deeper than the half-tints, it has still been too long exposed; if, on the contrary, it has been exposed too short a time, the lights are but slightly marked in black.
If the time has been just right, you will obtain a proof which will exhibit well-defined contrasts of black and white, and the light parts will be very transparent. The operation is sometimes accelerated by heating the gallic acid, and by this process the dark parts of the picture are rendered very black.
To fix these negative proofs, a very strong solution of hyposulphite of soda,—about 1 ounce of the hyposulphite of soda to 8 fluid ounces of water,—is employed, and the picture is allowed to remain in it until every trace of yellowness is removed from the paper.
The use of albumen on paper has not been extensive, nor do I conceive that it offers any peculiar advantages. The best mode of proceeding is to beat the white of eggs into a froth; then set the fluid aside, at rest, and, when perfectly clear, make with it the following solution :—
Pour the solution into a dish placed horizontally, taking care that the froth has entirely disappeared; then take the paper that you have chosen, and wet it on one side only, beginning at the edge of the dish which is nearest to you, and the largest side of the sheet, placing the right angle on the liquid, and inclining it towards you; advance it in such a manner as to exercise a pressure which will remove the air-bubbles. Place before you a light, so as to be able to perceive the bubbles, and to push them out if they remain.
Let the leaf imbibe for a minute at most, without touching it; then take it up gently, but at once, with a very regular movement, and hang it up by the corner to dry.
You prepare thus as many leaves as you wish in the same bath, taking care that there is always about a quarter of an inch in depth of the solution in the dish; then place your sheets (thus prepared and dried) one on the other between two leaves of white paper, and pass over them several times a very hot iron, taking out a leaf each time ; you will thus render the albumen insoluble.
The iron should be as hot as it can be without scorching the paper.
Then use this negative paper exactly like the first paper named ; only great attention must be observed that the immersion in the aceto-nitrate bath is instantaneous, and that the air-bubbles are immediately driven out; for every time you stop you will make stains on the paper. It is also necessary to warm moderately the gallic acid.
One of the best services rendered by the albumen to photo-graphy is, without doubt, its application to the preparation of the positive paper, to which it gives a brilliancy and vigour difficult to obtain by any other method. It is prepared thus :—
Take white of eggs, to which add the fifth part by volume of saturated solution of chloride of sodium; then beat it into a froth, and decant the clear liquid after it has settled for one night.
With this the paper is first washed, and then a strong solution of nitrate of silver applied.
The following is the process adopted by M. Blanquart Everard :—
The paper prepared by means of albumen possesses properties analogous to those prepared by means of serum, but in a much less degree; the former, like the latter, may be kept for an indefinite time after its preparation with the iodide of potassium, but after having been submitted to the action of the aceto-nitrate of silver, it will not keep good beyond the next day. The impressions obtained by means of the following preparation are admirable : though not so well-defined as those on glass, yet they are more beautiful, as the outline is less harsh, and they possess more harmony and softness. It must not be forgotten that the preparations with albumen change with comparative slowness.
Beat into a froth the whites of eggs, to which a saturated solution of iodide of potassium and bromide of potassium has been added in the proportion of thirty drops of the former and two drops of the latter for the white of each egg; let the mixture stand until the froth returns to a liquid state, filter through clear muslin, and collect the albumen in a large flat vessel. On this lay the paper to be prepared, and allow it to remain there some minutes. When it has imbibed the albumen, lift it up by one of its comers; let it it drain, and lastly dry, by suspending it with pins to a line or cord across the room. The subsequent preparation with the aceto-nitrate of silver is in every respect similar to that for the ordinary Calotype paper; care being taken not to dry it between the two folds of blotting-paper until it has become perfectly transparent. The exposure of the prepared paper to the light in the camera is done in the same way, and the same treatment with gallic acid is followed: it will, however, be found that the time required for exposure will generally be four or five minutes.
The positive paper prepared with albumen gives impressions somewhat shining, but of a very rich tone, well defined, and of perfect transparency; it is prepared in the following manner:— To any quantity of white of eggs add 25 per cent, by weight of water, saturated with chloride of sodium; beat into a froth, and filter as in the previous operation—only in this case leave the paper in contact with the albumen for half a minute; hang it up to dry, which it usually does in six to eight minutes; then lay it on a vessel containing a solution of 25 parts of nitrate of silver in 100 parts of water. Leave the paper on the solution for at least six minutes, then place it on a plate to dry.
The serum of milk has also been employed on paper as a quickening agent, and some of the French authorities speak highly of it; but I am not enabled, from my own experience, to speak of its advantages.