Alcohol .. .. .. 900 parts
Ether .. .. .. .. 1800 „
Pyroxyline .. .. .. 60 „
Plain collodion (as above) 700 parts
Alcohol........ 450 "
Ether .. .. ... .. 150 "
Cadmium iodide .. .. 14 "
Sodium bromide .. .. 10 „
Alcohol...... .. 100 „
We have given the proportions in "parts," which any photographer may interpret by gr. or dr. so as to suit his own idea as to the quantities desirable to be made at a time. But we must make one observation, which is this: It is necessary that the collodion be tough; but seeing that the longer a collodion is kept the less tough, or more rotten, it becomes, it is not desirable to make too large a quantity at a time.
The strength of the silver nitrate bath should not exceed 30 gr. to the oz. The developer, too, must be somewhat weak, consisting of 12 gr. iron proto-sulphate to the oz. of water, together with 1 dr. acetic acid and a few drops of alcohol, although the latter may be omitted if the developer flows smoothly.
The glass on which the picture is to be taken should have a coating of a solution of a wax or paraffin in ether, which must be rubbed off with a dry cloth. This leaves a very thin film that facilitates the removal of the collodion at a later stage. In exposing, according to the side of the negative that is turned toward the lens, so will the subsequent transparency be reversed or not; and it need scarcely here be said that the image, when finally placed upon the wood block, must be reversed, so as to print direct after it is engraved.
It is impossible here to give data for exposing, as this must be determined by a few trials. It is better to employ a lens with a small stop, and give a liberal exposure, having the negative directed either to a uniformly lighted portion of sky or backed at some little distance by a white card inclined backward. When developed, the lights must be absolutely transparent, and there must not be a trace of fog observable on the picture; nay more, the whole picture must be so thin and transparent as to permit the details of the shadows to be plainly seen when the plate is laid face down upon a sheet of paper. After fixing with cyanide and washing, tone by the application of a solution of platinum chloride, 1 gr. to 8 oz., or of a strength sufficient to penetrate throughout the thickness to the image in about a minute. It is recommended to add tartaric acid to the platinum solution, in the proportion of 5 gr. for each gr. of the metallic salt. When toned, the transparency, without being allowed to become dry, must be placed in a bath of diluted sulphuric acid, 1 oz. acid to a pint of water.
This serves to detach the film from the glass.
But previous to the operation just described, the wood block must have been prepared. Place in a porcelain vessel 80 gr. Nelson's gelatine, or that of any other good maker, and cover it with cold water. Allow it to stand for 2-3 hours to absorb as much as it can; then drain off the superfluous water, and add 10 oz. warm water. If this does not cause the gelatine to dissolve, place the vessel near the fire, and it will speedily liquefy. Having rubbed up 30 gr. zinc oxide in a mortar with a little water, add it to the gelatine, and filter through linen into a wide-mouthed glass bottle. A few drops of carbolic acid will prevent decomposition, if it is to be kept any considerable time. Next apply to the surface of the wood a paste made of zinc oxide and water, and rubbed by the palm of the hand, and then apply the gelatine by means of a broad camel-hair brush. This must be allowed to dry spontaneously.
Returning to the collodion picture in the acidulated water, it occasionally requires a little time, although sometimes only 1-2 minutes, to ensure the film becoming quite detached from the glass. When this is the case, a sheet of stiff waxed or paraffined paper is introduced, and the film is lifted out of the water by its agency. An easy way of doing this is to operate in a deep wooden dish having a plugged hole in the bottom. Lay the sheet gently down upon the collodion film, still in situ on the glass plate, although not now adhering to it; then, by withdrawing the plug, let the water run off, thus enabling the glass plate with the collodion film and the paper to be removed without disturbance.
The surface of the wood having been made wet by drawing a broad camel-hair brush dipped in cold water over it, the paper, to which the film now adheres in preference to the glass, is gently lifted up from the latter, and superimposed on the wood block, collodion side down. A sheet of blotting-paper is placed upon it, and over that a piece of rubber cloth, and moderately smart friction or pressure is applied to ensure the attachment of the collodion to the gelatined surface of the wood. By means of a penknife the margin of the paper is then raised, and the sheet lifted from the block, to which the film now adheres. This adhesion is rendered more firm by placing the block for a few minutes in a warm place, sufficient to impart tackiness to the still wet gelatine by which it was sized. To prevent the wood from warping, at this stage the back of the block should be sponged with water. Some operators effect the required adhesion by holding the surface of the block to the fire for a few seconds. But care must be taken not to let the collodion become dry.
The next operation consists in removing the collodion and leaving the image remaining on the wood. This is expeditiously effected by pouring over the surface first a little alcohol, following this by ether. If a good quality of soluble cotton has been employed, the collodion quickly dissolves by the method described. The wood is not effected by either alcohol or ether. When dry, the block is ready for being placed in the hands of the engraver, or in those of the artist to have the details supplemented by a few pencil touches, or for the removal of portions not desired to be engraved.
Although the film of gelatine upon the wood is so thin as not to clog the point of the graver, it may be rendered still more attenuated by increasing the proportion of water in the gelatine solution. (J. Traill Taylor.)