CHAPTER III.HALF-TONE INTAGLIOS—(Continued).
The next process under consideration is that in which the grain is given to the copper plate by dusting it with fine powder of resin (colophony) or of asphalt. To do this a dusting box is required, which may be an ordinary aqua-tint box hung on centres for the purpose of revolving it when desired to cover large plates with the powder. The following figure will illustrate a box suitable for small work.
Fig. 15.
Fig. 15.
The box should be eighteen inches high, twelve inches wide, and eight inches deep; it may be made of cardboard with the inside quite smooth. It should be closed all around except at A, where there is a small door four inches high; the bottom of the box is either studded with small nails or some coarse wire netting is stretched across, upon which the copper plates can rest, as near level as possible. To use this box, four ounces of resin or asphalt are powdered in a mortar until quite fine. The powder is placed inside the box, and the door closed; the box is now vigorously shaken, then placed on the table, the door opened, and a copper plate (previously cleaned and polished) is at once placed face up on the nails or wire netting. By the time the door is opened and the copper plate or plates are in position, the coarser particles of the resin will have subsided, leaving only the fine dust floating in the interior of the box; now close the door, and leave the copper plate within until it is judged that it is powdered sufficiently, which, of course, will be a matter for the operator to find out by practice.
When the plate has been sufficiently powdered, it is carefully withdrawn and placed upon a hot iron plate and allowed to remain there until the resinous{125}powder is just sufficiently melted to adhere to the copper plate, preserving as much as possible the separate existence of each atom of dust. Take care to stop the action of the heat before the resin is melted completely and made to run together and form a smooth surface.
The heat having acted long enough, carefully withdraw the prepared copper from the hot plate and allow it to cool. Any desired quantity of these plates may be prepared and stored away for future use. When they are required the grain may be selected to suit each picture, as the operator can prepare at will (after practice) either a coarse or a fine grained ground.
Instead of using a hot plate the resin may be melted over a gas flame, but the hot plate will be found the best.
There is another method of laying this aquatint ground that is simple and novel, viz., by using the air brush. Those who are in possession of this instrument need no further instructions than to be reminded that ordinary bitumen dissolved in benzole, or a resinous-spirit varnish, is placed in the color reservoir, and then the brush will coat the clean copper plate with an aquatint as fine or as coarse as required, according to the distance between the plate and the “brush.” Another advantage with this method of laying the ground is, the ground need not be even, as in the dusting method, but the artist may lay the ground coarse in some places and fine in others,verb sap.
The next step in this process will be to get a transparency of the subject. This may be made by any known process (at last, some one says we are getting away from the wet collodion) so long as it is clean, vigorous, and free from fog.
The transparency being at hand, that portion of it required to be transferred to the copper plate is carefully surrounded by a mask of tinfoil, or of non-actinic paper, which will serve as a safe edge to the carbon tissue to be used in making the negative resist, as well as to keep the margin of the plate free from extraneous picture.
This done, a piece of sensitive carbon tissue, a little smaller than the copper plate, which has been previously sensitized with bichromate of potash and dried, is placed upon the masked transparency and exposed to light until sufficiently printed; the tissue is then removed from the printing frame and placed in a dish of clean cold water; then one of the copper plates with aquatinted ground is also placed in the water, and the face of the tissue and the copper plate are brought in contact. After adjusting the tissue on the copper{126}plate, the two are withdrawn from the water, laid upon a bench, and a squeegee passed over the back of the tissue. Thus the driving away of the water from between the two surfaces will cause the tissue to adhere to the copper plate.
Now let the plate stand a few minutes, then immerse it in a dish of warm water at about 90° F. As soon as the tissue compound is melted, lift away the paper backing and throw it aside, then gently lave the copper plate with the warm water until all the soluble pigmented gelatine is dissolved; after that wash it under the tap, and place it in a strong solution of alum for fifteen minutes, and again wash and allow to dry.
When the gelatine image on the copper plate is quite dry, the margin outside the picture is coated with bitumen or spirit varnish, then the plate is placed in the etching bath of perchloride of iron (see page121), in which it is allowed to remain until the high lights under the thickest portions of the gelatine image just change color; then remove it and wash it under the tap; next clean off the resist image, and after drying and cleaning up, try a proof. If necessary, ink up as directed in Chapter I., page121,and rebite.
To succeed with this process it is only requisite to have a well-laid ground, a good, vigorous transparency, and the carbon tissue in good working order.
N. B.—In order not to confuse the above directions by interpolating the working details of carbon printing, these will be given in a chapter further on, together with formulæ for making suitable tissue.