II.—Graining the Copper Plate.
A grain is required on the copper plate so that the tones will be reproduced, as copper has not a sufficient grain of its own. The grain is given to the copper plate by dusting it with powdered Syrian asphaltum or resin. Have a paste-board box made 18 inches high, 12 inches wide and 8 inches deep, perfectly air-tight, with a small door running the whole length on the widest side, an inch or two from the bottom. Have the inside of the box perfectly smooth; place within the box 4 ounces finely powdered Syrian asphaltum (sold by Messrs. Theodore Metcalf & Co., Tremont Street, Boston, Mass.); it is difficult to find in New York. Shake the box vigorously, place on table, insert a piece of wood an inch high made in shape of cross (or open square, or have netting of wire raised an inch from the bottom of the box); the copper plate, previously cleaned, is at once placed face up upon it. Instead of shaking the box it can be arranged upon supports (see fig. 1), and revolved.
Fig. 1.
Close the door instantly, and let the plate remain about two minutes; carefully remove the plate and place it on a Florence oil lamp, holding the plate with a hand vise, watch carefully until the powder disappears from the surface and the plate slightly smokes, then stand aside to cool. Do not keep the plate too long on the heater, or the particles of dust will run together, forming an impenetrable varnish over the plate. This part of the process is not difficult, but requires practice. Preserve each atom of dust as much as possible, examine with magnifying glass and, when cool, test with finger nail; if it rubs off easily, it has not been heated enough; then the plate must be re-cleaned and again powdered. To get a good all-round working grain, suitable for medium subjects, the plate should be placed at once in the box after shaking; thus the coarser particles that fall first, and the finer, which gradually settle, will combine after two or three minutes.
Many combinations will be suggested to the student by practice to suit the subject; for instance, waiting for two minutes and then inserting the plate, gives a fine grain for delicate subjects. Powdered dragon's blood (resin) in combination with asphaltum makes a beautiful grain; a separate box may be used for the dragon's blood; the asphaltum first dusted on the plate, then inserted in the dragon's blood box for twenty or thirty seconds, then melted together. The dragon's blood melts first, then the asphaltum.
The air brush is also used by professionals; it throws a resinous spirit varnish, coarse or fine, as required.
CHAPTER V.
Development of Negative Resist on the Copper Plate, and Preparation for Biting with Acid through the Gelatine.
Have a wooden box made 24 inches long, by 12 inches high, 12 inches wide, with door 6 inches high on side, fastened with hinges, top and bottom of box open; cover top with sheet zinc. Place inside Florence oil lamp; the door is valuable to regulate the heat. On top of box place deep porcelain tray, 11 × 14, fill with water half full; in the water place two pieces of plate glass ½ inch high and 4 inches long, on which to place copper to keep it from the bottom of the tray. Slide the copper plate into the water, removing all air bubbles, keeping the fingers off the surface of the plate. Take the sensitized and exposed tissue and place face down in the cold water (65 deg.) sliding it gradually in at further end of the paper so as to avoid air bubbles; the instant the paper curls backward, place it over the copper plate and remove it quickly from the water. This has to be done with celerity or it will be found difficult to mount the tissue with the squeegee on the copper, and also it should be exactly placed with reference to the top and sides of the copper; all this, of course, to be done under water, never allow it to slip up out of it. Place plate on table and squeegee into place, stroking firmly from the centre, each way. Place face down on clean blotting paper, under heavy weight for fifteen minutes. While plate is under pressure (which is necessary to enable the gelatine to expand and attach itself to the plate), start the lamp gradually, and by the time the paper is ready the water should register on the thermometer 75 deg. Slide the plate under water removing air bubbles as they appear, with a ball of absorbent cotton; when the heat of the water reaches 90 deg. Fahr., the gelatine commences to ooze from all around the edges of the paper, and after the plate has remained in the water about ten minutes after the showing up of the gelatine (at the temperature from 90 to 95 deg.), take a pin and carefully raise the paper at the corner, gradually pulling away the paper toward the opposite corner, keeping the hand close to the water; should the gelatine which adheres to the plate appear to lift, wait a few minutes longer and start another corner. After the paper is stripped from the plate, gently develop the negative resist with a piece of fresh absorbent cotton, delicately rubbing the surface, edges first, and lave the plate up and down in the water, keeping the temperature steadily at 90 to 95 deg., by raising or lowering the lamp. (Should the paper be under-exposed and appear very black on the copper, develop at 100 to 110 deg., not over. If over-exposed it will appear very thin, and the heat of the water must not go over 90 deg.; it will strip at 88 to 90 deg.) Then the negative image gradually appears, the darks first, which are of course the brightest portions; when all detail appears in the shadows and the negative stands out clear and bright, take it out of the dark-room (which is lighted with an ordinary lamp), and gently wash under the tap with clean and cold water at 65 to 70 deg.
Dry the resist with alcohol, pouring it over the plate from one end, starting with half alcohol and half water, gradually adding more alcohol and eliminating the water, until a final flooding with absolute alcohol is reached; use fresh solutions of alcohol and water for each copper; don't use old alcohol for anything except cleaning the copper at the end, and for removing the spirit varnish. Stand up to dry against the wall, face out, and standing square on the bottom of the plate, in the same position as you flooded it with alcohol; it will be dry in twenty minutes if rightly flooded. The bare copper should now be protected by a strong varnish in alcohol (it must flow freely off the brush); a good varnish for this purpose, and the best I know of, is an etcher's asphaltum stopping-out varnish, sold by Messrs. Devoe & Co., New York; price 50 cents per bottle. Should it get thick as you come to the bottom of the bottle, add a little spirits of lavender until it flows again freely. Take an architect's ruling pen and carefully rule a line with the varnish up to the edge of the picture, making it exactly true with the sides of the plate and the space on each side of the work the same with the top, the bottom space slightly larger; make sure that it slightly comes inside the picture. Keep the rule away from the surface of the gelatine, as it is very delicate. Then cover all the rest of the copper, protecting the bare parts and bevel, and bringing the varnish up to the line. Allow to dry hard; about twenty minutes will do. Form a wall about the resist, with walling wax, about an inch high; make a lip at one corner, the further left hand one, for instance; see that there are no leaks. There are several grades of wax, but Liedel & Co.'s is the best; when ordering you should give the name as modeling wax; gray is a good color. Pans can be used made of tin and varnished, or porcelain trays, protecting the back and edges of the plate with varnish, but I find the wax very helpful, especially on large plates.
CHAPTER VI.
The Acid Baths.—How to Make Them and Method of Biting through the Gelatine.
Perchloride of iron C.P. is the acid generally used for this purpose; it is a still acid, and if the room is well ventilated no harm to health results, but care must be taken to air the baths after making to get rid of the surplus chlorine.
Four baths are used, each of different strengths, the strongest is used first, the weakest last. I quote from the catalogue of the Boston Art Museum, of the exhibition illustrating the technical methods of the Reproductive Arts and Photo-Mechanical Processes, held January 8, 1892: "Photo-aquatint (photogravure) for the production of half-tone intaglio plates from photographs from nature, paintings, etc. A dry aquatint ground is laid on a metal plate, and over this is mounted a gelatine negative film, made by the pigment printing process. To obtain this negative film a reversed positive on glass has first to be made. The reason why this positive must be reversed will become clear when the nature of the manipulations in the pigment printing process, which involves the turning of the film, are considered. The film mounted on the plate is a washout relief, thickest in those parts which are to show white in the impressions from the plate, and gradually growing thinner toward the darkest parts, where it is thinnest. The film acts as a 'resist' to the mordant, allowing it to pass freely in the thinnest parts, and less freely as it increases in thickness. If, however, the film were mounted on the bare plate, and the biting then proceeded with, the result would be of no practical use, as the plate would present merely shallow hollows, incapable of holding the ink, and which would therefore be wiped out in the attempt to clean the surface of the plate. This is, however, prevented by the aquatint ground, which allows the mordant to circulate only in the channels around the resinous particles of which it consists, and thus produces a grain precisely as in ordinary aquatinting. The mordant used in perchloride of iron, which is a 'still mordant,' i.e., one which does not evolve bubbles of gas. An effervescent mordant cannot be used as the bubbles rising under the film would tear it up. In biting, successive baths of varying strength are made.
"A strong solution of perchloride of iron penetrates only the thinner parts of the film, whereas a weaker acts also through the thicker parts. The biting, therefore, begins with a strong solution, which acts only in the darkest parts, and followed up with weaker and weaker solutions, which continue the biting in the darks and at the same time carry it on gradually toward the lights. If necessary, the plate is worked over with the burnisher to brighten the lights, and with roulettes, etc., to strengthen the darks."
Purchase nine (9) pounds of perchloride of iron in crystals (45 cents per pound), take a wide-mouthed gallon jar, place within half a gallon of distilled water, add the iron until it tests 30 deg. by a Beaumé hydrometer, pour off enough to fill a one-litre glass stoppered bottle, after filtering through absorbent cotton. Keep adding the iron to the jar until the strength of each bath is reached. To the strongest solution add half a drachm of C. P. muriatic acid, and to the weakest half a drachm C. P. nitric acid; the nitric acid is added so that in the last biting a good final nip is given to the copper.
I here give my own formula, with those recommended by others.
The four (4) baths should be well aired for a day (in broad pans) in the open air before filtering.