The plate is then ready for graining.
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.
