CHAPTER IV. The Cleaning and Graining of the Copper Plate, and Grade of Copper Necessary, and Where and How to Buy It at Reasonable Prices.

The best copper is recognized by its rosy lustre. Pure copper only should be used. It can be purchased ready polished and beveled from several firms in New York. The best way, if large quantities of plates are required, is to buy the copper in the rough of one firm and have it polished by another, and bevel it yourself if necessary with a file and burnish it by hand, or the firm who polish it will do the beveling for you.

The Scovill & Adams Co. supply copper-plates of the finest quality, ready polished, for photogravure work.

Total cost by this method about one-third less than by purchasing ready made. It makes a copper plate 1/16 grade, 9 × 11, cost about $1.10. Order your copper 1/16 in. grade up to 10 × 12; larger sizes 1/8 in. grade. If you use 1/16 in. grade above this size, the plate is liable to buckle. Be sure the plate is free from pits and scratches and with a high polish. Have what the polishers and engravers call a rouge polish. If they do not supply it, rouge it yourself with powdered rouge and turpentine, using a ball of absorbent cotton over a large piece of smooth cork. A good way to buy rouge is in the stick; it is more economical. Rub the wet cotton on it and the right quantity is assured. Pits in the copper may be taken out by tapping upon the back with a nail set, using a small piece of polished steel to lay the face of the plate on, and localizing the spot with a pair of calipers. The part raised by the tapping, cut away with the scraper, then rub the spot with Scotch stone and water, then a piece of engraver's charcoal (cut to a pencil point), with machine oil; then burnish with the regular engraver's burnisher and sperm oil, finishing with rouge and refined turpentine.

When the plate is well polished, make a strong solution of caustic potash (C.P.), which comes in sticks, as strong as possible, as long as it does not stain the copper. It should register about 40 deg. with an actinometer used to test silver solutions.

Take a piece of absorbent cotton and clean the copper with potash (by the way, use finger tips); rinse under tap for five minutes, then a fresh piece of cotton with alcohol at 95 per cent., rinse again with water, and place in warm water for final rinsing; stand up on corner, or place in drying frame usually used for negatives; allow to drain. Should any stains appear, it must be recleaned and all the operations repeated until it drains off without streaks, for these streaks and spots of stain are caused by the caustic potassa, which is difficult to remove. It is as hard to get rid of from the copper as hyposulphite is from a negative. These streaks retard the acid on the copper wherever they appear, and cause defects in the recording of the original tones of the negative.

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.

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.