Sensitizing the Tissue.
The carbon tissue comes only in rolls of 2½ feet by 12 feet, price $3.00, not cut. It is not sold in a sensitive condition. Full instructions with each roll for sensitizing. Tap water will do, but I would suggest distilled water for making the sensitizing solution of bichromate of potassium.
| Bichromate of potassium | 1 ounce |
| Water | 16 ounces |
| Alcohol | ½ drachm |
| Ammonia | 12 drops |
The best way to sensitize the tissue, is to place the tissue face up, keeping it flat so that the solution reaches all parts at once, removing all air bubbles, and rubbing in the solution with the fingers until pliant; the time of immersion is three minutes in winter, two minutes in summer. The hands should be washed directly after handling the solution, and care must be taken that there are no cuts on the fingers, as the solution is very harmful, but if due care is exercised and the hands well washed immediately with soap, little, if any, trouble will be experienced; use rubber finger tips as much as possible. Keep the temperature of the solution at 70 deg. both in summer and winter. Take a piece of glass free from scratches (an inch larger all round than the tissue); have the glass ready cleaned with ammonia and talcum powder of fine whiting, squeegee the sensitized carbon tissue directly from the solution on to the glass and place to dry at night in a light-tight box; it will be dry in the morning.
The tissue is in the best condition for three days after sensitizing; it can be used up to seven days; it gradually increases in sensitiveness from day to day. After a week or ten days has elapsed, it is hard to manage and becomes unreliable. When the tissue is dry take a sharp knife and cut inside the edge, and strip off one corner. Fairly good results can be reached by simply drying over a curved piece of pulp board, which is tied with string on each end, but squeegeeing on glass gives a sharper result. The addition of ammonia and alcohol to the sensitizing solution makes it easier to strip the paper from the copper plate, after the carbon tissue is mounted on the plate, and enables one to develop the resist in the water at a lower temperature than without it, thus avoiding pits in the darker portions and white specks or bubbles in the lights, should the water reach too high a temperature.
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.