The carbon tissue used as a resist, which is mounted on the copper plate, is made by the Autotype Company, London, England. No. 100 Standard Brown is the right grade to use, though I have reached good results with No. 103. The No. 100 is a heavier grade than No. 103, and requires two or three minutes longer exposure than the latter. Use a deep printing frame with a screw pressure to secure absolute contact, which is known by iridescent markings appearing on the glass of the printing frame. A Johnson's actinometer is very useful to time the exposure. From 4 to 6 tints are necessary. Experience here is the only guide, as the light varies as well as the density of the negative and the sensitiveness of the tissue. If one does not have an actinometer, a slip of albumen paper may be used; as soon as the paper has reached the darkest point, which is then called one tint, extend it so that a fresh portion comes out to the light, and so on for the different tints. In September, for instance, the darkest tint is reached in about 3 to 4 minutes; two tints and a half or 8 minutes in the shade at midday on a clear day in September is about right,—this is understood to be with medium negatives and No. 103 tissue sensitized within three days.

You should over-expose rather than under-expose, allowance being made when the acid is used. Print deeply, so that, on development, the negative tissue on the copper plate shows all the detail clearly in the shadows. The tissue should not appear very dark on the plate. The copper should show up through the gelatine clearly and brightly. The thinner the negative tissue, the quicker the biting of the acid.

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.

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.