Photo-engraving, Photo-etching and Photo-lithography in Line and Half-tone / Also Collotype and Heliotype

by W. T. Wilkinson

CARBON PRINTING.

In Chapters III. and IV. on engraving on copper, the resist and relief are made by what is called in photographic nomenclature the carbon process—that is, printing in gelatine to which has been added a suitable pigment. This pigmented gelatine, spread upon paper and dried, is known in commerce as carbon tissue, and is sold of various colors and degrees of density as regards the quantity of pigment incorporated with the gelatine. For our purpose that tissue containing the minimum quantity of pigment is the best (this only applies to the resist or relief on the copper, not to the transparency necessary for printing the resist from; in this case what is known as transparency tissue must be used, and that is made with the maximum quantity of pigment (India ink) incorporated with the gelatine), but as the subject of making transparencies in carbon has already been treated of, the present notes will be confined entirely to developing carbon prints upon copper for resists (Chapter III. for reliefs Chapter IV.).

The carbon tissue of commerce is not sold in a sensitive condition; therefore, the first care will be to render it sensitive to the action of light, which is done by immersion in a solution of bichromates of potash and ammonia.

First of all dissolve in eighty ounces of water one and a half ounces of bichromate of ammonia, and one ounce of bichromate of potassium, and filter. Then cut the carbon tissue into suitable sized pieces—i. e., pieces a little larger than the subject on the transparency or negative to be used, and a little smaller than the copper plate upon which it is intended to develop the carbon print; next select a sufficient number of glass plates quite free from scratches a little larger than the pieces of tissue, now clean these glass plates carefully by polishing with clean linen or silk cloth free from lint, then dust over with French chalk and again well polish; then coat with thin plain collodion, and when the film is well set place the collodionized plate in clean cold water until the greasy appearance that shows on the first immersion in the water has disappeared.

Place the filtered bichromate solution in a clean dish, then immerse the pieces of tissue, one at a time, taking care that no air bells are allowed to {107} remain upon the surface; if any form, remove them by bursting them with the fingers. After the tissue has been in the solution for three minutes, take one of the collodionized and washed plates from the water and remove one of the pieces of tissue from the bichromate bath, place the two face to face, lowering the tissue gently down upon the collodion film, then place the glass plate upon a bench, lay a piece of Macintosh cloth (larger than the plate) over the tissue, and with a squeegee gently scrape along the Macintosh; this will bring the carbon tissue into contact with the collodion film, and also expel superfluous bichromate solution, but care must be taken not to remove too much of the bichromate else the tissue will be insensitive and print hard. The plate is now placed upon a rack, and all the remaining pieces being done in the same way are also placed upon the rack.

The tissue being thus squeegeed into optical contact (the squeegee is made with a strip of soft rubber fixed between two pieces of wood) with the collodionized glass, it must be dried in the dark room in a current of warm air, not higher than 60° F., else the pigmented gelatine will melt and be useless (bye the bye, the bichromate solution in hot weather should be iced so as to keep the temperature below 50° F.).

When the tissue is dry it is stripped from the glass and stored in a tin tube, and care must be taken to keep it away from the light, as if the tissue be once exposed to the light even for a short time, the action will go on in the dark.

In treating of the transparencies, it is mentioned that the subject on the transparency or negative is marked, this being necessary to form what is called a safe edge to the carbon print. If such a safe edge be not provided, the carbon print will not adhere to the copper during development; therefore, the extreme edges of the tissue must be guarded from the light during the exposure in the printing frame.

The exposure to light under the negative or transparency must be timed by means of an actinometer, which may be improvised by dividing a strip of tracing paper into ten spaces, and numbering these spaces so: