By HENRY W. BENNETT, F.R.P.S.
Although it cannot be claimed that the carbon process is the most simple of photographic printing methods, it presents no special difficulty. If the work is undertaken in a systematic manner success is certain. The process is essentially different from all others, both in principle and in working. In commencing, new methods have to be encountered ; but it is novelty that presents itself, not difficulty.
The following notes have been written as an introduction to carbon printing for those who know nothing of the process. Everything essential is given so that successful results may be obtained from first essays. But no attempt has been made to give any other than the simple instructions necessary for preliminary work. The many variations possible would require more space than can well be given here.
The beauty and distinctive quality of carbon prints are too well-known to require description. These characteristic qualities induce most artistic workers to adopt carbon as their favourite medium ; and its flexibility or adaptability to varied tastes and requirements in the hands of those who have become familiar with its working, is an inducement for selecting it in preference to processes that are not capable of so much variation.
Henry W. Bennett, F.R.P.S.
Paper prepared for carbon printing is called carbon tissue. It consists of a stout strong paper carrying a film of coloured gelatine. This film is perfectly soluble, but becomes insoluble on exposure to light. By printing under a negative the degree of insolubility will vary in different parts of the film according to the extent of the action of the light transmitted through the various parts of the negative. After printing, the soluble portions of the film are dissolved away by soaking in hot water, the parts that remain form the picture. The colour of the finished print is determined solely by the pigments mixed with the gelatine when the film is manufactured. In desscribing the process it sounds clumsy ; in practice it is capable of yielding the finest and most delicate results.
Carbon tissue may be purchased either sensitive or insensitive, and in a variety of colours. Sensitive tissue will only remain in good condition for about ten days ; insensitive will keep indefinitely. Sensitizing is a simple operation, and the most satisfactory method for amateur workers is to keep a stock of insensitive tissue of various colours, and to sensitize to suit the requirements of the moment. The author has introduced an improved sensitizing solution, by means of which tissue may be sensitized so as to give results equal in every respect to that sensitized in course of manufacture.
The formula is :-
The potassium bichromate and citric acid should be dissolved separately in hot water, the solutions mixed, and sufficient ammonia added to change the colour of the solution from orange red to lemon yellow. It is imperative that the ammonia be added immediately after mixing the two solutions. The solution, if mixed as described, will keep indefinitely, and may be used many times in succession for sensitizing. It should be used at a temperature as near 60 degrees as possible.
For sensitizing, sufficient of the solution should be poured into a dish, a piece of tissue immersed and allowed to remain for ninety seconds. It should then be withdrawn, laid face downwards on a piece of glass or slate, and gently squeegeed so as to remove as much of the sensitizing fluid as possible. It should then be lifted from the glass, pinned to a lath by two corners, and hung up to dry. Drying will require from four to six hours. Sensitizing may be carried out in full daylight, but the drying must take place in a room or cupboard darkened to such an extent as not to affect silver printing-out paper. The most satisfactory method is to sensitize at night and remove the tissue from the drying bath the following morning.
In printing, the dark side of the sensitive tissue must be placed next to the film of the negative. No visible image is produced in printing ; consequently, the time of exposure in the printing frame must be gauged by means of an actinometer. Several types of actinometer may be obtained commercially, the most satisfactory being that which consists of a series of partially transparent numbered squares. These squares vary in density from the lowest to the highest number, so that by exposing a piece of silver printing-out paper to daylight for a short time a faint image of the square bearing the lowest number is produced. Longer exposures will strengthen the image of the first square, and produce faint images of the other squares. The method of using is to place a piece of silver paper in the actinometer and place the instrument at the side of the frame containing the carbon tissue when that is put out to print. When the carbon print is taken in, the actinometer is carefully examined. The highest square that can be seen on the actinometer print is the number that registers the printing of the carbon tissue. If this is found to be correct, future prints from that negative may always be secured with certainty by printing to that number in the actinometer. It is imperative to always use the same brand of silver paper for the actinometer.