The carbon and platinotype processes are regarded as the most permanent of all the photographic printing processes.
Carbon admits of a long range of colours, as black, brown, purple, red, and green. It depends for its effect upon the action of light on gelatine sensitized by immersion in a solution of bichromate of potassium. The colouring matter is incorporated with the gelatine previous to coating the paper.
When once sensitized the paper will not keep for any length of time - for about a couple of weeks; therefore it should only be bought in the coated and sensitive condition in such quantities as will allow of its being quickly used up, or in the coated but insensitive state, and sensitized as required. The beginner will do well to obtain his paper ready prepared for use for early attempts, and as proficiency is acquired to sensitize his own; ultimately, in special cases, he may even prepare the coating for the paper.
To print, take a piece of plain glass and attach, by means of paste or gum, a strip of opaque paper about a quarter to half-inch wide round the margins. This is to make what is called a "Safe Edge." When the paste has dried, this piece of glass is placed in a printing-frame paper side up; upon it is laid the negative to be printed from, then a piece of carbon tissue, and lastly the back. It is preferable to put a pad between the paper and back, the same as suggested under the heading "Platinotype Printing," and for the same purpose.
It is now put out to print in a good light. Carbon - unlike P.O.P. and platinotype - cannot be examined during printing, so that it becomes necessary to have some kind of guide, by means of which it can be ascertained when the tissue has had sufficient exposure. One way is to select a negative having similar characteristics to the one from which a print is to be made. Under this second negative expose a piece of P.O.P. Place the two frames side by side to print. Examine the P.O.P. from time to time, and when sufficiently printed the carbon may be considered so too. A few experiments on these lines will soon put the worker in possession of some practical knowledge for future guidance. The other and usual way is to use some form of actinometer, or print-meter, of which there are several kinds on the market.
This is done in weak light, preferably in the evening, by gas or lamp light. The tissue, having been printed, has to be developed to bring out the image. For this purpose will be required two or three deep porcelain dishes, a squeegee, a piece of glass, hot and cold water, a saturated solution of alum, and a piece of "Single-Transfer Paper." This latter should be obtained at same time as the tissue; the sheets of paper are generally cut a little larger than the tissue, and have a prepared surface.
The piece of glass is placed at the bottom of one of the dishes, which is nearly filled with cold water. Put in a sheet of single-transfer paper and let it soak for two or three minutes, then put in the printed tissue, which will curl inwards; as soon as it flattens out bring the two surfaces of paper and tissue together by lifting up the plate of glass from the bottom, drain off superfluous water, adjust the tissue in centre of the paper, place on the developing table, and squeegee into absolute contact.
Now place between several thicknesses of blotting-paper, and allow to remain for a quarter of an hour or so under pressure. At the end of this time they are immersed in hot water (about 100 deg. Fahr.). In a short time the soluble coloured gelatine will begin to make its appearance from between the paper and tissue. Now take hold of the two at one corner, and whilst keeping under the water carefully pull them apart; if they do not part easily, wait a little longer and try another corner. When they have separated, the support, which has until now carried the pigmented gelatine, may be rejected, as its place has been taken by the single-transfer paper. Development is completed by pouring the hot water over the print. The print when dry is darker than when wet, so that development should be stopped just before the required depth of colour is reached. Place in the dish of cold water for a few minutes, and then transfer to the solution of alum; afterwards wash and dry.
The single-transfer process reverses the position of things on the print. If an item of interest occupied a position to the right of a picture if printed on P.O.P. it would be on the left in a single-transfer carbon.
In many landscapes, if required for pictorial purposes only, this would not matter in the least. In other cases, however - such as buildings, etc. it is necessary to have things in their right places; to get this, "Double-transfer" has to be resorted to. Films for use in cameras are very useful for carbon printing, because, owing to their thinness, it is possible to print from the back as well as the front, and so obviate the necessity of double-transfer.
This is carried out on the same lines as the single, only, instead of using the single-transfer paper, a temporary support is used in its place. These temporary supports may either be rigid or flexible. Ground opal is used for the former. This has to be first rubbed over with French chalk, dusted, and then coated with weak collodion.
The flexible supports have to be prepared by waxing with a solution of beeswax and resin in a solvent, as turpentine. From the temporary support the print is transferred to a final support, which is purchased ready prepared.
"Gum Bichromate," and "Ozotype" belong to the same class as carbon.