Carbon Processes. Carbon printing is a general term applied chiefly to those printing methods in which a pigment, which may be, and often is, carbon, is mixed with gelatine, gum, or albumen and applied as a coating on paper. The film being made sensitive by soaking in a solution of an alkaline bichromate - or in some cases the sensitising material is added to the organic mixture in the first instance - exposure to light makes the bichromated organic matter insoluble, while the unexposed portions can be washed away. In the most usual form of carbon or pigment printing - also frequently called autotype printing - the starting point for the amateur or occasional worker is a material sold as carbon tissue, or autotype tissue, and this consists of paper coated with a thick layer of gelatine, somewhat heavily coloured with suitable pigments. The preparation of this tissue on a small scale is seldom worth undertaking, but particulars will be found farther on. The tissue may be bought ready sensitive, the bichromate being added during manufacture; but as its keeping properties are very uncertain and variable, it is generally best to obtain the unsensitised tissue, which may be had in various colours, as black, engraving black, standard brown, standard purple, portrait brown, portrait purple, sepia, red chalk, and special transparency tissue for lantern slides. The paper, or tissue, is sensitised in the following bath : -
Bichromate of potash ... ... ... i oz.
Liq. ammon. fort. *88o ... ... ... 5 drops.
Distilled water ... ...... ... 20 ozs.
The tissue is immersed in this for two minutes in hot weather or three minutes in cold. It must be dried, and kept protected from light, air, and damp. From the colour of the tissue it is evident but little, if any, image can be seen; therefore the duration of exposure is judged by means of an actinometer of ordinary chloride albumenised paper, and according to the density of the negative the tissue is exposed whilst the actino-meter registers one, two, or three tints. The action of light continues in the dark. Allowance must, therefore, be made for this fact if the tissue is not to be developed at once. As the exposure to light renders the gelatine insoluble (and as the action of development is to remove the unacted-upon gelatine), it is obvious that it will be useless to try and develop the picture