Development.—The tissue must be developed, either by artificial light, or in a room protected by blinds, or otherwise, from strong daylight. The face of the tissue has become practically insoluble and, in order to develop it, we must transfer it to another surface and meanwhile remove the soluble pigment at the back of the film. This surface is termed the temporary support, and may be either a polished zinc plate, glass, or prepared paper according to the surface desired. The zinc plate, if employed, must be very smooth and especially free from scratches, each one of which would betray itself on the final print. The flexible temporary support is the best for many reasons—a smooth shellac-coated paper, which some hours before use is coated with a waxing solution.
Resin . . . 1 8 gr.
Yellow Beeswax . 6 „ Spirits of Turpentine . „ . . oz.
or celluloid sheets rubbed over with a piece of flannel steeped in Spermaceti . . . . . . . . zo gr.
Benzole . . . . . . oz.
or, in default of these, common rubber solution will do. Remove the print from the printing frame and place face downwards in cold water, with a piece of flexible support of rather larger size. When the tissue begins to uncurl and float flat the two surfaces are brought into contact under water, and squeegeed together on a plate glass or other level surface, after which they are laid between blotting paper for about a quarter of an hour, or longer if desired, with a weight, such as a large book, over them.
At the proper time the print is placed once more in water heated to a temperature of ioo° to 110° Fahr., which will soften the gelatine. As soon as the coloured gelatine begins to ooze out at the edges of the paper, the latter may be carefully lifted off, and the dish rocked steadily to assist dissolution of the mass of unexposed tissue. Warmer water may be applied, if the tissue fails to loosen sufficiently to release the paper ; but the water must never be of higher temperature than is just sufficient to develop the print, or a weak, feeble image will result. The backing paper must always be removed under water, and should come off spon taneously when one corner is lifted between the finger and the thumb.
When the picture is satisfactorily developed, wash the print in cold water, and then for five minutes in a dish containing a 5 per cent. solution of alum to harden.
Final Support.—The final support may be of paper, wood, or any similar material. The surface must be larger than the print, and should be coated with a weak solution of Nelson's gelatine 1 oz., water 20 oz., to which has been
added either 1 oz. of chrome-alum solution or enough bichromatc of potassium to colour the whole a clear yellow colour. The support after coating is exposed to the light. These supports, like the rest of the materials for the carbon process, may be obtained commercially.
When the print attached to its temporary support is dry, or at any subsequent time, it is once more placed in warm water (I Id" to i2o') with the permanent support, which has been previously soaked just long enough in cold water to render the surface slimy to the touch. Bring the two together, avoiding air-bubbles, lift them out, squeegee between blotting boards, and leave them to dry. When dry the temporary support will peel off, leaving the print in its permanent position.
Single Transfers.—This process is simpler, and is resortcd to when the owner is willing to disregard the fact that the final print will be a reversed picture, or when a reversed negative has been prepared for the purpose. Such a negative is most essential as a saving of time whenever a large number of prints are required. Film negatives are therefore advan tageous for carbon work. The tissue when taken from the frame is brought into contact under water with the permanent support, which for single transfer purposes should contain a rather larger proportion of chrome alum than the amount given for preparing the final support as above. Develop ment then proceeds in the same way as with the double transfer, except that after the alum bath the print is rinsed in cold water for a short time, and all is then complete.
Tissue may be obtained in a great variety of colours, but the most useful are engraving black, engraving brown, sepia, and red chalk. Fine effects may sometimes be obtained by using a coloured base for the final support.
Carbon Transparencies or Lantern Slides.—Glass plates, free from scratches or marks, are cleaned thoroughly with powder and acid, and then coated with Nelsons' No. i Gelatine . . . . . oz.
Water . . . . zo „ Potassium Bichromate . . . 30 gr.
When dried they are exposed to light until the coating is insoluble. Or, dry plates are freed from silver in the fixing bath, and hardened in a 2 per cent. bichromate solution, when they are ready to be used as supports in the single transfer process. Special transparency tissue is manufactured for this purpose. Carbon lantern slides, if not overprinted, have the advantage of crisp detail as well as permanence.