At the proper time the print is placed once more in water heated to a temperature of 100° to 1100 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 spontaneously 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 Nelsons gelatine 1 oz., water 20 oz., to which has been added either 1 oz. of chrome-alum solution or enough bichromate 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 (1100 to 120°) 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 resorted 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 advantageous 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. Development 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. 1 Gelatine . . . . . . 3/4 oz.

Water ......... 20 „

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.

Three-Colour Carbon Prints

Tissues of the proper colours - yellow, blue, and pink - are supplied by the makers. The tissue may be sensitised in a bath of one part 10 per cent. bichromate solution and one part methylated spirit, and squeegeed under blotting-paper on to ferrotype sheets. Drying may, if necessary, be accelerated by heat. The yellow tissue will take about twice as long to print as blue, and the pink about one and a half times the yellow, but in all three colours the image will be more or less visible.

When the tissues have been separately developed on their temporary support, the yellow is transferred first to the final mount, and adheres without special treatment. A thin coating of Nelson's gelatine (gelatine 30 gr., water 1 oz.) must then be laid on, then the blue print, then another coating of gelatine, and finally the pink print. The register must be secured by needles passing through holes at measured spots in the temporary supports. This precaution is of course unnecessary with the transparent celluloid temporary supports introduced by the manufacturers of carbon tissue.

Glazed Surface Prints

The tissue must be somewhat underprinted, as its depth will be increased by the action of the collodion. On removing from the printing-frame, coat the tissue with a thin solution of enamel-collodion and transfer as usual to temporary support. The enamel must be very evenly laid on; inequalities will show as markings on the finished print.

Difficulties And Defects

If the print, or part of it, refuses to adhere to the support it is probable that grease of some kind has found its way into the dishes, and so reached one or other surface. Spots are produced either by dust or bubbles, especially in under-exposed prints. Such spots, if not too numerous, may often be filled up with a fine brush and melted tissue of the right consistency. Over-exposure and under-exposure may to a great extent be compensated for by the use of hotter or colder water in development; but greatly over-exposed tissue will generally cockle up or leave the paper in uneven patches. Tissue which has been sensitised for more than a fortnight is usually worthless Care and exactness are as necessary in the carbon process as in those involving a series of chemical reactions.