After development is contleted the bichromate salt is discharged and the image rendered perfectly insoluble by well washing in cold water and placing in a dish containing a 5 per cent. solution of potash alum, after which it is again washed and dried.
Double Transfer this the development is the same, and the whole process similar, except that the tissue is squeegeed first on to a temporary support, and from this transferred to its permanent support. By this means we get a non-reversed print from an ordinary negative.
For the temporary support we may use paper,* zinc, or glass, and treat with a waxy solution of Yellow resin 6 drams Beeswax 3 ounces Turpentine f 20 ounces This solution is rubbed on with a piece of flannel and then polished with another clean piece of soft flannel. The carbon tissue is squeegeed on to this support in the same manner as described in the single transfer process, and after it has been developed, washed, fixed in alum, washed again, and dried, it is ready for transferring to the permanent support. The transfer may take place at once whilst wet, or the pictures may be hung up to dry and transferred at some future time.
The double transfer paper or permanent support is a paper coated with gelatine and barium sulphate. A piece of this paper cut to the right size is steeped in Potash alum. ounce Water pint some time previous to the first operation. This is then squeegeed in the usual manner to the developed print. It is then allowed to dry, and a knife point inserted under one corner will cause the temporary support to separate from the other, leaving the image firmly imbedded in the gelatinous insoluble couche of the permanent support.
The temporary support may be employed again by re-waxing in the manner described.
The pictures may be given a brilliant finish by applying a few drops of methylated spirit to the surface, rubbed over with a piece of flannel. Another method is to squeegee on to glass. This method will be found described in finishing bromide prints, and can be adopted with any picture in which the film or image is a gelatinous one.
Among the defects that will arise in carbon printing, it will be well to mention a few of the more important. If the tissue becomes insoluble, it is due to its having been dried in warm, damp air. It may also be caused by the following : Acid sensitizing bath, too long keeping after sensitizing, and exposure to light, gas fumes or dampness. Frilling is caused by a neglect of the safe edge, placing the film on the temporary support too soon after the waxing of the latter, or insufficient pressure after development. Spots are due to air bubbles, or dust between the tissue and the support. If the prints refuse to leave the temporary support, it is owing to imperfect waxing.
The carbon or autotype process is also largely used for making lantern slides, opal pictures, and transparencies. See under these headings. See also Artiques Process and Papier Velours.
Several methods have been devised for printing upon carbon tissue without the necessity of a transfer. A transparent or translucent material, such as tracing paper, celluloid film, etc., was coated over with the gelatine, containing the pigment, and exposure made through the paper or film. Valentine Blanchard recently described a similar method./ The system is this. The sensitive tissue is immersed in a petroleum oil until the paper is saturated, the gelatinous coating being unaffected by the oil. After the superfluous oil has been removed, the tissue is exposed behind the negative, with its paper side next it. The exposure is necessarily long—three tunes that required when the tissue is exposed in the usual way— because the light has to pass through the paper, and that is stained by the bichromate to a non actinic color. With this method much of the fine detail is lost, and the texture of the paper will be visible. It can only be employed for broad, artistic effects. A reversed negative is also necessary.