Sensitive tissue must be kept very carefully, or it will deteriorate even more rapidly than the time previously given. It should be kept in the wrapping paper or envelope in which it is packed by the makers, or else in waxed paper, and placed under moderate pressure in a printing frame, but it is desirable to use it within seven or eight days.
If prints are not developed as soon as they are taken from the frames they may be kept in a similar manner to the tissue, or in a calcium tube similar to that used for storing platinotype paper. If they are placed in a tube they should be rolled film inwards.
There is a curious property in the carbon process in which it is unique among ordinary printing methods, and that is that a print after exposure to light continues gaining in depth even if kept in absolute darkness. This con-tinuating action is almost stopped by keeping the prints under pressure as advised, and it is entirely arrested by the preferable plan of storing them in an absolutely dry atmosphere, such as that of a calcium tube. Prints may then be kept for several hours between printing and development without the slightest loss of quality; or in extreme cases for two or three days. If kept more than a few hours the extreme dryness renders them very brittle and difficult to handle; they should be taken from the tube some time before required and allowed to absorb moisture from the air—in darkness, of course. Prints that have become dry in this manner require longer time in the cold water before they are in condition for squeegeeing to the transfer paper.
In the case of prints being taken from the frames before they are sufficiently exposed, advantage may be taken of this continuating action by leaving them exposed to the air, in darkness, when they may become similar to those fully exposed. Although expert printers find this quality an advantage in bad weather when printing is slow and difficult, it is so uncertain and beyond control that at the best it should be avoided as too unreliable for the ordinary worker.
Two other defects that may occur are frilling and blistering, the former being the film leaving its support at the edges during development, the latter a similar result, but in circular patches in any part of the picture.
The most usual cause is prolonged development, as the film holds to the transfer paper principally through the latter being so coated with insoluble gelatine that water cannot penetrate from the back to loosen the film. With long development or very hot water the gelatine coating may fail to resist the penetrating action, and the film fail to hold in consequence.
There are several contributory causes, however, and these may make a print blister or frill with normal treatment, while without them it might withstand any reasonable degree of development.
Careless handling of the print during development may bend the transfer paper in places and so break the surface coating. Wherever such a break or sharp bend occurs, blisters are almost certain to appear.
Imprisonment of air between the film and transfer paper when squeegeeing into contact may frequently cause blisters during development. For this reason the water in which they are immersed for transferring should not be drawn from a tap immediately before required, but allowed to stand first.
The method of bringing the surfaces into contact is also very important. If the tissue and transfer paper are brought together under water, this cause of blistering should not exist, and in small sizes it is the most satisfactory method. In large work it is not always practicable on account of difficulty in handling, but an alternative plan is to lay the transfer paper face upwards on the squeegeeing board, well flood it with water, lift the limp tissue from the water in which it is soaking, place it face downwards on the transfer paper, and squeegee into contact.
If the tissue is allowed to soak too long before squeegeeing into contact there is a very great risk of its leaving the transfer paper either when the backing paper is being removed, or in the form of frilling during development. It should be squeegeed down before it becomes quite flat.
The alum bath used after development serves two purposes. The first is the removal of a slight yellow stain due to the bichromate in the paper. With a thin transfer paper and moderate development there should be very slight yellowness to remove, and there is this difference between carbon and silver printing in this respect, that in carbon any stain due to incomplete removal of the sensitising salt is seen at the time of development, and can be removed easily. If not visible then it will never appear afterwards. The print should be kept in the alum solution until all yellow stains have disappeared, or in any case not less than four or five minutes.
The second purpose is the hardening of those semi-soluble portions of the film which have been allowed to remain to form the image, and which have now become the face of the film, and four or five minutes in the solution is sufficient for this. After drying the surface is reasonably hard.