The prints must be protected from daylight until they are put into the water ; after this they have lost their sensitiveness to light, and all subsequent work, development, etc, may be carried out in full daylight. This assists considerably in determining the extent to which they shall be developed.
On first putting the print in water it has a tendency to curl with the sensitive side inwards, though this tendency will vary according to the dryness of the print. If it should curl it will uncurl again as the film absorbs water, and the best time for lifting it out to squeegee down is just before it becomes flat. If the tendency to curl is very slight, it should be carefully watched, and as soon as it shows the slightest sign of flattening itself, taken out. Under ordinary conditions the time necessary for immersing in water may vary from fifteen seconds to one minute, depending entirely on the dryness of the tissue. Under no circumstances must it be allowed to remain until thoroughly flat or curling outwards, the best time to take it from the water being as soon as it is sufficiently limp to allow the squeegeeing into contact to be performed satisfactorily.
As soon as the squeegeeing is done the prints are to be placed between stout blotting paper, and put under moderate pressure for about twelve to fifteen minutes to partially dry. If several prints are dried together they must be separated by pieces of blotting paper.
They should on no account be left drying much beyond this time, as there is a very decided tendency to become insoluble. They should then be put in a dish of cold water, from which they may be taken one at a time as required for development. Soaking the prints in cold water at this stage is a departure from the usual practice, but it allows the worker to develop them singly without undue hurry. If several were left drying while others were developed, those taken last would probably be so insoluble that good results would be almost an impossibility. The cold water will not injure the prints in any way, but greatly facilitate the subsequent development. That first developed may have two or three minutes' soaking only, that taken last half an hour or more, but there will be no difference in the working or final result.
For development, hot water only is required. Those who work the carbon process largely use a specially made deep dish or developing tank, resting on supports, so that a gas ring or spirit lamp standing underneath keeps the water at a uniform temperature. For the occasional worker, however, an efficient substitute can be found in the usual household appliances. For small sizes, any moderate sized bowl will answer, provided it be nearly twice as long as the prints and at least five inches deep ; for larger work, a small bath or similar appliance will answer well. The objection to a shallow dish is that the water cools so very rapidly ; by keeping a body of water four or five inches deep the temperature may be maintained at a moderately uniform point.
The standard temperature for working is 105 degrees, and if carbon printing is to form part of any amateur's regular work a thermometer should be added to his photographic outfit, as it ensures uniformity in working, which, in carbon especially, is essential to success. If a thermometer is not available for preliminary trials the degree of warmth must be gauged by the hand, though it is difficult to give more than a rough test. It should be somewhat hotter than the usual temperature for a warm bath.
The beginner should not attempt to develop more than one print at a time, and if the temperature of the water in the developing bowl is correct when a print is commenced, the slight falling off during development will not be of the slightest consequence, unless the print be over-exposed, when more hot water should be added. A small kettle of boiling water will serve to raise the temperature before commencing a fresh print. A little should be added to that already in the developing bowl : the discolouration of that that has been used is unimportant ; however many prints may have been developed in it they will still emerge bright, clean, and unstained. Prints of different colours may be developed in the same water—it has absolutely no staining effect.
A print should be taken from the dish of cold water and put into the developing bath, transfer paper downwards, and kept beneath the surface of the water. After a few seconds —fifteen to forty, according to the temperature and the condition of the tissue—the colour will be seen to ooze out from the edges of the print. A corner of the backing paper should be gently lifted from the transfer paper, and if it lifts easily, gently pulled away, leaving the gelatine film on the transfer paper. If it does not come away easily it should be left a little longer, and a second attempt made from a different corner.
The backing paper should be removed with a steady even movement, and, if possible, without stopping; and it is very important that the print should be kept beneath the surface of the water during the operation. The backing paper should be thrown away, as it is absolutely worthless.
The print at this stage looks as though a passable result were a physical impossibility, as it consists of a soft semifluid, semi-jelly-like mass of coloured gelatine. But what is seen is the back or fully soluble portion of the film, and this semi-fluid condition is an indication of the ease with which it may be washed away.
Development is effected by simply soaking the print in the hot water. It should be lifted out from time to time, and the loosened gelatine and water allowed to run off, so that its progress can be seen ; but apart from this it should be kept below the surface of the water, and water made to flow gently over it by the motion of the hand. Although the soluble portion of the film is so soft and easily removed, the insoluble part forming the image is fairly tenacious, and with the treatment suggested there should be no risk of injury. When the print is lifted from the developing bath it may be supported on the hand or a piece of glass, and some of the water flowed over it; or it may be simply drained, so that its progress can be judged, and put back again. The picture will gradually become more clearly defined as the gelatine unaffected by light dissolves away, and the operation should be continued until the print is sufficiently light, when it should be placed face downwards in a dish of cold water to rinse off any remaining solution or gelatine.