Tissue sensitized by the formula given is approximately equal in sensitiveness to Ilford or Imperial P.O.P. A rough print may be taken of a portion of the negative on either of these papers, using the same paper in the actinometer, and noting the actinometer number when the print from the negative is sufficiently dark for the effect wanted and not over-printed as for toning. By printing a piece of carbon tissue to the number thus obtained, it will be found to be correctly exposed.
Many workers may prefer to use ready-sensitized tissue for their I first experiments in carbon printing. This removes one possible source of failure by ensuring perfect material. It should be noted, however, that ready-sensitized tissue is more rapid than that sensitized by the method given. Autotype tissue will require about half the exposure, and Barnet about three-fourths.
Before printing in carbon, the negative must be provided with a " safe-edge." This is an opaque edging to the negative, or, as a substitute, a thin mask may be interposed between the negative and tissue, or attached to the glass side of the negative. Its object is to protect the edges of the carbon tissue from the action of the light ; so that the margins of the print are white. Whatever form the safe-edge may take, the tissue should overlap it to the extent of one-eighth of an inch, at least. This is an absolute necessity for successful working.
After printing, the exposed tissue should be stored in a calcium tube, or under pressure until ready for development. A printing frame will answer sufficiently well for a few hours. A piece of transfer paper will be required for each print. Transfer paper may be obtained when the tissue is purchased. Several kinds are prepared commercially, varying in texture and tone from thin smooth and perfectly white to thick rough white and toned drawing papers. The thinner papers will require soaking in cold water for about five minutes before using, the thicker kinds for half an hour. Before putting the pieces in water, it is desirable to mark the back with a pencil.
When the transfer paper has soaked sufficiently, a piece of exposed tissue should be immersed in cold water face downwards, the corresponding piece of transfer paper being immersed in the same dish, under the tissue, and face upwards. As soon as the exposed tissue becomes moderately limp - before it has become sufficiently limp to lie flat in the water - the tissue and transfer paper are withdrawn together, face to face and firmly squeegeed into contact. They are then placed between blotting paper under moderate pressure for twelve to fifteen minutes - not longer. Any number of prints may be partially dried in this manner, at the same time. After fifteen minutes the prints are placed in cold water for five minutes or longer, until required for development. Half an hour's immersion will not hurt them, and it is not desirable for the inexperienced to develop more than one print at a time.
For development the print is placed in moderately hot water. The best temperature is 105 degrees. After a few seconds' immersion colour will be seen oozing from the edges of the print. It will, of course, be recognised that the print is now a film of coloured gelatine imprisoned between two papers, the original paper and the transfer paper. The original paper support should be uppermost when the print is placed in the developing bath. When the colour is seen oozing from the edges, a corner of the original backing paper is gently-lifted away from the transfer paper. If it comes away easily it should be steadily pulled entirely off, and thrown away. If it lifts with difficulty, the print should be left for a few seconds and another attempt made from a different corner. Care must be taken to keep the print below the surface of the water during the operation of removing the backing paper. The coloured film will now be left on the transfer paper, and at this stage it presents a most unpromising appearance. It is simply a soft, jelly-like mass of coloured gelatine. By keeping it in the hot water, and gently splashing the water over it for a few minutes, the soft and soluble portions of the film are washed away and the picture remains on the transfer paper. It should be lifted from the water from time to time to drain off the semi-liquid gelatine and colour, so that the progress of development may be seen. As soon as the picture is sufficiently light, it should be rinsed in cold water, and then placed in a solution of alum. A good strength for this solution is one ounce to a pint of water. Prints on thin papers should remain in the alum solution for about five minutes, those on thick papers for ten to fifteen minutes. When taken from the alum bath the prints should be washed in three or four changes of cold water and hung up to dry. On no account should their surface be touched until dry. After drying their surface is comparatively hard ; they may be re-wetted for mounting and touched without injury.
This method of working is called " single transfer," because the film is transferred from the original paper support to the single transfer paper. It possesses the disadvantage of reversing the picture. For many subjects this is unimportant ; for those in which it is inadmissible a second operation is necessary, the re-transferring of the film from the support on which it is developed to a paper which is to be its final support.
For development, a specially prepared paper is used instead of the single transfer paper. It is called " temporary support," and must be prepared for use by rubbing a little waxing solution* over its surface a few hours before it is required. It may be used for many prints in succession by re-waxing each time. The work of squeegeeing, developing, aluming and drying is performed in exactly the same manner as for single transfer.
After drying, the print on the temporary support and a piece of specially prepared paper called " final support for double transfer," are immersed in cold water for ten or fifteen minutes. They are then placed in hot water-about 100 degrees - for a few seconds, until the surface of the final support is soft and yielding. The two are then withdrawn from the water together, face to face, placed on the squeegeeing board, final support uppermost and firmly squeegeed into contact. When thoroughly dry the final support bearing the image can be easily pulled away from the temporary support.
* The waxing solution may be purchased ready for use or may be made as follows :-
Melt the wax, add the resin a little at a time stirring meanwhile, then remove from the fire and add the turpentine. - E. J. WALL.