Carbon-tissue, like albuminized paper, has become an article of commerce, and may be prooured of almost any colour. Its preparation involves technical difficulties so grave as to render it inad visable for photographers to attempt to make the tissue for thenn,elves. It will be sufficient in the preeent instance, thereforo, simply to outline the method by which it is manufactured.
The paper designed to carry the tissue must be tough, smooth, and not too heavily sized. Suitable paper is made in rolls by Rives, Stienback, and other manufacturers. The gelatine should neither be too soluble, nor too hard. It must be free from fat, chalk, acid, and alum. The gelatine should be rendered flexible by the addition of soap and sugar. Indian-ink, or other suitable oolouring matter in a fine state of division, iB next provided. All colours whose permaneney is doubtful should be avoided. The baeis must he carbon. This may be mingled with Indian red, oxide of iron, alizarine, purpurine, umber, indigo, Vandyke brown, Venetian red, bone-blaek, and sa on. The gelatine and pigment, when mixed in proportions suitable for the sort of tissue required aro consigned to a tank, and kept at a given temperature. The roll of paper to be coated is passed over the surface of the fluid at an accurately adjusted speed. The impel- thus takes on a uniform coating. The speed at which this operation is carried through regulates the thickness of the coating. The roll with its film is passed into a drying-chamber, kept at a uniform temperature, and scrupulously free from noxious futues. When dry, the tissue has the appearance of American cloth, or patent leather.
Sensitising Carbon-Tissue.—The 'dry tissue out the required size, is immersed in 1 oz. of potas sium biohromate and 50 oz. distilled water. This may be done in a metallic trny, several sheets being immersed at a time, aud care taken to remove air-bubbles from the surface of the tissue. The sheets should remain until they have become limp and flat, after which, they may be removed with wooden or bone forceps, as the potassium bichromate is poisonous. Each sheet, as it is brought out, should be plaeed faoe down on a plate of clean glass, and an indiarubber squeegee passed over the baok, to remove superfluous moisture. This done, the tissue may be fixed to wooden olips, and hung up to dry in a dark room, free from dust.
The proportion of potassium biehromate should be varied to suit the season of the year, and the oharader of the negative to be printed. In warm weather, a weak bath should be used. When the
negative is hard, the bath should be strengthened. A strong solution yields soft prints, whereas with a weak bath, satisfactory results are obtained from a weak negative.
In timing the exposure of a carbon print, it is necessary to use a photometer, as the tissue undergoes no visible change during the process of printing. The photometer consists of a square box, having a lid with a slit, and beneath, a ribbon of sensitized albuminized paper. The slit is formed in a eoatiug of brown paint on a diso of glass set into the lid. When in use, the end of the ribbon is brought beneath the slit, and the box is closed. The photometer is then placed beside the printing-frame. The sensitized paper will speedily darken in the light, and assume the tone of the brown paint. Thus one tint is registered, and the ribbon is moved, so as to present a new surface in the slit.
By dint of practice, the number of tints required to print a negative is ascertained to a nicety, so that the process may be repeated successfully.
The ordinary printing-frame may be used in this process, although special frames are constructed for printing ovals and tinted margins to " card " and " c,abinet " photographs.
It is essential in printing carbon-tissue to attach a narrow mask of blank or orange paper to the edges of the negative, so as to leave a margin all round the print unaffected by light.
Single Tranefer.—Carbon prints by single transfer require reversed negatives; with ordinary negatives, the priuts will be reversed. Reversed negatives are made in a variety of ways. By the aid of a mirror in frout of the camera-objective, reversed negatives may be taken direct. They may also be obtained by the " dusting-on " process. Obernetter's formula is 1 dr. dextrine, lf dr. white sugar, dr. ammonium bichromate, and 3 oz. water. A glass plate is coated with this solution, drained, and dried over a spirit-lamp. It is then placed in contact with the negative, and printed in diffused light in 15-20 minutes. The plate is then laid on a sheet of white paper, and dusted over with a broad soft brush, charged with finely-powdsred graphite. The print develops gradually with repeated dusting. Should the image develop at once, and indistinctly, the ex posure has been too short ; if on the other hand, the image is thin and full of detail, exposure has been unduly prolonged. After development, the reversed negative is coated with thin collodion, and may be placed in the printing-frame.