Three-colour Carbon Prints.—Tissues of the proper colours—yellow, blue, and pink—are supplied by the makers. The tissue may be sensitised in a bath of one part io per cent. bichromate solution and one part methylated spirit, and squeegeed under blotting-paper on to ferrotype sheets. Drying may, if necessary, be accelerated by heat. The yellow tissue will take about twice as long to print as blue, and the pink about one and a half times the yellow, but in all three colours the image will be more or less visible.
When the tissues have been separately developed on their temporary support, the yellow is transferred first to the final mount, and adheres without special treatment. A thin coating of Nelson's gelatine (gelatine 3o gr., water i oz.) must then be laid on, then the blue print, then another coating of gelatine, and finally the pink print. The register must be secured by needles passing through holes at measured spots in the temporary supports. This precaution is of course unnecessary with the transparent celluloid temporary supports introduced by the manufacturers of carbon tissue.
Glazed Surface Prints.—The tissue must be somewhat underprinted, as its depth will be increased by the action of the collodion. On removing from the printing-frame, coat the tissue with a thin solution of enamel-collodion and transfer as usual to temporary support. The enamel must be very evenly laid on ; inequalities will show as markings on the finished print.
Difficulties and Defects.—If the print, or part of it, refuses to adhere to the support it is probable that grease of some kind has found its way into the dishes, and so reached one or other surface. Spots are produced either by dust or bubbles, especially in under-exposed prints. Such spots, if not too numerous, may often be filled up with a fine brush and melted tissue of the right consistency. Over-exposure and under-exposure may to a great extent be compensated for by the use of hotter or colder water in development ; but greatly over-exposed tissue will generally cockle up or leave the paper in uneven patches. Tissue which has been sensitised for more than a fortnight is usually worthless Care and exactness are as necessary in the carbon process as in those involving a series of chemical reactions.
Ozotype.—This is a variation of the carbon process invented by Mr. T. Manly, and is founded on the property which an image on a bichromated surface has of inducing insolubility in gelatine when pressed into contact with it. A special sensitising solution (protected by patent), con taining bichromate and manganous salts, is spread evenly with a broad hoghair brush over any suitable drawing paper, and then hung up to dry in a place free from damp or actinic light. The sensitised paper does not keep unless enclosed in chloride of calcium tubes or boxes, like platino type paper. When exposed under the negative a brown image prints out to the required depth, and no actinometer is therefore necessary. All details ought to be visible except light clouds.
The print is next washed in cold running water until the margin covered by the rebate of the printing-frame is quite clean and white, but care must be taken not to wash too long, or the brown image will lose in strength. The
exact time of washing may be from 6 to io minutes in summer, and from 20 to 30 minutes in winter. After washing, the further operation of pigmenting may be deferred if necessary for months.
Pignzenting.—When ready for pigmenting, the print is placed face downwards in a special acid bath in which the pigment plaster has been soaking for from 3o secs. to a minute. The two are brought quickly into contact face to face, and squeegeed together, as in ordinary carbon paper. Development and subsequent operations are the same as with single transfer carbon, which this process indeed greatly resembles, except that the picture is not reversed, and an ordinary negative can be used.
The acid bath is made up as follows : Stock Solution.
Warm Water . . . . . . . to oz.
Sulphate of Copper (pure) . . . 20 gr.
Glacial Acetic Acid . . . . • . • 5 Glycerine Iiydroquinone . . . . . 4 „ Working Bath.
Concentrated solution . . . . . . 4 dr.
Water . . . . . . . . 40 oz.
The diluted bath will keep for three days, and may be used for any number of prints in succession. The stock solution will keep for many months.
Ozobrome.—This is a further invention of Mr. Manly, by means of which an ordinary bromide print may be con verted into a carbon print. A piece of carbon tissue is soaked until saturated in a patent pigmenting solution containing potassium bichromate, potassium ferricyanide, and a bromide salt. It is then squeegeed into contact with the bromide print, which has been previously hardened, either by alum or formaline. A chemical reaction takes place between the silver image and the ferricyanide, the bichromate salt is deoxidised, and ultimately a chromate image is formed which penetrates well into the tissue. After being left in contact under blotters for about zo minutes the whole is developed at a temperature of Ho°, like an ordinary carbon. Lastly, the bleached silver image may be fixed out in the ordinary hypo bath, or redeveloped if required to give greater intensity to the picture.
Transfer Process.—An alternative method is to place the bromide print with the pigment plaster adhering to it in cold water, and separate them from each other by carefully and steadily pulling at the former. The pigmented paper holding an impression of the silver image may now be pressed into contact with a fresh support and developed in warm water in the usual way. The original bromide print may be redeveloped, and used again as a matrix for a succession of prints.
The process, especially in the first form, is an exceedingly valuable one for improving the appearance and increasing the chances of permanence of bromide paper enlargements. Many variations may be suggested. For instance the bleached silver image, instead of being redeveloped, may be sulphide toned in a solution of sodium sulphide 30o gr., water 20 oz., or toned to any desired colour ; and being pro tected from the atmosphere by the semi-transparent layer of hardened gelatine, is almost everlasting.