832 PHOTOGRAPHY shellac in boras, on which wax or resin was nibbed. The advantage of this flexible support" is that the dark parts of the picture have no tendency to contract from the lighter parts, which they were apt to do when a metal plate was used, as was the case in Johnson s original process. With this patent, and minor improvements made since, carbon-printing has arrived at the state of perfection in which we find it to-day. According to Liesegang, the carbon-tissue when prepared on a large scale consists of from 120 to 150 grains of gelatin (a soft kind), 15 grains of soap, 21 grains of sugar, and from 4 to 8 grains of dry colouring matter. The last-named may be of various kinds, from lamp-black pigment to soluble colours such as alizarin. The gelatin, sugar, and soap are put in water and allowed to stand for an hour, and then melted, the liquid afterwards receiving the colours, which have been ground with a mallet on a siab. The mixture is filtered through fine muslin. In making the tissue in large quantities the two ends of a piece of roll-paper are pasted together and the paper hung on two rollers ; one of wood about 5 inches in diameter is fixed near the top of the room and the other over a trough contain ing the gelatin solution, the paper being brought into contact with the surface of the gelatin by being made to revolve on the rollers., The thickness of the coating is proportional to the rate at which the paper is drawn over the gelatin : the slower the movement, the thicker the coating. The paper is taken off the rollers, cut through, and hung up to dry on wooden lathes. If it be required to make the tissue sensitive at once, 120 grains of potassium dichromate should be mixed with the ingredients in the above formula. The carbon-tissue when prepared should be floated on a sensitizing bath consisting of one part of potassium dichromate in forty parts of water. This is effected by turning up about 1 inch from the end of the sheet of tissue (cut to the proper size), making a roll of it, and letting it unroll along the surface of the sensitizing solu tion, where it is allowed to remain till the gelatin film feels soft. It is then taken off and hung up to dry in a dark room through which a current of dry warm air is passing. Tissue dried quickly, though not so sensitive, is more manageable to work than if more Printing slowly dried. As the tissue is coloured, it is not possible to with ascertain by inspection of it whether the printing operation is carbon- sufficiently carried out, and in order to ascertain this it is usual tissue. to place a piece of ordinary silvered paper in an " actinometer, " or "photometer," alongside the carbon-tissue to ascertain the amount of light that has acted on it. There are several devices for ascer taining this amount, the simplest being an arrangement of a varying number of thicknesses of gold-beater s skin. The value of 1, 2, 3, &c., thicknesses of the skin as a screen to the light is ascertained by experiment. Supposing it is judged that a sheet of tissue under some one negative ought to be exposed to light corresponding to a given number of thicknesses, chloride of silver paper is placed alongside the negative beneath the actinometer and allowed to remain there until it takes a visible tint beneath a number of thicknesses equivalent to the strength of the negative. After the tissue is removed from the printing-frame supposing a double transfer is to be made it is placed in a dish of cold water, face downwards, along with a piece of Sawyer s flexible support (already described). When the edges of the tissue begin to curl up, its surface and that of the flexible support are brought together and placed flat. The water is pressed out with an india-rubber squeezer called a "squeegee " and the two surfaces adhere. About a couple of minutes later they are placed in warm water of about 90 to 1 00 Fahr., and the paper of the tissue, loosened by the gelatin solution next it becoming soluble, can be stripped off, leaving the image (reversed as regards right and left) on the flexible support. An application of warm water removes the rest of the soluble gelatin and pigment. When dried, the image is transferred to its permanent support. This usually consists of white paper coated with gelatin and made insoluble with chrome alum, though it may be mixed with barium sulphate or other similar pigments. This transfer- paper is made to receive the image by being soaked in hot water till it becomes slimy to the touch ; and the surface of the damped print is brought in contact with the surface of the retransfer-paper in the same manner as was done with the flexible support and the carbon-tissue. When dry the retransfer-paper bearing the gelatin image can be stripped off the flexible support, which may be used again as a temporary support for other pictures. Such is a brief outline of carbon-printing as practised at the pre sent day, subject, of course, to various modifications which need not be entered into here. We ought, however, to mention that if a reversed negative be used the image may be transferred at once to its final support instead of to the temporary flexible support, which is a point of practical value, since single-transfer are better than double-transfer prints. Printing Printing vrith Salts of Iron. BIT John Herschel and Mr Hunt in with salts sundry papers and publications entered into various methods of of iron, printing with salts of iron. At the present time there are two or three which are practised, being used in draughtsmen s offices for copying tracings. When a ferric salt is exposed to light it be comes reduced to the ferrous state, and when this latter compound is treated with potassium ferri-cyanide a blue compound is formed. If, therefore, a solution of a ferric salt be brushed over a paper, and the latter be dried, and then exposed behind a tracing, the parts of the ferric salt on the paper exposed beneath the white ground are converted into a ferrous salt, and if potassium ferri-cyanide be brushed over the paper, or the paper floated upon it, the tracing shows white lines on a blue ground. Another method is to mix ferri-cyanide of potassium with a ferric salt, and expose it behind a tracing or drawing. Where the light acts, the mixture is con verted into a blue compound. The resulting print is the same as the foregoing. Another method of producing blue lines on a white ground is to expose paper coated with gum and a ferric salt to light, and then treat it with potassium ferro-cyanide. This body forms an insoluble blue compound with the ferric salt, whilst the ferrous salt is inactive, or only gives a soluble body. A further development of printing with salts of iron is the beautiful platinotypc process. Sized paper is coated with a solution of ferric oxalate and a platinous salt, and exposed behind a negative. It is then floated on a hot solution of neutral potassium oxalate, when the image is formed of platinum black. This process was introduced by Mr W. Willis in 1874. The rationale of it is that a ferrous salt when in solution is capable of reducing a platinum salt to metallic platinum. In this case the ferrous salt is dissolved by the potassium oxalate, and at the moment of solution the platinum salt is reduced and forms the image. Photo-mechanical Printing Processes. Allusion has already been made to the invention of Poitevin, who claimed to have discovered that a film of gelatin impregnated with bichromate of potash, after being acted upon by light and damping, would receive greasy ink on those parts which had been affected by light. But Paul Oreloth seems to have made the discovery previous to 1854, for in his patent of that year he states that his designs were inked with printing ink before being transferred to stone or zinc. Tessic de Motay (in 1865) and Marechal of Metz, however, seem to have been the first to produce half-tones from gelatin films by means of greasy ink. Their general method of procedure consisted in coating metallic plates with gelatin impregnated with bichromate or tri- chromate of potash or ammonia and mercuric chloride, then treat ing with oleate of silver, exposing to light through a negative, washing, inking with a lithographic roller, and printing from the plates as for an ordinary lithograph. The halt-tints by this process were very good, and illustrations executed by it are to be found in several existing works. The method of producing tin- plates, however, was most laborious, and it was not long before it was simplified by Albert of Munich. He had been experimenting for many years, endeavouring to make the gelatin films more dur able than those of Tessie de Motay. He added gum-resins, alum, tannin, and other such matters, which had the property of hardening gelatin ; but the difficulty of adding sufficient to the mass in its liquid state before the whole became coagulated rendered these un manageable. It at last occurred to him that if the hardening action of light were utilized by exposing the surface next the plate to light after or before exposing the front surface of the film and the image, the necessary hardness might be given to the gelatin without adding any chemical hardeners to it. In Tessie de Motay s process the hardening was almost absent, and the plates were consequently not durable. It is evident that to effect this one of two things had to be done: either the metallic plate used by Tessie de Motay must be abandoned, or else the film must be stripped oft the plate and exposed in that manner. Albert adopted the transparent plate, and his success was assured, since instead of less than a hundred impressions being pulled from one plate he was able to take over a thousand. This occurred about 1867, but the formula was not published for two or three years afterwards, when it was divulged by Ohm and Grossman, one of whom had been employed by Albert of Munich, and had endeavoured to introduce a process which resembled Albert s earlier efforts. The name of " Lichtdruck " was given about this time to these surface-printing processes, and Albert may be considered, if not the inventor, at all events the perfecter of the method. Another modification of " Lichtdruck " was patented in England by Ernest Edwards under the name of " heliotype. " This consisted in coating a glass plate, the surface of which was very finely ground, with bichromated gelatin to which a certain amount of chrome alum had been added. The film itself was much thicker than that of the Albert type, since it had to be detached from the surface of the glass by stripping, which was rendered possible by the previous application of a waxing solution to the plate. After the film was stripped off it was exposed under a negative for the time necessary to give a good image with printing ink, after which the inner side was exposed to light for almost the same length of time. The gelatin sheet was then transferred to a pewter plate, to which it was cemented by thick india-rubber cement and soaked in water till all the soluble bichromate was extracted. After this it was placed in a type printing-press and inked with a lithographic or gelatin roller, and an impression pulled on paper in the same manner as in printing with type, save that a greater pressure was brought to bear on the surface. This pressure was necessary Pho mec cal ] in. Hel typt
