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the negative, representing the lines of the subject, it renders the colloid coating insoluble throughout the thickness of the coloured film, so that the lines withstand the solvent action of the warm water, which entirely removes the rest of the coloured film from the ground and parts which have not been influenced at all by the light. If, however, instead of a negative of a line subject, on which the lines are transparent and the ground opaque, we take a negative of a subject in half tones, possessing various degrees of translucency in the lights and shadows of the picture, and make a print from it on a piece of the pigmented paper, we shall find that the light will only be able to penetrate through the entire thickness of the colloid film in the deepest shadows, represented, as before, by nearly clear glass; in the darker half-tones it will penetrate nearly through the coating; in the middle tones about half-way through, and in the lightest tones the light will be able to act only on the surface of the gelatine. We shall therefore have a print with an insoluble surface of varying depth, and underlying this a more or less soluble layer; it will thus readily be understood that when exposed to the action of warm water this layer will dissolve and carry away with it the partially insoluble surface-film forming the half shades of the picture, leaving only the stronger shades and giving a rough, hard, and unfinished appearance to the print.

For a long time this difficulty proved a stumbling-block in the way of the progress of permanent printing and gave the silver-printing processes a supremacy of which it has now become difficult to deprive them. The Abbé Laborde was the first to see the necessity for adopting the principle of exposing on one side and developing on the other. Blair, Fargier and Swan applied this to the carbon process, and the latter finally succeeded in introducing a practical method of pigment-printing applicable to the same class of subjects as silver-printing. Swan prepared a tissue by coating paper with a thick layer of gelatine mixed with bichromate of potash and coloured with any suitable pigment. After the exposure to light the gelatinous surface of the tissue was caused to adhere closely to a second piece of paper coated with india-rubber. The whole being immersed in hot water, the paper on which the gelatinous layer was originally supported, became loosened and could be removed, allowing the hot water to gradually dissolve away the unaltered and soluble gelatine. In this manner the exposure to light takes place on one side of the gelatine film, while the washing away of the superfluous gelatine is effected from the other, or unexposed side, without disturbing in any way the exposed parts of the film, and thus the most delicate shades in the half tones are perfectly preserved. Since its introduction by Swan this process has been much improved by Messrs. J. R. Johnson, R. Sawyer and other members of the London Autotype Company which acquired Swan's patents, and under

the name of the 'Autotype' process, it has been worked on a large commercial scale for the reproduction of works of art, and is now fairly beginning to come into active competition with silver-printing for all ordinary purposes of portrait and landscape photography.

On the Continent, the pigment-printing process is largely used by the well-known houses of Braun and Goupil for the reproduction of works of art, and is also coming into extended use for general purposes.

The following is an outline of the operations as now practised by the Autotype Company.*

The pigment tissue is prepared by coating long bands of paper with a moderately thick layer of gelatine coloured with any suitable pigment, and is sold ready for use either in an insensitive or sensitive condition.

The tissue is sensitised by immersion for a minute or two in a 5 per cent solution of bichromate of potash in water, to which some alcohol may be added with advantage, especially in hot climates; the bath should also be cooled down with ice if its temperature exceeds 65°. The tissue is then carefully dried, and when dry is ready to be exposed under the negative. This is done in a printing-frame in the usual way, the only precaution necessary being to paste slips of thin grey paper round the edges of the negative, so as to cut off a great portion of the light and form what is called the 'safe edge'. As the tissue generally appears black all over, the progress of the printing cannot be ascertained by inspection, and it is necessary to use a little instrument called an 'actinometer', by means of which, the degree of exposure necessary for any negative having been once ascertained, it is easy to give the same amount of exposure to successive prints. Up to this point the operations are the same whatever may be the nature of the support upon which the picture finally rests. The subsequent operations, however, differ accordingly as the image is developed on a final support, by what is called the 'single transfer' method, or on a temporary support, by the 'double transfer' method. In any case, some support is indispensable to retain the image and preserve it from injury during the washing.

In the single transfer process the support is paper coated with a gelatinous substance which, though insoluble in water, retains sufficient adhesive power when moistened to enable it to hold the picture during development and afterwards permanently.

After exposure under the negative the pigmented tissue having been immersed in cold water, together with a piece of the transfer paper, the two surfaces are applied to one another under water, and both drawn out together. They are then laid on a zinc plate, tissue uppermost, and brought into close

* See "The Autotype Process", 6th edition. Also Monckhoven's, Vidal's and Liesegang's treatises on Carbon-printing.

contact, all intervening air being driven out by means of an india-rubber scraper, or 'squeegee', which also removes all superfluous moisture. The prints and support are allowed to remain together for a short time, and are then immersed in warm water. After a little while the soluble gelatine will soften and become partially dissolved, when the paper forming the original support of the layer of gelatine may be gently removed, leaving a dark slimy-looking mass on the transfer paper. The soluble gelatine gradually clears away by the action of the hot water and reveals the image in more or less perfection of details according as the exposure has been properly timed. When fully developed, the print is washed with cold water, then passed through a solution of alum, rinsed again with water and allowed to dry.

Instead of paper, any other suitable permanent support may be used, but whatever the support may be, a reversed negative must be used if it is desired to obtain non-inverted pictures by the single transfer method.

When it is inconvenient to use a reversed negative, and it is desired to obtain a non-inverted picture-the development of the tissue-prints must be conducted by the double transfer method upon a temporary support, either rigid or flexible. The discovery that the pigment pictures might be developed upon any impermeable surface is due to Mr. J. R. Johnson, who also found that if such surface previously receive a coating of some fatty or resinous compound, the picture may be transferred, after development, to a final support.

The most suitable surface for the temporary support is a sheet of zinc, which may be either polished or grained; opal glass, or porcelain plates may also be used with advantage.

The plate employed as the temporary support first receives a coating of a solution of wax and resin in turpentine, and some operators coat the plate with collodion after the waxing, in order to improve the surface. The pigment tissue carrying the image is attached to the support under water in much the same way as in the single transfer method, and after remaining for a time, is developed in the same way and allowed to dry. The plate with the picture on it is then rinsed in water, and a piece of what is called double transfer paper-a fine paper coated with an enamel surface—having been soaked in water till quite soft, is laid on the wet plate, avoiding air-bubbles, and pressed into perfect contact with it by means of the indiarubber scraper. The picture with the transfer paper attached is now dried carefully, and when dry separates of itself from the temporary support.

Mr. J. R. Sawyer of the Autotype Company has introduced a flexible support, consisting of paper coated with a solution of gelatine rendered insoluble with chrome alum. When dry this is coated again with an alkaline solution of shellac, dried and well rolled under powerful pressure-it is. afterwards coated with a waxing compound. The use of this flexible sup

port is said to be advantageous with small pictures, but I have not found it answer very well in this country.

complicated, are in reality very sensitive to light a great many The number may, moreover, be

All these operations, which seem so simple, and as the sensitised tissue is very prints can be produced in a single day. increased by a plan proposed by Capt. Abney, R. E., of exposing the print for only half the usual time and then letting it lie by in the dark for some hours. The decomposing action set up by the light goes on in the darkness, and on development a picture is produced quite as good as if it had received a full amount of exposure and been developed at once. This discovery is largely utilised by those working the process in England, and enables an amount of work to be done in the winter months which would otherwise be impossible.

The single transfer process has been successfully worked at the Surveyor General's Office in Calcutta for the production of photographs of the convicts transported for life to the Andamans. No great difficulties were met with in working it, even in the hot weather, but it was found necessary to ice the solution of bichromate of potash used for sensitising the tissue, and to add a certain proportion of spirits of wine to it, in order to keep the gelatine from softening too much. Messrs. Bourne and Shepherd, the wellknown Indian photographers, have made arrangements for working the Autotype process at Simla, the climate of Bombay having been found unsuitable.

The pigment prints are perfectly permanent for all practical purposes, and, though they may under certain circumstances change colour slightly or lose their brilliancy, there is no such absolute fading and loss of details as in silver prints. The process may be applied in all cases to replace silver printing where permanency of results is an object. As I have mentioned before, the process is not quite suitable for the reproduction of coloured or shaded maps, owing to difficulties in obtaining prints comprising large surfaces of clean white paper together with the delicate half tones of hillshading. For maps in line the simple carbon process is more suitable, or if many copies are required, photozincography would be better.

Anilin Printing.-Before proceeding to the consideration of the processes employed for producing prints in the printing press, mention may be made of an ingenious process of printing which depends upon the use of salts of chromium, and is largely used in Europe for the reproduction of maps and plans. It is known as the 'Anilin printing process' and is the invention of Mr. J. Willis, who has patented it.

Paper is impregnated with a solution of bichromate of potash to which a little phosphoric acid has been added. After exposure to light under a transparent positive, such as a drawing on thin paper or vellum cloth, or even

an ordinary engraving or manuscript, it is exposed in a closed box to the vapour of anilin, which developes a greyish image. The print is then fixed by merely washing with water. As a positive original yields a positive print, maps or drawings may be copied without the necessity of making a negative by means of a camera, which is a great recommendation in certain cases. The process has hitherto been worked only by the inventor and his licensees and has not come into general use.

V.

PHOTOLITHOGRAPHY AND PHOTOZINCOGRAPHY.

In all the processes noticed in the last section, it is necessary to repeat the printing operation by exposure to light for every print produced. The rate of printing will consequently be more or less dependent on the sensitiveness of the paper, the strength of the light at the time of exposure and the state of the weather; the printing operations can, moreover, only be carried on during the few hours of daylight. In the photo-mechanical processes, now about to be described, these grave disadvantages are obviated, and, once the photographic image has been produced upon the printing surface, prints may be made in any numbers, quite independently of light or weather.

The simplest and most generally useful of these mechanical processes is photolithography, or the analogous photozincography, the principal difference between the latter and the former being merely the substitution of a thin smooth plate of grained zinc for the thick heavy lithographic stone. For maps of large size, zinc is certainly the most suitable and offers in other respects all the advantages of stone, but the latter being better known is generally preferred for ordinary work of moderate size.

In ordinary lithography, the image may be produced on the stone or zinc either by transfer from a drawing on paper with the solution of resinous soap known as 'autographic ink', or by drawing direct on the stone with a similar ink or crayon; so in photolithography there are two similar methods of obtaining the photographic image-either by transfer from a photographic print in fatty ink-or by impressing the image direct on the stone, by applying a photographic negative on a suitable coating sensitive. to light and removing by means of a solvent the parts unaltered by light. The transfer method being the most convenient is the one in general use.

The first photolithographic process on record is that proposed by Jobard, of Brussels, who, in 1839, obtained lithographic proofs from stone or zinc plates that had been treated with iodine or bromine. This process never came into practical use and has been quite superseded by two distinct methods-one dependent on the alterability of asphaltum under the influence of light-the other on the reactions of the alkaline bichromates upon gelatine and other colloid. substances.

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