Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/869

 PHOTOGRAPHY 833 for two reasons, the relief of the image would be too great if only a moderate pressure were used, and the entire surface was so large that a heavy pressure was requisite to make the paper bite on the ink. Between each pull the gelatin film was damped, the surface moisture taken off with a dry cloth, and the inking proceeded with. The drawback to this process is undoubtedly the great relief that is given from the film being so thick, but it is a more manageable process in some respects than that of Albert, since the support is unbreakable. We should mention that Edwards also patented the use of two or more inks of different degrees of stiffness. The stiffest, which was generally black, adhered to the most deeply printed parts of the image, the next stiffest to the next most deeply printed parts, and so on. By this means the least deeply printed parts acquired a different tone from that of the deeper printed parts, which was an advantage as regards artistic effect. The same method of inking could be applied to Albert s process with the same results. Since the time of the heliotype patent many im provements have been made in the minor details of the operations, and various firms now produce prints in greasy ink very little if at all inferior to silver prints. I- Woodbury Type. This process was invented by Mr W. Wood- bury about the year 186-4, though we believe that Mr J. W. Swan had been working independently in the same direction about the same time. In October 1864 a description of the invention was given in the Photographic News. M. Gaudin claimed the principle of the process, insisting that it was old, and basing his pretensions on the fact that he had printed with translucent ink from intaglio blocks engraved by hand ; but at the same time he remarked that the application of the principle might lead to important results. It was just these results which Mr Woodbury obtained, and for which he was entitled to the fullest credit. Woodbury type is a combination of the principle upon which intaglio printing is based with that upon which a carbon -print is obtained. The general features of the procedure will be understood from the foregoing de scription of the carbon-process. An image is obtained on bichro- matized gelatin from a negative of the usual kind by exposing a thick layer of gelatin to light and then washing away all its soluble parts from the back of the exposed print. This is the mould which it is necessary to obtain. At first Woodbury made electrotypes from the mould, from which he could obtain prints mechanically. The intaglio was placed on a specially devised printing-press, and the mould filled with gelatin containing colouring matter such as Indian ink. A piece of paper perfectly even in thickness was placed in con tact with the mould, and a piece of flat glass under pressure brought down upon this. The excess of pigmented gelatin was squeezed out, and, when slightly set, it adhered to the paper and was brought away from the mould. After drying, a perfect picture was obtained in pigment, the image being reversed as regards right and left ; but that difficulty was surmounted by using a reversed negative, and also by a modification of the process subsequently introduced by Mr Woodbury. The gelatin relief was made as before, and then by means of very heavy pressure in a hydraulic press the mould was squeezed into soft metal, from which the prints could be after wards taken off. This is the same principle as that on which nature-printing is conducted, and at first sight it seems strange that material such as gelatin should be able to impress metal. Mr Woodbury found that it made very little if any difference in the sharpness of the image if the relief was reversed and the back of the relief pressed into the mould. This of course made the print correct as regards right and left. He has not, however, been con tent with his original operations, but has further simplified them, the outcome being what is known as the &quot; stannotype process.&quot; In 1880 he read a description of it before the French Photographic Society. The modification consisted in taking a mould in gelatin from a positive on glass. The mould, when hardened by chemical means (as was indeed the case with the original AA r oodbury-type process), was attached to a sheet of flat glass, and then covered by the foil and passed through a rolling press the cylinders of which were covered with thick india-rubber. This forced the tinfoil into every crevice of the mould, yielding a block impervious to moisture and ready to have gelatin impressions taken from it. At first Mr Woodbury took an electrotype from the relief, covered with tinfoil, obtained from a negative, but he abandoned this for a simpler plan. He took a positive on glass in the ordinary manner adopted by photographers, from which he made a mould in gelatin. This he covered with tinfoil and printed direct from it. Photo-Lithography. on gelatin impregnated with bichromate of potash, where by the gelatin becomes insoluble, and also incapable of absorbing water where the action of the light has had full play. It is this last phenomenon which occupies such an important place in photo-lithography. In the spring of 1859 Asser of Amsterdam produced photographs on a paper basis in printer s ink. Being anxious to produce copies of such prints mechanically, he conceived the idea of trans ferring the greasy ink impression to stone, and multiplying the impressions by mechanical lithography. Following very closely upon Asser, J. W. Osborne of Melbourne made a similar application ; his process is described by himself in the Photographic Journal for April 1860 as follows. &quot; A negative is produced in the usual way, bear ing to the original the desired ratio. ... A positive is printed from this negative upon a sheet of (gelatinized) paper, so prepared that the image can be transferred to stone, it having been previously covered with greasy printer s ink. The impression is developed by washing away the soluble matter with hot water, which leaves the ink on the lines of print of the map or engraving.&quot; The process of transferring is accomplished in the ordinary way. Early in 1860 Colonel Sir H. James, R.E., F.R.S., brought forward the Southampton method of photo-litho- South- graphy, which had been carefully worked out by Captain ampton de Courcy Scott, R.E. We give a detailed description of m it as practised at Southampton. Preparation of the Paper. The mixture consists of 3 oz. of Nelson s &quot; fine art &quot; gelatin and 2 oz. of potassium bichromate dissolved in 10 oz. of water and added to the 40 oz. of water with which the gelatin, after proper soak ing, has been previously mixed. Good and grainless bank post-paper (chosen on account of its toughness) of medium thickness is made to float on this solution (after it has been strained) for three minutes, when it is hung up in the dark to dry. It is again floated on the solution and hung up for desiccation by the corners opposite to those which were previously uppermost, and then passed through a copper-plate or lithographic press to obtain a smooth surface. The paper is next placed upon a negative and printed in the ordinary manner, the negative being very dense in those parts which should print white, and perfectly transparent where the black lines have to be impressed. From about two minutes exposure in sunshine to an hour in dull light is requisite to give sufficient intensity to the prints, which are next covered Avith greasy printer s ink, made from lithographic printing ink, pitch, varnish, palm oil, and wax. The inking is best done by covering a lithographic stone with a fine layer by means of a roller, and then passing the paper through the press as if pulling a lithographic print, an operation which may have to be repeated twice to ensure the whole surface being covered, and yet not too thickly. The inked print is placed face uppermost on water of a temperature of about 90 Fahr., and, when the soluble parts of the gelatin have taken up their full quantity of water, the paper is laid on a sloping glass plate, inked surface uppermost, and a gentle stream of warm water poured over it. This removes the soluble gelatin and the greasy ink lying on it, the removal being helped by the application of a very soft sponge. When all the gelatin and ink except that forming the image have been removed, the paper is allowed to dry till ready to transfer to stone. The method admits of several variations in detail, such as coating the gelatin with albumen and removing the soluble albumen by cold water, some of them being excellent, especially where the relief of the developed print is small, as relief is an enemy to the production of fine work on a lithographic stone, since the ink, in passing through the press, squeezes out and produces broad lines which should be otherwise fine. Another method of producing a transfer, called the Papyro- &quot;papyrotype process,&quot; was published by Abney in 1870, type in which the ink is put on to a surface of gelatin by means m&amp;lt; of a soft roller ; and this has the great advantage that the ink can be removed at pleasure if any part is not satisfactorily inked, without the basis of the print being XVIII. 105
 * o- Reference has already been made to the effect of light