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Asphaltum methods.-In 1852, MM. Lemercier, Lerebours, Barreswil and Davanne, proposed a method of litho-photography, in which a stone was coated with a solution of bitumen in ether, exposed to light under a reversed negative, and developed with ether, which dissolves the parts not affected by the light, while the exposed parts being insoluble remain and form the image. (Benzole, chloroform or turpentine may also be used instead of ether). After development the stone was prepared with acid and gum and inked in the same way as an ordinary lithographic drawing.” Since then many other similar asphaltum processes have been proposed and have been worked with great success, both for subjects in line and halftone ; but, owing to the length of exposure required and the uncertainty of the results, this process is not well adapted for general use, and has, I believe, been almost abandoned in favour of the collochromate methods. Collo-chromate Processes.—Paul Pretsch, whilst working out his photogalvanographic process, hereafter to be described, discovered that if a. mixture of gelatine and bichromate of potash be spread upon a suitable support and when dry exposed to light, then again moistened and inked in with a roller charged with printing ink, the ink would only take upon the parts altered by the light, and thus impressions could be obtained by transferring the design to zinc or stone. Pretsch does not seem to have made any practical use of this discovery, but shortly afterwards, in 1855, Poitevin independently worked out a photolithographic process on the same principle, which has been the foundation of all the present processes of photolithography and photocollotype and is worked to the present day for the reproduction of the Belgian topographical maps. Poitevin impressed his photographic image direct upon the stone and not by transfer. The first practical transfer process of photolithography seems to have been suggested by Asser, of Amsterdam, early in 1859. He coated unsized paper with starch, and then floated it on a strong solution of bichromate of potash. When dry it was exposed to light under a well intensified negative. The print was next heated with a flat iron, then moistened and inked in with transfer ink, by means of a roller, and thus an impression was obtained which could be transferred to stone or zinc.f The next transfer processes were the Southampton process of photozincography, which was founded on Asser's, and Mr. Osborne's process of photolithography. These two processes, though quite independent one of the other, were identical in principle and almost so in details; the only difference being that Mr. Osborne added a certain proportion of albumen to the mixture of gelatine and bichromate and then treated his prints with boiling water, in * See Davanne, Chimie Photographique, p. 456, f Photographic News, Vol. III, p. 146.
order to congulate the albumen and leave a slight coating of it on the paper, so as to obtain a “grip’ on the stone during the process of transfer.” At the Ordnance Survey Office, Southampton, and at the various photographie offices in India, in which the process has been introduced from Southampton, photozincography is used, with the best results ; but in Australia, America and the Continent of Europe photolithography is more usual and it is also used at Madras. These processes have occasionally been used with fair success for the reproduction of shaded maps, architectural views and other subjects in halftones, but they are not by any means suitable for such subjects, and are best adapted for the reproduction of maps and drawings boldly executed in dot or line alone. They may also be used for copying prints or engravings of all kinds on the same, larger or smaller scales, but engravings, and even many lithographs, are generally more or less unsuitable for the purpose. The best results are obtained from original drawings specially prepared to suit the requirements of photographic reproduction, in strict accordance with the rules already given. The following outline of the Southampton method will give an idea of the operations. Like the pigment-printing process, already described, it depends upon the property possessed by a dried layer of gelatine and other colloids, when mixed with an alkaline bichromate, of becoming insoluble and repellent of water under the influence of light. The procedure, too, is much the same as in the simple pigment-printing process, except that, instead of the fatty ink which forms the image on the photo-transfer print being mixed with the gelatime, it is applied to the surface of the print after exposure to light. The inked print is them washed in hot water, by which the colloid coating in the unexposed parts is dissolved and carries away with it the superfluous ink not retained by the lines forming the image. The negatives are obtained by the methods already described applicable to the reproduction of subjects in line. Having obtained a suitable megative, the next operation is to produce from it a photograph in greasy ink which may be transferred to zinc or stone. To prepare the sensitive paper, a sheet of bank-post paper is coated twice with a mixture of 6 parts gelatine and 4 parts bichromate of potash, dissolved in 100 parts of water, dried in the dark and glazed to give it a smooth surface. It is then exposed to the light under a negative for one or two minutes in the sun, or until the finest lines are distinctly visible. When sufficiently exposed, which may be ascertained by the whole of the detail appearing in brown upon a bright yellow ground, the print is taken out of * Photographic News, Vol. IV, p. 374,
the printing frame and passed through a lithographic press in contact with a polished stone, or zinc plate, which has been coated with a lithographic transfer ink, and thus receives an even coat of the greasy ink. The inked print is immersed for a few minutes in tepid water, to soften the gelatine still remaining soluble in the parts not acted on by light, and then laid on a sloping glass or metal plate and gently washed with a sponge and warm water till all the unaltered gelatine is removed, carrying the superfluous ink with it. The lines, on which the light has acted, remain insoluble and retain the ink, forming a clear image of the subject in a greasy transfer ink, precisely similar to the ordinary lithographic transfer drawing. When all the details are clearly and sharply defined, and the ground is quite free from ink, the print is rinsed in clean water and dried. It is then ready for transfer to stone or zinc. It often happens that a map is too large to be photographed in a single section. In this case the transfer prints of the different negatives are carefully joined together with gelatine and transferred to the stone or plate; or if too large to be printed in one sheet, the joined-up transfers may be cut up into as many convenient-sized sections as may be necessary. Zinc plates possess great advantages over lithographic stones on account of their superior lightness, cheapness, facility for storage and less liability to breakage, and are therefore to be preferred in reproducing plans of large size. For fine work stone is considered by some to give better results than zinc, but I believe that if due care be taken as good prints may be made from zinc as the best from stone. The plates used for this purpose are about so of an inch in thickness, and have one side carefully planed and smoothed ; but in order to give a somewhat porous surface to the plate, so that it may be more absorbent of moisture and hold the greasy ink better, the planed side of the plate is grained, or roughened by grinding it evenly all over with very fine sand and water. After the transfers are made, the plate is etched with a preparation of gum and decoction of gall-nuts to which a little phosphoric acid is added. If the transfers are made to a lithographic stone instead of to a zinc plate, the operations are exactly the same as for transferring an ordinary lithographic transfer-drawing, except that the stone need not be heated. The operations of printing, whether from zinc or from stone, are precisely the same as in ordinary lithography. Various modifications have been introduced, but the above process is still one of the best and most simple, and, if care be taken with suitable subjects, results may be obtained by it not to be surpassed by any other method. Full details regarding it will be found in Sir H. James' ‘Photozincography, also in the Photographie News, Vol. XII, page 280 et seq. The accompanying specimen of a reduction from an old engraved map will give an idea of the results that may be obtained. In the Southampton process the whole of the unaltered gelatine is removed from the paper, and the objection has been made that, in consequence of this, the ink on the lines being left on ridges of gelatime is more liable to spread in transferring, that the fine lines are liable to be washed away by the dissolution of the gelatine beneath them, and that the prints are liable to slip during transfer. To remedy these defects various methods have been proposed for retaining the gelatine on the paper. One of the best of these methods has been perfected by Capt. Abney, who has patented it under the name of ‘ papyrotype.” A tough paper is coated with gelatine, and subsequently treated with alum or chrome alum. It then receives a coating of gelatine and bichromate of potash as in the Southampton process. After exposure to light the print is drawn through cold water, and is them “squeegeed' down on to a smooth metal plate, and inked in with a soft gelatine roller charged with transfer ink. The ink ‘takes' only on the parts exposed to light, while the ground of the print remains clear. When the image is fully inked up, the print is dried and exposed to light, to harden the gelatine thoroughly by the action of light on the bichromate Salt still remaining, and is then ready for transfer to stone or zinc. Among the advantages claimed for this process, the principal are that— The ink which forms the lines is not left on ridges of gelatine, as in the Southampton method. The fine lines are not liable to be removed. The surface of the transfer will have no tendency to slip during transfer. In practice this method was not found to answer in this country so well as the ordinary one, but a modification of the latter has lately been introduced in the Surveyor General's Office, with the same object as the papyrotype, and seems to answer well. The paper is prepared as usual with two coats of gelatine and bichromate of potash. It is then put away for a few days, in order to allow the gelatine to become hard and insoluble. When required for use, it is coated again with a mixture of gelatine and bichromate of potash of about one-third the usual strength, and is then exposed to light and inked in the usual way. The washing is done with cold water instead of with hot. Instead of allowing the gelatine to harden by keeping, which takes from 3 to 12 or 14 days according to the season, the hardening action may be hastened by laying the sensitive paper face downwards on a board, and allowing the light to act on the back surface for a minute or two. This may be dome, either after the print has been obtained from the negative, or just after the preliminary coating has been given to the paper. The gelatine may also be hardened with alum or chrome alum, It has been found that this method has the advantage that a base of hard insoluble gelatine remains on the paper and retains the finest lines, while the fresh and easily soluble final coating preserves the clearness of the ground. It is necessary that the underlying gelatine should be thoroughly hardened, otherwise the transfers stick to the zinc plate in transferring, and are difficult to remove ; the soft gelatine is also liable to spread over the lines and prevent their transfer. Another advantage is that warm water is not required for washing the prints, and the ink is not so liable to become pasty as in the usual mode of working. The lines are found to keep crisp and the spaces between them free from scum, thus giving clearer and sharper transfers. |Mr. Herbert Deveril, Government photolithographer in New Zealand, found that, in working Osborne's original process of photolithography, which is still generally used in the Australian Colonies, great inconvenience arose from the use of boiling water to coagulate the albumen added by Mr. Osborne to the gelatine in order to produce an insoluble surface with a ‘grip’ on the stone. He has therefore substituted the following method of producing his transfer prints. Paper is first coated with gelatine to which a small proportion of chrome alum has been added. This is allowed to dry and is then sensitised in a solution of bichromate of potash. The prints are exposed and inked as in the Southampton process, and are washed off in cold water.* Mr. Deveril claims for this method the further advantage that the sensitive paper can be kept in good condition for a long time. The keeping properties of papers coated with gelatine and bichromate are, however, very dependent on climatic conditions. The results which I have seen by the process are exceedingly good. A method of photolithography by transfer which yields excellent results in line, and even reproduces half-tones fairly well, is a modification of Asser's process, invented by Mr. Toovey, of Brussels, who coats paper with a solution of gum arabic mixed with bichromate of potash, and after exposure to light under the negative in the usual way, places the transfer-print face downwards on the stone with several thicknesses of wet blotting paper over it, and leaves it under pressure for some hours in a powerful press. The gum on the parts not exposed to light being soluble is forced into the stone and prepares it, while the lines being hardened and rendered insoluble leave the stone quite free from gum and ready to take printing ink from a roller when passed over them, thus producing an image which may be printed from as soon as the soluble bichromate salt has been washed out, because the bichromated gum is a most powerful preparation for the stone * “Photographie News, Vol. XIX, p. 585.