A NUMBER of processes have been devised for applying photography to the production of blocks or plates, from which impressions may be taken by purely mechanical methods. These processes are of three kinds. They consist of blocks or printing surfaces in which (I) the parts intended to show up in print are sunk, cut, or etched below the even surface. These are known as intaglio plates or blocks; or else (2) the printing surface is a film so treated that various parts thereof differ in their capacity for taking or rejecting ink, the picture or design in ink being afterwards transferred to paper by contact; or (3) the portions to be printed stand out like type, receive ink, and are printed in the ordinary manner of letter-press, the half-tone in such cases being produced in some sort of stipple or grain, as in various kinds of engraving.
It will be seen that these three varieties of photo-mechanical printing have affinities respectively with the methods of copper-plate, litho-graphic and letter-press printing.
In all photo-printing processes the great difficulty has been the production of half-tone. To secure two tones, that is, black and white, as in pen and ink drawing, is a simple enough matter ; but it becomes altogether a different affair when the attempt is made to seize all the gradations of tone, all the subtle shades of chiaroscuro, intervening betwixt the deep shadow and broad light, as seen in nature, and reproduced in a good photograph.
Not all the processes hereafter to be described succeed in overcoming this difficulty; perhaps indeed only one of them is completely successful. That process is the one known as Woodburytype, from the name of the inventor, Walter B. Wood-bury, who took out his first patent for the method in 1866. Roughly speaking, the process consists in obtaining an intaglio impression of the image which it is required to copy. The mould thus secured is filled in with transparent coloured ink, which, transferred to paper, makes the picture. The deepest portions of the mould naturally take the most ink, and represent the darkest shadows, while the shallowest portions take the least, and represent the most delicate tones ; so that all the gradations of tint are truly exhibited.
Others had worked at such a process before the actual inventor, and Woodbury himself was not in the outset completely successful. The first step in the process is to obtain an image in relief by means of a film of bichromated gelatine. This is dried, exposed under a negative, and developed as in the Autotype process, the result being an image in considerable relief. The unexposed portions of the film are left soluble, while the other parts are rendered insoluble to depths corresponding to the intensity of the light.
The relief thus obtained is now covered with lead, and the two are pressed together in a hydraulic press which, strange to say, produces a reverse or mould of the image in the soft metal without injuring the gelatine relief. The intaglio mould thus produced is placed in a press of simple construction, ink in the form of a pigmented gelatine is poured upon it, and a strongly-sized sheet of paper laid over it. A platen of perfectly flat glass is now brought down upon the paper and mould, and held tightly in position by a clutch. The pressure forces out all the superfluous solution, while that in the mould adheres to the paper. When the gelatine has had time to set, the platen is raised and the paper removed. Then, to finish off the print, it is placed in a solution of alum which hardens the gelatine.
Another process which was successfully worked out by the author of the above method is that known as Stannotype, or printing from a tin surface. It is thus described by the inventor : “A positive is first made from the negativepreferably by the carbon process. From this carbon or other transparency a negative is made, also in carbon; but in this case the tissue possesses much more body and much less colour, so as to obtain a certain amount of relief. This (gelatine) relief negative is then coated with a thin indiarubber varnish. A piece of tinfoil is laid over it, and the whole passed through a pair of indiarubber rollersa species of mangle in fact. We have now a printing mould ready for placing in the press and printing from in gelatinous ink.” Although the Stannotype process does not give the beautiful pictures obtained from Woodbury’s first method, it yields results of a very high quality.
Another modification of the Woodbury process is one much used for photo-engraving. The relief is stamped into lead or type metal as in Woodburytype with this differencethat the bichromated gelatine has had powdered glass mixed with it, and this produces a grain. From the lead block a copper plate is obtained by double electroplating, and printing is done as in copperplate. The excellent photogravures produced by the Goupil process are said to be obtained by a modification of the above method. A variation of Woodbury’s invention was introduced in I880 by Major Woodhouse; but it does not appear to offer any special advantages.
The only process which differs materially from Woodbury’s or the still earlier one of Fox-Talbot seems to be that brought out by Obernetter, which is thus described by Burton: “A positive is produced on a film of gelatino-bromide of silver very rich in the silver salt. The silver of the developed and fixed image is converted into chloride of silver by the action of the mixture of perchloride of iron and chromic acid. The film is then wetted and brought into contact with the surface of the copper-plate, which it etches, the chlorine leaving the chloride of silver to combine with the copper-forming chloride of copper, which is soluble in water.”
Although Fox-Talbot’s engraving process (patented in 1852) did not, as regards results, come up to those described above, excellent prints were obtained by it, and it is believed that many of the secret processes practised here and on the Continent are but modifications of his photoglyphy. The process consists in printing the negative on a bichromated gelatine film, washing away the unaltered gelatine, and making an electrotype from it. He even proposed to obtain half-tone by impressing on the gelatine film, prior to exposure under the negative, ” the image of a piece of folded gauze or other suitable material ” the means still largely used for obtaining gradations of tone.
In later years Talbot hit upon the idea of improving his method by adopting the use of powdered resin for the production of grain. He does not appear to have been very successful in its application ; but the idea was worked out with good effect in a process based upon his and patented long after by Clic, the main difference between the two being that, whereas Talbot pro-posed to spread the resin over the gelatine relief, the former applied it in the form of a fine powder on the surface of the copper-plate, using heat to make it adhere.* The process is much used both in England and abroad.
The name of Poitevin has been frequently mentioned in these pages. To him photography owes many original ideas. He was one of the first to experiment with bichromatised gelatine. In 1855 he discovered that gelatine so treated, after exposure to light, will ” take ” greasy ink, while it repels water. Upon this principle he based a process of photo-lithography, which, as Collotype in England and Lichtdriick in Germany, has become one of the most useful forms of photo-mechanical printing.
The method of working the collotype process is as follows: A sheet of glass is coated with a film of bichromated gelatine. When dry it is exposed under a negative, the result being a faint image. On being washed in cold water, those parts of the plate that have not been acted upon by the light swell up. Between the extreme blacks and whites of the print there is a complete gradation of tones produced by a reticulation or grain in the gelatine. The process of printing from a collotype plate is the same as in lithography; that is to say, it is damped, inked, covered with a sheet of paper, and so passed through the press.
There have been a number of modifications of the collotype process, only a few of which have proved commercially successful. They vary in minor details only, and need not detain us here. In 1861, however, Col. De C. Scott, R.E., and Sir Henry James brought out a process which proposed to overcome one defect in collotype printing, namely, the limited number of impressions which could be taken from a plate. To this they transferred the map or planit being designed chiefly for the copying of line subjectsto zinc or stone, from which an indefinite number of copies could be printed. Others applied them-selves to the same idea with useful results. Captain Abney made some advances on his predecessors in the process to which he gave the name of papyrotype, and upon his method J. Husband, R.E., based a procedure for securing a half-tone by means of soluble crystals embedded in the bichromated gelatine. A full description of the process is given by Burton.*
We now come to the third method of applying photography to the production of printing surfaces, that, namely, for making blocks to be used with type in ordinary letter-press printing. This is the form in which photo-engraving comes most under the popular eye, being employed not only by the newspapers for their often crude illustrations, but also in the highest class magazines, in which it has almost wholly superseded wood-engraving, as well as in book-work. The process is one of relief, and while it is an easy matter to reproduce line subjects in this way, it is quite another thing to secure half-tone. For a long time all attempts to meet this difficulty were in vain ; but of late years better success has attended the efforts of experimenters.
The earliest and most thoughtful workers in this branch of photo-engraving were Poitevin and Pretch. Woodbury also worked at the production of relief blocks for typographic printing, using a screen of gauze or network to produce a grained impression ; but in this respect he seems to have been more successful in giving out ideas for others to develop, than in perfecting a workable process himself. Two methods, both with more or less resemblance to Woodbury’s, if not actually based on it, take the lead at the present time. These are the Ives and the Meissenbach processes.
The best known in England is the Meissenbach process, which, after having been worked by Messrs. Bullock and Swan for a number of years, was patented by Meissenbach in 1882. This process is thus described : ” A transparent plate is etched or stippled in parallel lines. A transparent positive is made of the object, the two plates are joined, preferably face to face, and from the combined plates a definite negative is photographed in the ordinary way. In order to cross-hatch and break the lines of the shading, the hatched or stippled plate may be shifted once or twice during the production of the negative. The photographic negative thus obtained may be applied either directly to a zinc plate, or a lithographic transfer may first be made in the usual manner, and the plate subsequently bitten by acid to form a block in relief.”
The Ives process is a very original one in so far as the production of half-tone is concerned, and its excellent results may be seen in many of the American magazines, which until very recently left the English periodicals far behind in regard to their illustrations. Of late, however, the English magazines have been gaining ground very rapidly. Burton thus describes the general principles of the method: ” The process depends on the presence of a half-tone relief on a surface more or less compressible, and consisting of a series of slight eminences forming a grain, as, for example, paper having a surface something like bookbinders’ cloth. The eminences are flattened out in pro-portion to the height of the relief, and if by any means the flattened portions be inked, the result is a grain representing a half-tone on account of the greater size of the dots or grains in the darker than in the lighter parts. In fact, in the deepest shadows the eminences are entirely flattened, giving an even black tone; in the brightest lights they are not touched at all.” The image thus produced was transferred to a zinc block, which was then etched.
An improvement upon his first method was subsequently introduced by the inventor, in the substitution, in place of the grained paper, of an india-rubber pad with V-shaped grooves in it, and a plaster of paris reproduction of the Woodbury relief for the relief itself.
A number of other processes might be referred to, including that of Zuccato, a very ingenious one, having some resemblance to the last named, but they are all more or less alike in principle and need not detain us here.
For the reproduction of line drawings there are many different processes, including the bitumen method invented by Niepce, and employed by St. Victor to copy engravings, which, with but little modification, is still used. Another simple method is to get a proof of a line drawing on stone in transfer ink, and transfer it to the polished surface of a zinc plate in the ordinary litho-graphic manner. After the transfer the plate is slightly wetted, and the lines well inked ; they are then dusted with powdered bitumen, and the plate immersed in an acid bath, in which the uncovered parts are quickly etched away. When other than transfers are to be dealt with, such as pen and ink drawings, engravings, or any other subjects in line, the picture is reduced to the required size by photography. It is then treated as for photo-lithography, transferred to stone, retransferred to zinc, and so proceeded with as above.
A more direct, and, at the same time, sharper process is one in which bichromated albumen is employed. The zinc plate is thinly and evenly coated with albumen in which a quantity of bichromate of potash has been dissolved. A strongly photographed negative, taken in reverse from the drawing, is then put on the plate and exposed. Development is effected with cold water, albumen being, unlike gelatine, thus rendered soluble. The method is remarkable for the clean sharp lines it gives. The plate is etched in the usual way. There are also several gelatine processes which are similar in general procedure to the above. Zinc is generally employed because it is cheap, and because it is readily bitten into by the acid; but sometimes copper is had recourse to for finer and more ex-pensive work.
In the finer and more artistic forms of photo-gravure England seems so have been left almost entirely behind, as in so many other departments where exceptional knowledge of technique is required, by the French and the Germans; nearly all the best reproductions of large pictures by photogravure being done at Paris or Berlin. It is to be hoped, however, that now that the technical schools are turning their attention to photography, and to the various ways in which it may be applied to the industrial arts, photogravure will not be entirely lost sight of.