Photographic Printing Processes
( Originally Published 1898 )
IT will be remembered that Niepce's attention was first turned to the problem of fixing the image of the camera obscura by his experiments in lithographic printing; and his earliest successes in photography were in the reproduction of engravings. In other words, they were experiments in photo-printing. Fox-Talbot went a step still further, and his calotype may be said to have been the first effective printing process in which photography is the primary agent. Although improvements have been made in the method, it has continued to be employed to some extent to the present time. One of the chief modifications of the original process was the addition of albumen to give a surface to the print. The albumen fills the pores of the paper, and keeps the sensitive salt of silver, subsequently added, upon the surface.
The credit of the introduction of albumenised paper is usually given to Fox-Talbot, but it seems to belong more properly to Le Gray. Gelatine is now much used in place of albumen.
Since the first publication of Fox-Talbot's method, the number of printing processes gradually evolved out of the photographic art is very great. To deal with them in anything like an exhaustive manner would require a considerable volume. It need hardly be said therefore that only a brief general account can be given of them here.
Photographic printing processes are of two sorts, direct and mechanical. The simplest example of the direct process was that practised by Thomas Wedgwood in 1795 and by Davy in 18o2, though they were unable to fix the image obtained. Their method, as well as that of Fox-Talbot, was a process of silver printing—a process which still largely holds the field in photographic printing.
At the present time silver printing is done for the most part upon fully prepared and sensitized papers. This, however, was not always the case; and even now many photographers sensitize their own paper, even if they do not do the albumenising themselves. The prepared, though unsensitized, paper is floated on a solution of silver nitrate and then dried. The silvered surface is exposed to the action of light in a printing frame, the length of the exposure being timed to the brightness of the light and the character of the negative. Toning is effected by a solution of chloride of gold, though sometimes a platinum chloride (chloro-platinite of potassium) is used. After fixing with hyposulphite of soda, the picture needs to be quickly, though thoroughly, washed. The papers chiefly used for silver printing are of foreign make, and are most commonly prepared with gelatine emulsions of chloride of silver.
The impressions obtained from silver printing can hardly be exceeded in beauty; but there is one drawback to their enjoyment, and that is the doubts that have been thrown on their durability. On account of this tendency of silver prints to fade, efforts were early directed to the discovery of a more permanent method.
Various suggestions were thrown out. and many experiments were made, but the first method discovered that promised real permanency of impression was that of Swan, who introduced his carbon process in 1864. He was, however, but an improver on the ideas of others. As early as 1839. Mungo Ponton made known the fact that under certain conditions bichromate of potash is sensitive to light. In other words, it was found that paper steeped in the bichromate, and then dried, changed its colour when exposed to the light. Another fact that subsequently appeared was that light does not merely alter the composition of the bichromate but that it oxidizes the gelatine of the paper. Upon this discovery is based the much-used carbon process—perhaps the best known of all methods for obtaining durable prints. Gelatine is not the only substance that thus becomes insoluble when exposed to light in conjunction with bichromate of pot-ash, or other chromium salts, starch, gum, and albumen, amongst other like compounds, being similarly affected.
When gelatine is soaked in cold water it absorbs the water and swells; then, if the water is heated, it melts. If bichromate of potash be added to the cold water it also is absorbed. By this absorption of the bichromate the gelatine has been so altered that, on being dried and exposed to the light until the stain caused by the chemical is changed in colour, it no longer possesses the quality of swelling in cold water or dissolving in hot. The action of the light upon the bichromated gelatine has been such as to render it insoluble.
The importance of this discovery as the germ of a practical printing process seems to have struck many minds about the same time, including Poitevin and Beauregard, and several devoted themselves to the task of working it out. It was not done all at once, however, and the various processes that were the result have not all proved equally successful. But to this principle of chemically treated gelatine we owe the " Auto-type " or carbon " process, sometimes styled the pigmented gelatine process. It is one of the most important methods known, and upon it are based the Collotype, the Woodburytype, and even some forms of photo-zinco engraving and photogravure.
There were, however, many difficulties to be overcome before the " carbon " became a practical process. Into these it is not necessary to enter here further than to state that, after many experiments, Swan perfected his method even to the getting of the long desiderated " half-tone." In its final form gelatine was dissolved in water, with which bichromate of potash was mixed ; then finely powdered carbon was added by way of pigment. Paper was coated with the mixture and dried, and was then ready for exposure under negatives.
There being no visible picture, as in a silver or iron print, an actinometer had to be used to time the exposure. The print was now coated with an india-rubber solution and placed upon a sheet of paper similarly treated. After having been dried and passed through a press the whole was soaked in warm water, both the paper and the soluble portions of the gelatine being then removable, and nothing being left but the image. This was necessarily in relief, the depth or shallowness thereof being proportionate to the gradations of the negative, the half-tone being adequately represented. The film was now spread upon the paper that was to be its final support, albumen serving for cement.
A valuable improvement was made in this process by J. R. Johnson in 1869. He found that when the Autotype tissue had been exposed it only required to be soaked in cold water and then placed on a waterproof support in order to adhere by atmospheric pressure. This discovery proved so important that it made of the carbon process a practical method of printing. Johnson used rigid supports; but Sawyer went a step further by introducing flexible supports in the form of paper varnished and so rendered waterproof;
Meanwhile, during the gradual development of the Autotype process, another permanent method, although one of more restricted application, had been brought out (1857). This was the powder process of Messieurs Salmon and Garnier, based on the fact that "ferrous salt resulting from ferric citrate is more hygroscopic than the ferric citrate itself." * Herschel, as will be seen, had thoroughly tested the relative sensitiveness of the different salts of iron, finding that the double citrate of ammonia was more readily acted upon by light than any other, next to it coming the double oxalate of iron and potassium. Upon these properties he built up the processes of printing already referred to, and concerning which more will have to be said later.
In the Salmon and Garnier process paper covered with ferric citrate is exposed under a trans-parent positive, and then brushed over with a fine dark pigment like plumbago. In the parts not acted upon by the light the citrate of iron is sticky, or "tacky," as is the photographic phrase, and so causes the powder to adhere to it, while the parts which the light has effected become so dry that the pigment obtains no hold. The result is improved when some preparation of sugar, gum, or other sticky substance is mixed with the citrate. The process is faulty as regards half-tones; but for the reproduction of engravings it is, in a modified form, still occasionally used with advantage. The same method has been employed, with some variations of procedure, for transferring the photographic image or picture to glass or porcelain in the form of enamel, and with very fine results.
Another method which calls for mention is that based on the sensitiveness of uranium salts, discovered by Gehlen in 1804. Niepce St. Victor and J. C. Burnett seem both to have turned their attention to this metal as a possible means to a printing process about the same time ; albeit Harrison accords priority to the Englishman (1857-8). By this method paper is floated on a solution of uranic citrate, and when dry exposed beneath a negative in the usual way. Development takes place in a solution of nitrate of silver or chloride of gold; a very fine image in metallic silver or metallic gold, as the case may be, being the result.
A variation in the uranium process was introduced by Wothly, and hence called Wothlytype, in 1864. He employed uranic nitrate with other salts in collodion, and appears to have been the first to employ collodin on paper for positive printing. Starched paper was coated with the collodionised nitrate, and was printed until the picture came out as a bluish black, being then intensified by means of gold. Although a company was started to work the process, it did not prove a success. About the same time a method of positive printing was discovered by Mr. G. Wharton Simpson, general manager of the Great Western Railway, in which collodion was used for holding silver chloride in suspension. By means of the film thus produced very good results were obtained both on opal glass and paper, but the process seems never to have become popular in Great Britain.
A method closely allied to the carbon process has recently attracted considerable attention both in this country and abroad, and bids fair to become rather popular, at least where expense is no object. This is the one known as the bichromated gum process or photo-aquatint, which is based on the principle, above referred to, that gum or any colloid substance impregnated with a chromic salt becomes insoluble in proportion as it has been affected by the action of light. The paper is treated with a sensitive mixture of this description, to which plumbago or any colouring matter may be added. Development is effected by water alone. As there is no visible image, exposure has to be timed by the actinometer.
The process places in the hands of an artist a power of developing or suppressing parts of the latent image which enables him to produce by purely photographic means any effect he may wish to secure, from absolute obliteration to varying degrees of half-tone and shadow. It is a method, of course, whereof only the most skilful can avail themselves successfully. A particular development of the gum bichromate process is that of the late M. Artigue, whose papier velours is a distinct advance on anything of the kind yet produced.
Although its chief value is for monochrome, the method is much used on the Continent, particularly in Vienna, for printing in colours. Almost as many colours as desired may be employed. The process, however, is extremely difficult and laborious, necessitating successive printings or exposures, together with extremely careful registration, and notwithstanding the resulting polychrome is often very fine, it is doubtful whether it repays for the ingenuity and patience demanded for its production.
Next to the carbon and allied processes the most valuable is perhaps the invention made by Mr. W. Willis, and patented by him in 1873. It is based on the quality possessed by platinum of being deposited from some of its chemical preparations in an extremely fine black powder when brought in contact with one of the iron salts acted upon by light. Herschel had previously explained a method of obtaining prints in platinum, and others had worked in the same direction, but nothing satisfactory was effected until Willis turned his attention to the subject. In its latest development—patented improvements having been brought out in 1878 and 1880—the platinotype paper is sensitized by means of a mixture of chloro-platinite of potassium and ferric oxalate. When exposed under a negative the ferric salt is reduced to the ferrous state wherever the light has reached it. A faint image is the result. By now " floating the exposed paper on a solution of neutral potassium oxalate, which is a solution of the ferrous oxalate, the platinum salt in contact with it is reduced to the metallic state, and an image is thus built up." The prints, which are extremely delicate and beautiful in colour, are fixed by immersion in dilute hydrochloric acid.
There is no question as to the permanency of platinotype, which, moreover, besides its engraving-like colour, possesses the advantage of extreme simplicity of manipulation. The process is fully described in a little work by Pizzighelli and Hubl, to whom it is indebted for some improvements. By an invention of the former the developer is added to the sensitizing solution applied to the paper, and development takes place simultaneously with the printing by means of atmospheric influences. For the platinum process a dead or "matt " surfaced paper is used, which gives a much more engraving-like appearance to the prints than can be obtained on the glossy surface of gelatine or albumen-faced papers. To this fact is largely attributable the popularity of that process.
Another method, known as the Blue Printing process, though based on the action of light on ferrous salts, is much older than the platinotype. It is the Cyanotype of Sir John Herschel. Twenty-four grains of ammonio-citrate of iron is dissolved in an ounce of water; a given quantity of this is mixed with an equal quantity of a solution composed of forty-eight grains of ferri-cyanide of potassium to an ounce of water, and with it one side of a sheet of white paper is washed by means of a sponge. The paper thus sensitized is used for the most part in the copying of plans and drawings which appear in white lines on a. blue ground. Pictures from photographic negatives can be taken in the same way, and recently for a short time small landscapes printed by this method became rather popular.
There are other processes in use amongst photographers which are more or less adaptations or modifications of those already mentioned. Of such is the iodide of silver on paper (Fox-Talbot's calotype without the oiling of the paper), much used at one time on account of the pictures it gave after brief exposure. Another method still used to some extent for certain descriptions of enlarging work is a transparency on wet collodion films, which are transferred from glass to paper. Both these processes were eventually superseded by the employment of the gelatino-bromide and the gelatino-chloride papers. Swan patented the first-named process in 1879; but W. T. Morgan appears to have been the first to put a gelatinobromide paper on the market, and it soon became extensively used, both for direct enlargement and contact printing.
The gelatino-chloride of silver paper was introduced by Marion & Company in 1884, and soon became popular, partly by reason of its cheapness. It was turned to good account in connection with Urie's automatic printing machine, by means of which some two hundred copies an hour could be printed. The paper gives warm and varied hues, but generally needs toning.
John Urie, of Glasgow, does not appear to have been the first to devise a method of automatic printing. He was anticipated in the United States by Fontayne, and he has had imitators in Germany and France. In England, however, the idea has been developed to an extent and in a direction hitherto apparently undreamed of in the ingenious photo-printing machine of W. Friese-Greene, which, as will be seen from a description of the process in the next chapter, bids fair to revolutionise the whole process of letter-press printing.