PHOTOGRAPHY 831 Jlodio- >I)ride of minute subdivision of the gold may be conceived when it is stated that, on a couple of sheets of albuminized paper fully printed, the gold necessary to give a decided tone does not exceed half a grain. Collodio-chloridc Silver Printing Process. In the history of the emulsion processes we have already stated that Gaudin had attempted to use silver chloride suspended in collodion, but it was not till the year 1864 that any practical use was made of the suggestion so far as silver printing is concerned. In the autumn of that year Mr George Wharton Simpson worked out a method which has been more or less successfully employed, and is still one of the best with which we are acquainted. The formula appended is the original one which Mr Simpson published : 1 Alcohol . ... 1000 To every 1000 parts of plain collodion 30 parts of No. 1, previ ously mixed with 60 parts of alcohol, are added ; 60 parts of No. 2 are next mixed with the collodion, and finally 30 parts of No. 3. This forms an emulsion of silver chloride and also contains citric acid and silver nitrate. The defect of this emulsion is that it con tains a large proportion of soluble salts, which are apt to crystallize out on drying, more particularly if it be applied to glass plates. The addition of the citric acid and the excess of silver nitrate is the key to the whole process ; for, unless some body were present which on exposure to light was capable of forming a highly- coloured organic oxide of silver, no vigour would be obtained in printing. If pure chloride be used, though an apparently strong image would be obtained, yet on fixing only a feeble trace of it would be left, and the print would be worthless. The collodio- chloride emulsion may be applied to glass, as before stated, or to paper, and the printing carried on in the usual manner. The toning takes place by means of the chloride-of-lime bath or by ammonium sulpho-cyanide and gold, which is practically a return to the sel d or bath. The organic salt formed in this procedure does not seem so prone to be decomposed by keeping as does that formed by albumen, and the washing can be more completely carried out. This is a beautiful process, and deserving of more attention than has hitherto been given to it. latino- Gclatino-citro-chloridc Emulsion. A modified emulsion printing ro- process was introduced by Abney in 1881, which consisted in sus- [oride pending silver chloride and silver citrate in gelatin, there being no ul- excess of silver present. The formula of producing it is as follows : 40 parts. 40 , " 1 Sodium chloride 1. Potassium citrate ( Water 500 -, ( Silver nitrate 150 ,, - "( Water 500 ( Gelatin 300 ,,
Water 1700
Nos. 2 and 3 arc mixed together whilst warm, and No. 1 is then gently added, the gelatin solution being kept in brisk agitation. This produces the emulsion of citrate and chloride of silver. The gelatin containing the suspended salts is heated for five minutes at boiling point, when it is allowed to cool and subsequently slightly washed, as in the gelatino-bromide emulsion. It is then ready for application to paper or glass. The prints are of a beautiful colour, and seem to be fairly permanent. They may be readily toned by the borax or by the chloride of lime toning-bath, and are fixed with the hyposulphite solution of the strength before given, inting Printing with Salts of Uranium. The sensitiveness of the salts th of uranium to light seems to have been discovered by Niepce, and anium the fact was subsequently applied to photography by Burnett in Its. England. One of the original formulae consisted of 20 parts of uranic nitrate with 600 parts of water. Paper, which is better if slightly sized previously with gelatin, is floated on this solution. When dry it is exposed beneath a negative, and a very faint image is produced ; but it can be developed into a strong one by 6 to 10 per cent, solution of silver nitrate to which a trace of acetic acid has been added, or by a 2 per cent, solution of auric chloride. In both these cases the silver and gold are deposited in the metallic state. Another developer is a 2 per cent, solution of ferro-cyanide of potassium to which a trace of nitric acid has been added, sufficient to give a red coloration. The development takes place most readily by letting the paper float on these solutions. otnly Wothly Type. A variation was introduced in the uranium pro- pe. cess by Herr Wothly in 1864, when he employed uranic nitrate with other salts in the collodion, and then coated starched paper with the product. The paper was printed until it assumed a bluish- black image, which was subsequently intensified by means of gold. The most generally used Wothly-type formula, however, consisted of a triple salt of silver nitrate, uranic nitrate, and ammonic nitrate, which were dissolved in collodion. This compound was applied to paper sized with arrowroot, and, after drying, the printing pro ceeded in the usual manner, the image being subsequently fixed with hyposulphite of soda. The prints produced by this method were very beautiful, but for some reason they found no great favour with the public. Printing with Chromatcs. The first mention of the use of Printing potassium bichromate for printing purposes seems to have been with made by Mungo Ponton in May 1839, when he stated that paper, clironi- if saturated with this salt and dried, and then exposed to the sun s ates. rays through a drawing, would produce a yellow picture on an orange ground, nothing more being required to fix it than wash ing it in water, when a white picture on an orange ground was obtained. In 1840 M. E. Becquerel announced that paper sized with iodide of starch and soaked in bichromate of potash was, on drying, more sensitive than unsized paper. Joseph Dixon of Massachusetts, in the following year, produced copies of bank-notes by using gum arable with bichromate of potash spread upon a litho graphic stone, and, after exposure of the sensitive surface through a bank-note, by washing away the unaltered gum and inking the stone as in ordinary lithography. The same process, with slight modifications, has been used quite recently by Simonet and Toovey of Brussels, and is capable of producing most excellent results. Dixon s method, however, was not published till 1854, when it appeared in the Scientific American, and consequently, as regards priority of publication, it ranks after Fox Talbot s photo-engraving process (see below), which was published in 1852. On 13th Decem ber 1855 M. Alphonse Poitevin took out a patent in England, in which he vaguely described a method of taking a direct carbon- print by rendering gelatin insoluble through the action of light on bichromate of potash. This idea was taken up by Mr Pouncey of Dorchester, who perhaps was the first to produce veritable carbon -prints, notwithstanding that Testud de Beauregard took Carbon- out a somewhat similar patent to Poitevin s at the end of 1857. prints. Mr Pouncey published his process on 1st January 1859 ; but, as described by him, it was by no means in a perfect state, half tones being wanting. The cause of this was first pointed out by Abbe Laborde in 1858, whilst describing a kindred process in a note to the French Photographic Society. He says, " In the sensi tive film, however thin it may be, two distinct surfaces must be recognized an outer, and an inner which is in contact with the paper. The action of light commences on the outer surface ; in the washing, therefore, the half-tones lose their hold on the paper and are washed away." Mr J. C. Burnett in 1858 was the first to endeavour to get rid of this defect in carbon-printing. In a paper to the Photographic Society of London he says, " There are two essential requisites ... (2) that in printing the paper should have its ifwprepared side (and not its prepared side, as in ordinary printing) placed in contact with the negative in the pressure- frame, as it is only by printing in this way that we can expect to be able afterwards to remove by washing the unacted-upon portions of the mixture. In a positive of this sort printed from the front or prepared side the attainment of half-tones by washing away more or less depth of the mixture, according to the depth to which it has been hardened, is prevented by the insoluble parts being on the surface and in consequence protecting the soluble part from the action of the water used in washing ; so that either nothing is removed, or by steeping very long till the inner soluble part is sufficiently softened the whole depth comes bodily away, leaving the paper white." This method of exposing through the back of the paper was crude and unsatisfactory, and in 1860 Fargier patented a process in which, after exposure to light of the gelatin film which contained pigment, the surface was coated with collodion, and the print placed in warm water, where it separated from the paper support and could be transferred to glass. Poitevin opposed this patent, and his opposition was successful, for he had used this means of detaching the films in his powder-carbon process, in which ferric chloride and tartaric acid were used. Fargier at any rate gave an impetus to carbon -printing, and J. W. Swan (to whom electric lighting owes so much) took up the matter, and in 1864 secured a patent. One of the great features in Swan s innovations was the production of what is now known as "carbon-tissue," made by Carbon- coating paper with a mixture of gelatin, sugar, and colouring tissue, matter, and rendered sensitive to light by means of bichromate of potash or ammonia. After exposure to light Swan placed the printed carbon-tissue on an india-rubber surface, to which it was made to adhere by pressure. The print was immersed in hot water, the paper backing stripped off, and the soluble gelatin containing colouring matter washed away. The picture could then be re- transferred to its final support of paper. In 1869 J. R. Johnson of London took out a patent in which he claimed that carbon- tissue which had been soaked in water for a short period, by its tendency to swell further, would adhere to any waterproof surface such as glass, metal, waxed paper, &c. , without any adhesive material being applied. This was a most important and fruitful improvement. Johnson also added soap to the gelatin to prevent its excessive brittleness on drying, and made his final support of gelatinized paper, rendered insoluble by chrome alum. In 1874 J. R. Sawyer patented a flexible support for developing on ; this was a sized paper coated with gelatin and treated with an ammoniacal solution of
