In 1851, an American clergyman named Hill claimed to have discovered photography in colours, but eventually the process was proved to be useless. In 1855 and 1856, Testud de Beauregard brought forward the view that a silver photograph without colour possessed a latent power for colour ; but no one has ever been able to produce colours by his process— save the discoverer.
Poitevin (Comptes Rendus, 1865, p. describes his process for obtaining colours on paper, and the following is his final method : I form upon non-albumenised paper a film of ordinary silver chloride by floating one side of the paper on a io per cent. solution of salt ; when dry, I float this on an 8 per cent. solution of silver nitrate, or the back of the paper is painted with a mixture of equal volumes of a saturated solution of potassium chromate, and a io per cent. solution of cupric sulphate, dried in the dark, and then floated on the silver bath. Chromate of silver is now formed ; I wash with plenty of water in order to remove excess of the nitrate salt and add to the last washing water a few drops of ordinary hydrochloric acid till the red chromate salt is converted into white silver chloride. both these methods of preparing the silver chloride film are equally good. Now, in order to obtain the violet subchloride, I pour into the dish, which contains the paper soaked in water, a small quantity of 5 per cent. stannous chloride solution ; about 20 ccs. (about / oz.) are required for a whole sheet. Now I expose the sheet, without taking it out of the bath, to the action of light, and preferably in the shade than in the sun ; its surface quickly discolours and after five or six minutes, has assumed the desired dark violet colour. It is not advisable to allow the light to act still longer, otherwise a greyish black tone would be obtained, which is not suit able for heliochromy. After the action of light, I wash the sheet in several changes of water, and then allow it to dry in the dark. In this condi tion it is very little sensitive to light, and can be kept for a long time." Poitevin used water acidulated with sulphuric acid, or a very dilute solution of mercuric chloride diluted with sul phuric acid for fixing, and subsequently glazed the pictures with albumen.
Wharton Simpson followed with a suggestion to the use of an emulsion of silver chloride in collodion (1867). Saint Florent (Bull. Soc. Franp., 1874) also suggested a method of using collodio-chloride paper, which was exposed to sunlight for 8o to too seconds, or till reddish black ; immersed in a bath of alcohol ioo ccs. (3f oz.), glycerine 7 ccs. (126 mins.), I per cent.
tincture of iodine 7 ccs. (126 mins.), and am monia 6 drops, for io minutes ; dried in a dark place ; exposed under a coloured transparency for about one hour in sunshine ; fixed in a io per cent, solution of " hypo " ; washed and dried in sunshine. In the fixing bath the colours disappear, but reappear when exposed to the sun, or ironed with a hot iron.
Veress, in 1890, followed up on Poitevin's lines, and in the following year Kopp, of Munster, certainly made some advances, which promised well. The best report of his work is found in Valenta's work, Die Photographic in lichen Farben, 1894.
He floated raw paper on salt solution, then on silver nitrate and again on salt solution, thus obviating any excess of silver nitrate, which both Becquerel and Poitevin had pointed out as pre judicial to the purity of the colours ; the paper was then well washed and exposed, under an acidulated •I per cent, solution of zinc chloride, to diffused daylight till it had assumed a blue grey colour. The paper is washed and dried, and made sensitive to all colours, as well as white and black, by treating with a solution made as follows : Potassium bichromate .1,940 grs. 15o g. Cupric sulphate . .1,94o „ 15o „ Distilled water to . . 20 oz. r,000 ccs.
Dissolve by the aid of heat, and when boiling, add Mercuric nitrate . .1,94o grs. r 5o g.
dissolved in as small a quantity of water as pos sible, and acidulated with nitric acid. A red precipitate forms, which should be filtered out when the solution cools down ; then make the filtrate measure 20 oz. or r,000 ccs. by adding water. This solution will keep well in well closed bottles. The blue-green paper should be immersed in this for half a minute till completely decolorised, drained and immersed in a 3 per cent. solution of zinc chloride till it again becomes blue, then well washed in running water, super ficially dried between blotting-paper, and exposed Whilst still damp. The yellow and green of the spectrum appear at once—that is, with about 3o minutes' exposure—and the picture should then be coated as to these colours with a shellac or celluloid varnish, well heated, immersed in a 2 per cent, solution of sulphuric acid till all the colours appear, thoroughly washed and dried between blotting-paper ; fixed in the above-given mercury bath, in which the colours disappear, and finally immersed in a sulphuric acid bath and coated with a solution of gum arabic con taining 5 per cent. of sulphuric acid. Valenta's improvement was the use of •5-1 per cent. solution of sodium nitrite instead of the zinc chloride solution, and excellent results are attainable.