THIS is another interesting laboratory process of no practical value as, so far, no means of fixing the images has been discovered. It is named after J. T. See-beck, who, prior to 1810, sent to the poet Goethe a treatise on the action of light on silver chloride, in which he disclosed the fact that under the influence of the spectral rays it assumed the colors incident upon it. Sir John Her-schel in 1840, Robert Hunt, J. W. Draper and others followed on the same lines. Later, E. Becquerel of Paris paid particular attention to the subject and so far modified the methods of the prior workers as to use copper plates silvered on their surface. The plate was protected on the back with varnish to protect the copper base, and was hung by wires in a one to eight solution of hydrochloric acid and connected with a two-cell bichromate battery, the opposite pole being a platinum plate. The acid was electrolytically decomposed, the chlorine attacking the silver and forming silver chloride. The action was so regulated that a definite amount of chloride was formed, and the plate was then dried and heated until the originally white silver chloride turned to a delicate rose color. On exposure to the spectrum, or colored light, it assumed the colors more or less correctly. Becquerel was followed by Niepce de St. Victor, who also used silvered plates, and A. L. Poitevin, who did not publish the first account of his work with paper as the support until 1865, though it was begun in the early 5o's. The chloride was produced by alternate baths of salt, and silver nitrate and salt, as for the success of the process an excess of silver nitrate must not be present. To accelerate the action, the paper was treated with an alkaline bichromate and cupric sulphate, which were used with the idea of absorbing the chlorine set free by the action of light. Practically this process is the basis of all subsequent processes.
Wharton Simpson, in 1866, published a method in which collodio-chloride of silver emulsion was coated on glass, exposed to light until it turned slate color, then under the colored original. To those desirous of making their own paper on the above lines, the following directions may be useful. Dissolve 20 g of silver nitrate in 12 ccm distilled water by the aid of heat, and add to 250 ccm of 96% alcohol; then add 10 g of pyroxylin, and then 250 ccm of ether. Also add 10 g of pyroxylin to 250 ccm of alcohol, dissolve in this 8 g of crystallized strontium chloride, and add 250 ccm of ether. Pour the second solution into the first in small quantities, shaking after each addition. Allow to stand twenty-four hours, and it is fit to use. After coating the paper or glass it should be dried in front of a fire.
Any chloride may be used instead of the above in equivalent weights, for instance, 4.3 g of potassium, 5.8 g of sodium, 6.3 g of ammonium, 6.1 g of calcium or 7.0 g of magnesium chloride; all being the anhydrous salts.
It is not, however, such an easy matter for the novice to coat collodion on paper, and any commercial printing-out paper may be used, if treated according to the following method, which was patented by Kopp in 1891, although it is practically Poitevin's 1866 process. Immerse the paper in a 5 per cent solution of salt for fifteen minutes. The purpose of this is to convert into silver chloride the excess of silver nitrate and organic salt, usually citrate or tartrate, that are always present in these papers. Then hang up to dry, and in this condition it will keep for a long time in the dark. Then in a clean dish immerse in the following:
Zinc chloride 1 g.
Sulphuric acid 12 drops.
Distilled water 1000 ccm.
Expose to diffused daylight until the paper turns bluish-green, taking care that the surface of the paper is always beneath the solution, and that the bluish-green color is not overstepped. After this the paper should be well washed, dried off between blotting papers, and dried in the dark. In this condition it will keep for a long time. The following solution is then prepared:
Potassium bichromate 150 g.
Cupric sulphate 150 g.
Distilled water to 1000 ccm.
Heat until this boils, and then add:
Mercuric nitrate 150 g.
Nitric acid 5 ccm.
Distilled water q. s.
The nitrate should be placed in a porcelain evaporating dish, the acid added, and then distilled water in small quantities, heat being applied and more water added in small quantities until solution is effected. This should then be added to the boiling copper solution with constant stirring. A reddish precipitate will be formed, and the solution should then be allowed to cool and filtered, and either water added or the solution evaporated till it measures 1000 ccm. This will keep well in the dark in a stoppered bottle. The blue-green paper should be immersed in this solution for thirty seconds, or until it becomes colorless, well drained and then immersed in a 3 per cent solution of zinc chloride and rocked till it turns blue again, then well washed in running water, blotted off, and allowed to hang up for about ten minutes and exposed while still damp. Yellow and green soon make their appearance, and when sufficiently intense,' the paper should be removed and the yellow and green parts painted with a varnish. An ordinary negative varnish may be used, and two or three coats should be given, the paper being heated after each coat so as to thoroughly dry it. The other colors can be seen but faintly, being covered with a yellow fog, and to remove this the print should be immersed in a 2 per cent solution of sulphuric acid and rocked until the colors are visible, then rapidly washed, blotted off and dried.
In this condition all the colors, as well as black and white, should be well represented, but they are not permanent, nor is it possible to make them completely so; but they can be made more so by immersion for five minutes in the mercury-bichromate bath, in which all color disappears, then in the sulphuric acid bath, which again develops the color. The print should be thoroughly blotted off and painted with gum arable acidulated with 5 per cent sulphuric acid. This can be made by dissolving 20 g of gum in 100 ccm of water, adding 5 ccm of strong sulphuric acid, with constant stirring, and then filtering. This acts as a provisional fixer or desensitizer and also brightens up the colors by giving them more gloss.
Instead of the zinc chloride solution, advised above, a one per cent solution of sodium nitrite (not nitrate) may be used. Undoubtedly the above process is the most satisfactory if correctness of coloring is considered, but some fairly interesting results can be secured, with far less trouble, by treating commercial printing-out paper with the salt or zinc chloride solutions to insolubilize the excess of silver, then merely exposing to light until it turns violet or slaty blue, and exposing under the colored original. This does not however, give whites or blacks, and better results are obtained by exposing under the nitrite first.
The results obtained by Kopp's method may be examined by weak daylight, and they will keep in the dark unchanged for years; some have been kept for twenty years.