| Double hyposulphite of gold and sodium (sel d'or) | ½ | grain. |
| Hypo. | 1 | " |
| Water | 1 | ounce. |
Upon the print assuming a purple-gray color it was withdrawn and fixed in a sixteen per cent. solution of hypo. to dissolve the unacted upon silver chloride. Gold, when in a fine state of subdivision, is of a rich purple color. The layer obtained by deposition upon a silver image is very finely divided; hence the color. The only object in continuing the toning action beyond the stage at which a good surface color has been reached is to obtain a deposit of sufficient density to completely neutralize the red color of the organic silver image beneath; therefore, it is preferable, in forming a judgment of toning action, to examine proofs by transmitted light rather than by reflected only.
Before dealing with the various formulæ for alkaline toning I should like to step out of the golden track to say a few words on platinum tetracloride, PtCl4.
Experiment 3.—Platinum Toning.—The value of a platinum salt as a toning agent for silver images has been thoroughly demonstrated before you by Mr. Henderson, when he initiated us into the secrets of ceramic photography. My trials with this salt as a toning agent for paper proofs have only been partially successful. By that I mean that toning does take place when a dilute solution is employed, but the action is too tardy for demonstration here to-night, since anything like a black tone could not be obtained under half an hour. You will observe that the surface becomes covered with chloride, showing the necessity for copious washing. Yellow or discolored prints are bleached when toned in this bath, the whites becoming very pure. The formula here given is capable of producing a very good shade of brown in less time, and should be permanent, since platinum is a metal practically unaffected by the atmosphere; and I think there is good reason to suppose that if a thin coating of platinum could be deposited on the silver image, the protection offered would be more economical as well as stable. Something has already been done in this direction, but not in recent years.
The following is the composition we are now using:
| Platinum tetrachloride, sirupy solution, color of old East India sherry | 5 | minims. |
| Hydrochloric acid | 150 | " |
| Water | 20 | ounces. |
Wash away the free silver thoroughly, warm the toning solution to 70° Fahr., and fix in a twenty per cent. hypo. bath.
Mr. A. Watt, in the second volume of the News, gives a formula which runs as follows:
| Solution of platinum | 30 | minims. |
| Hypo. | 3 | grains. |
| Hydrochloric acid | 5 | minims. |
| Water | 5 | ounces. |
This bath is said to act instantly, but I have not had an opportunity to test it. The strength of the platinum solution here given is indefinite, but any of our experimental members can soon ascertain the amount of dilution necessary to obtain the most favorable results.