The washed prints are placed one by one into this bath, and pass from an initial red-brown through dark brown and purplish-black to purplish-brown. Toning may be stopped at any stage, and after fixing, washing and drying, the print will be red-brown, warm black, or pure black ; slight variations will occur from one paper to another. As the prints reach the desired tone they should be removed one by one and placed in water, where they may be left until sufficient have accumulated for fixing.
535. Sulphocyanide Toning. If a solution of a thiocyanate l (sulphocyanide) is added pro gressively to a solution of a gold salt, the red precipitate of auric sulphocyanide which is at first formed re-dissolves in the excess of sulpho cyanide to give the complex colourless sulpho cyanide. This double salt slowly (or rapidly under the influence of heat) changes into aurous sulphocyanide.
These solutions can be used for toning print out papers (lleynier, 1863), and, under certain conditions, to be explained later, development papers also (§ 593). Tones which are almost black can be obtained by replacing the sulpho cyanide by thiocarbamide or thiourea 2 (A. Helain, 1902).
As these compounds are solvents of silver chloride, a fresh toning bath attacks the silver chloride image to some extent, causing very rapid toning. In addition, the finished image may have a very much greater proportion of gold in it than it would have had if it had been toned in an alkaline' bath ; these properties are obviously not to be found in an old bath saturated with silver chloride. It seems well
established, however (R. E. Blake Smith, 1913), that the toning is not due exclusively to the gilding of the image, but that a small portion at least is due to sulphuration during the fixing, which follows the toning ; the tone is modified considerably in the process of after the solution of the silver chloride is complete, and the image becomes first a yellow-brown, and then slowly changes to purple.
536. As the sulphocyanide toning bath is very readily exhausted, and as its action is very rapid even when the solution is weak, the best method of working is to use only the amount of chemicals corresponding exactly with the treatment of a given number of prints, according to the method long recommended years ago by the Eastman Kodak Co. for gelatine P.O.P.
Prepare two stock solutions— A. Gold chloride 17.5 gr. (2 grin.) Water, to make 20 oz. (I,000 C.C.) B. Ammonium sulphocyanide 175 gr. (20 grin.) Water, to make 20 or. (1,000 C.c.
and according to the number of prints to be toned, and having regard to the colour required, take of each solution A and B—