Silver sulphide is the most insoluble compound of silver, and consequently if a silver image or a silver halide salt is treated with sulphur or a sulphide, respectively, they will at once be transformed into silver sulphide. Silver sulphide has a color varying from light brown to black, according to its state of subdivision, and the transformation of the image into silver sulphide is by far the most popular method of toning developing-out paper prints, the prints so toned being generally known as "sepia" prints. There are two general methods of transforming the image into silver sulphide:

Portrait Film Negative, Artura Print By M. B. Nicliolson Kansas City, Mo.

Portrait Film Negative, Artura Print By M. B. Nicliolson Kansas City, Mo.

A. Direct toning, with the hypo alum bath; and B. Bleaching and redevelopment.

A. As was explained in the article dealing with fixing, when an acid is added to a solution of hypo, it tends to precipitate sulphur. Now, a solution of alum in water is weakly acid, so that if alum is added to plain hypo without any sulphite present, the solution will, after a time, become turbid and precipitate sulphur. This solution of alum and hypo at the point where it is ready to precipitate the sulphur may be considered as having free sulphur in solution, and if prints are immersed in a hot solution of alum and hypo, the silver image will be converted directly into silver sulphide and the prints will be toned brown. Only one precaution is necessary in order to obtain successful results with the hypo alum toning bath. The bath tends to dissolve the image and consequently if a fresh bath is used, it will weaken the print, eating out the high-lights. In order to prevent this a little silver must be added to the bath, either in the form of silver nitrate or by toning a number of waste prints or by throwing in old Solio prints, which contain free silver. A bath lasts for a long time, and as a general rule a hypo alum which has been somewhat used works better than a fresh bath.

B. The greatest objection to the hypo alum bath is that the bath has a somewhat disagreeable odor, sulphur compounds being liberated from it, and it is rather troublesome to use a bath which has to be heated, so that while hypo alum toning is used on the large scale, smaller quantities of prints are commonly toned by bleaching the silver bromide print in a bath of ferricyanide and bromide, and then treating the bleached print, after washing, with sodium sulphide, which converts the silver bromide directly into silver sulphide.

Sodium Sulphide occurs in white, transparent crystals, which have a strong affinity for water and so quickly deliquesce unless kept carefully protected from the air. It is best kept in a strong stock solution. It is a chemical which very often contains impurities, chiefly iron, and only "tested" sulphide should be used. Old sodium sulphide often contains hypo, since hypo is produced in the oxidation of sulphide, and if hypo is present in any considerable amount, some of the silver bromide will be dissolved by it and the print will lose strength in the high-lights and give a very inferior result.

Portrait Film Negative, Artura Print By M. B. Nicholson Kansas City, Mo.

Portrait Film Negative, Artura Print By M. B. Nicholson Kansas City, Mo.

All sulphides give off a certain amount of hydrogen sulphide, which smells offensively, and which is extremely dangerous to photographic materials, since a very small amount of hydrogen sulphide will convert enough of the silver bromide or chloride of the material into sulphide to produce a severe fog. No photographic materials should therefore be stored in a room where sulphides are kept or where sulphide toning is done.

It has already been explained that the color of silver sulphide depends upon its state of division, and since the state of division of the toned image depends upon that of the untoned image and this again upon the treatment of the material, it is evident that the exposure and development of the print will have an effect upon the result obtained. As a general rule, it may be stated that to get good colors in sulphide toning it is necessary that a print should have been fully developed and not overexposed; a print which is very fully exposed and then developed for a short time will not give a good tone.