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The Thiourea Toner is actually a sulfide toner. In alkaline solution, thiourea (thiocarbamide) reacts to yield sulfide ions, the active ingredient in the toning bath. The advantage of this process is the avoidance of the foul smelling sodium sulfide bath.
To obtain beautiful rich brown, sepia-like tones, the print is first bleached using a ferricyanide-bromide bath then redeveloped in a sulfide bath. In this process, the silver metal in the print is first converted to silver bromide then to brown silver sulfide. The resulting print can give the impression of a print made in the late 1800's.
Table of Contents
Sulfocarbamide Sulfourea Thiocarbamide
White powder or white/colorless, prismatic crystals The substance is soluble in water and ethanol
In gold toning baths Clearing yellow stains from prints In Mordant solutions for dye toning
Thiourea is neither toxic nor corrosive but, unfortunately, it is a potential carcinogen. Use rubber gloves when mixing or handling this compound or its solutions. Wash the work area, trays, and all mixing utensils with water followed by soap and water. Should a solution containing thiourea be spilled on the skin, wash immediately with water followed by soap and water.