A toning solution is composed of chloride of gold in water with
enough carbonate of soda to make it slightly alkaline, and a pinch
of common salt. The action of toning is merely the deposition upon
the metallic silver in the print of a certain minute quantity of
metallic gold from the solution, which deposit causes the change
of color that may be seen while the toning is taking place.
The normal toning solution is varied endlessly by the addition of
acetates, chlorides and nitrates of various metals and salts and
of borax and chalk and such, the object being to produce unusual
tones, such as sepia, blue, blue-grey, brown and black, but the
richest and most agreeable tones may be produced as well by the
simplest means.