Uranium Nitrate Toning [Orange-Red Tone].

The orange-red tone resulting from replacing some of the silver image by uranium ferrocyanide was commonly used in the early years [and, like Prussian blue, was improved to yield a primary dye for several experimental 2-colour processes in the 1920's and became a common 2-colour primary in the 1930's}. The single solution most commonly used seems to have been Uranyl nitrate with Potassium ferricyanide [about 2-4 gms per litre each, acidified by Hydrochloric acid] and were probably stored in separate solutions to be mixed just before use like the Prussian blue. Later formulations contain Oxalates and Alums and keep better.

Modern materials seem to respond less well to this procedure and the colour is less saturated than old examples even after 50 or 60 years.

As will be seen below uranium toning was also used as a starting point for some dye toning procedures.