Prussian Blue Toning.

Prussian Blue toning was very commonly used and was probably in most cases carried out in a single solution. The silver image is converted to a mixture of silver ferrocyanide, which is colourless or white [but does have some density to light] and ferric ferrocyanide or Prussian blue. Prussian blue, when fresh is a strong blue but seems to fade slowly in time. It also is susceptible to fading, sometimes entirely, with the effect of ultra violet light from projector lamps.

Two separate solutions, one of 1-2 gm per litre of potassium ferricyanide with 2 ml per litre of concentrated sulphuric acid, the other of 1-2 gm per litre of ferrous ammonium sulphate or citrate with 2 ml per litre of sulphuric acid, were mixed in equal proportions immediately before use and the positive film placed in it. When the process had continued to the extent the operator wanted, the film was removed and washed, sometimes in a dilute hydrochloric acid to help remove the yellow stain also produced.

The whole process was critical in many ways.

If the original print still retained some fixing salts the process would not work at all or only patchily.

If stopped before completion then some silver image remained and the image was a dark "navy" blue. If washing wasn't sufficient, the base retained a yellow stain. It was also known that if washing was too long the strong blue colour could be washed out.

It as possible that the yellow base stain [which often fades to pink in time] was actually required as part of the effect or that the examples seen today of what we consider is a Prussian blue tone on a pink tinted base is actually a Prussian blue tone with insufficient washing fading in time from yellow to pink!

Prussian blue, ferric ferrocyanide, is a blue-cyan in colour, and close to the subtractive primary Cyan, and in the 1930's was used as primary colour for several 2 and 3 colour processes. During this period more stable solutions were used which lasted longer and did not stain the emulsion so much. A typical example consisted of Ammonium persulphate, Ferric alum, and Oxalic acid as well as Potassium ferricyanide.

Oxalates were used in 1920's Prussian blue toning processes presumably to increase stability.