(Source : Basic Photography - Michael Langford)
Caution : Blue toning is prone to fading and so cannot be considered as archival.
Water 700 ml
Sulphuric acid (conc) 4 ml
Pot. Ferricyanide cryst 2 g
Water to 1 litre
Sol B
Water 700 ml
Sulphuric acid (conc) 4 ml
Ferric Ammonium citrate 2 g
Water to 1 litre
Note: Always add the acid to the water slowly whilst stirring. It is hazardous to add the water to the acid.
Working sol. : 1 part A + 1 part B.
This toner has an intensifying action, so start with a light print. Immerse prints until required tone is reached, then wash gently until the whites no longer have a yellow stain. Over-washing begins to bleach the blue. While this can be reduced by adding salt to the wash water, a final wash in fresh water is need to remove to salt.