Nickel as a toning option for Cyanotype is not often mentioned. I first noticed it when reading Mike Ware - the “Cyanotype II” inventor’s extensive book on Cyanotypes.
The source for Mike Ware’s book is research done by Holtzman H, and his article “Alkali resistance of the Iron blues” published in Industrial Engineering Chemistry 37, 9 (1945).
As many of you probably have noticed, alkali buffered papers work really bad for cyanotypes - and exposing the print to an alkaline solution like bleach or sodium carbonate is a part of the process for “bleach first” toners. Nickel toning was/is used to enable Prussian blue to be used in alkaline environments. For cyanotype printers - another interesting side effect of Nickel, is the green/turquoise tone it makes on the print
The results are striking, even with weak solutions. After about 15 minutes of toning in a 1% bath a split-tone happens where highlights are drawn against a green/turquoise that works really well with the dark Prussian blue.
Mike Ware suggests Toning with a 10% solution and after one hour of toning the entire image should be shifted towards green - I will absolutely try that in a later experiment.
Artwork by Flugswamp. X marks the s pot. Print on the right toned with Nickel (II) Sulphate 1% ~ 15 minutes