Ferroblend [cupro-cyanotype]

FerroBlend is a duo-tone contact printing process that combines the chemistry of cyanotype and cuprotype to produce split-tone prints. This results in cool blue shadows and warm reddish-brown highlights in a single image. It was developed by Raghavendra Udupa and it's one of the newest processes in the iron-based [siderotype] family.

What interests me about ferroblend is the tonal range you get without a separate toning step. Most iron-based processes produce a single color. You can tone afterward if you want something different. FerroBlend builds two distinct pigments during development, so the split-tone character is inherent to the process rather than added after the fact.

The sensitizer is the same as a classic cyanotype [ferric ammonium citrate and potassium ferricyanide] mixed in equal parts and coated onto paper. Exposure works the same way too: UV light through a contact negative. If you can make a cyanotype, you already know most of the physical workflow.

The difference is in what happens after exposure.

Instead of washing in water, the print goes into a dedicated developer that introduces copper ions. During development, two pigments form simultaneously: Prussian blue [the same blue as cyanotype] and Hatchett's brown [a reddish-brown copper compound]. The blue settles into the shadows, the brown builds in the highlights. A clearing bath and water wash follow.

The process offers some creative control over the balance of warm and cool tones through developer additives, and an optional toner can shift the shadows toward black. Paper choice matters — as it does with all iron-based processes — and unbuffered or pre-acidified papers tend to give the cleanest results.

How I use this process

I'm drawn to ferroblend because it extends the palette of what iron-based printing can do without adding complexity to the coating and exposure steps I already use. You can see my ferroblend work and compare it with my cyanotype and kallitype prints to see how the processes differ.