Cyanotype kit (ferric ammonium citrate)

Description

Classic cyanotype to Herschel’s 1842 formula: two solutions mixed before use, brushed onto paper and exposed to sunlight. It develops in plain water.

How it works

Ferric ammonium citrate is reduced to iron(II) by ultraviolet light, which reacts with potassium ferricyanide to form Prussian blue. No developer is involved at all — the image is already there, and water simply washes out the unexposed sensitiser.

What you need

Cotton paper without optical brighteners, a brush or glass rod to coat with, a contact negative the size of the print, and sun. First tests usually run 5–20 minutes in sunlight; a UV lamp makes the timing more predictable.

Questions and answers

How does the citrate kit differ from the oxalate one?

Citrate is the classic formula: simpler, cheaper, with that characteristic deep blue. The oxalate kit is faster and holds a wider tonal range.