Ferric Ammonium Oxalate

Ferric ammonium oxalate ((NH₄)₃[Fe(C₂O₄)₃]·3H₂O; CAS 13268-42-3) is the UV-sensitive iron salt used in platinum/palladium printing and in Mike Ware's "New Cyanotype" formulation.^[1] Chemically it is the oxalate analog of ferric ammonium citrate: both are iron(III) complexes that photoreduce to iron(II) under UV light, which then drives the noble-metal reduction (Pt/Pd) or Prussian-blue formation (cyanotype). The oxalate complex is more photosensitive than the citrate and produces cleaner highlight rendering with greater contrast — the reason Mike Ware selected it for his reformulated cyanotype in 1995.

Photographic uses

• Platinum/palladium printing: Ferric ammonium oxalate is the standard sensitizer iron salt, typically formulated as a 20–27% solution that is mixed with chloroplatinic acid or palladium chloride sensitizer just before coating.^[2]
• Ferric oxalate (alternative form): Some formulas use plain ferric oxalate (without ammonium) for similar but slightly different contrast response.
• New Cyanotype (Ware formulation): Mike Ware's 1995 cyanotype reformulation uses ferric ammonium oxalate plus potassium ferricyanide, producing extended tonal range and higher contrast than classical cyanotype.
• Gravure / photogravure: Used in some copperplate photogravure workflows where sensitivity and sharp threshold response matter more than sensitizer cost.

Practical notes

Supplied as bright green to emerald-green crystalline powder. Extremely light-sensitive — even brief exposure to room light initiates slow photoreduction that darkens the crystal. Store in amber bottles in the dark cabinet; buy only what you'll use within a year. Solutions similarly photodegrade; prepare fresh or store in tightly-sealed amber bottles refrigerated.

Ferric ammonium oxalate dissolves best in distilled water — hard-water mineral content interferes with the sensitizing chemistry and is the single most common cause of Pt/Pd process failure for beginners. Use distilled or de-ionized water exclusively.

Related compounds

Ferric ammonium citrate is the citrate analog, preferred for classical cyanotype but lower contrast. Ferric oxalate (without ammonium) is the plain oxalate iron(III) complex.