Ferric oxalate, also known as iron(III) oxalate, is a chemical compound composed of ferric ions and oxalate ligands; it may also be regarded as the ferric salt of oxalic acid. The anhydrous material is pale yellow; however, it may be hydrated to form Fe2(C2O4)3·6H2O which is bright green in colour; Like many oxalates, ferric oxalate has been investigated as a short term treatment for dentin hypersensitivity. It is used in certain toothpaste formulations; however, its effectiveness has been questioned. It is used as the light-sensitive element in the Kallitype photographic printing process.