The vicissitudes of vivianite as pigment and corrosion product

Abstract

Vivianite, hydrated ferrous phosphate, Fe2+ 3(PO 4)2˙8H2O, can be formed in the soil under reducing conditions. It occurs in wet clay soils, in the presence of a source of phosphate ions such as decomposing bone, and in the absence of sulphur compounds. As an iron corrosion product it sometimes preserves iron finds in excellent condition. When exposed to air, it undergoes oxidation to Fe3+ -containing species, which cause a shift in colour from pale blue to darker blue or blue-green. Vivianite has also been used as a pigment, and the number of published occurrences has been increasing steadily over the last few years. It can discolour, a problem encountered in both Romanesque wall paintings and seventeenth-century Dutch paintings. The occurrence of vivianite as a pigment is sufficiently rare to provide information on dating and provenance. All published occurrences of the pigment to date are reviewed.