Mineral impurities in azurite pigments: artistic or natural selection?

Abstract

Nineteen natural specimens of azurite from European mining locations used in medieval times were analysed by Raman microscopy to investigate the existence and identity of any impurities. Malachite, hematite, goethite and other commonly occurring minerals such as cuprite, rutile and anatase were detected in a significant proportion of the specimens. Other minerals detected, albeit less frequently, include quartz, calcite, cerussite, orthoclase, beudantite and jarosite. These findings indicate that any iron oxides and malachite detected as minor impurities in azurite‐containing museum objects should be taken as a consequence of the natural make‐up of azurite specimens used for the pigments rather than a deliberate addition by the artist. Furthermore, the apparently sensible assumption that black and orange‐brown impurities in azurite pigments are mainly copper oxides (cuprite and tenorite) is incorrect as these particles actually correspond to the iron oxides, goethite and hematite. It is possible with considerable further work that the concurrent association of the less common impurities within a single pigment sample might help to ascertain the provenance of an individual azurite‐containing pigment by relating it to a known historic azurite source. If such a provenance study was carried out, we expect specific localization of an azurite pigment would only be possible if a source region or mine is known to contain azurite with particularly exotic or unusual composition. © 2014 Victoria and Albert Museum, London first published in the Journal of Raman Spectroscopy by John Wiley & Sons, Ltd.