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.

Unusual pigments found in a painting by Giotto (c. 1266-1337) reveal diversity of materials used by medieval artists

“…It is one of the green-blue particles in the right hand side of the section the source for azurite in Florence was Hungary or Germany. Aru et al analyzed some minerals from places that may have been the sources for azurite have been analyzed but there is not enough information to be able to locate the source for this pigment [7] since its purity does not provide any evidence for the source of Giotto's supply.…”

“…2a shows that the azurite used in this work is very pure, and does not contain any detectable transition element impurities or substitutions, such as zinc or manganese, which are sometimes found in azurite or malachite [5,6]. The cross sections do not contain red particles which are often observed in azurite paint, which have been shown to be naturally occuring iron oxide impurities [7].…”

An investigation of a unique group of painted silk banners from a Polish collection

“…The Cu detected by XRF is probably due to the presence of tenorite (CuO), which is a black copper oxide. This mineral could be found in the oxidized portions of copper deposits where it is associated with other secondary minerals such as cuprite, malachite, azurite, goethite and hematite . Rich copper deposits occur in Poland in the area of the Lower Silesian Voivodeship close to the Sudety Mountains.…”

“…This mineral could be found in the oxidized portions of copper deposits where it is associated with other secondary minerals such as cuprite, malachite, azurite, goethite and hematite. [32] Rich copper deposits occur in Poland in the area of the Lower Silesian Voivodeship close to the Sudety Mountains. The data indicate that the Battle Banner was probably prepared with use of local pigments just before the battle of the Silesian army.…”

Applications of Raman spectroscopy in art and archaeology

“…Nineteen natural specimens of azurite from European mining locations (exploited in medieval times) were analysed by Aru et al . using Raman microscopy to investigate the existence and identify the impurities.…”