Page 127 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I FINGERPRINTING AZURITE Because of its inherent interest
as a pigment, analyses have been carried out on natural azurite to determine the mineral's
common trace elements, such as gold, holmium, samarium, rhenium, palladium, and mercury.
Nothing appears in the literature on the use of these trace elements for investigating the prove­
nance of particular samples of azurite, so this may not be achievable. Recent developments
in laser ablation-inductively coupled plasma-mass spectrometry have made possible the
determination of many additional trace elements, however, some of which may help future
researchers "fingerprint" the azurite used by a particular artist or originating from a certain
source or type of deposit. 9

Conservation issues Azurite can be altered to other products by environmental fac-
for azurite tors, impacting the conservation of materials that incorporate
this mineral. For example, azurite can be darkened by exposure
to sulfur fumes, perhaps due to sulfide formation, as seen especially in mural paintings. Gettens
and Stout (ΐ96β) reported cases of suspected azurite alteration to malachite or, rarely, to para-
tacamite (probably clinoatacamite); and Riederer (i985) found that azurite pigment from the
facades of the elder temple of Aphaia at Aegina, Greece, had been partially transformed to para-
tacamite. Thick layers of azurite in oil paintings often become greenish or dark with age. This
probably results from the reaction of the copper with the oil medium, producing a variety of
copper organometallic compounds, such as copper résinâtes or oleates.
Gutscher and colleagues (i989) investigated the alteration of azurite into tenorite in wall
paintings, a conversion that has also been reported on polychrome sculpture excavated from an
alkaline environment. The alteration of azurite to copper trihydroxychlorides, such as ataca-
mite, in wall paintings is not uncommon and is probably due to the slow ingress of saline solu­
tions or groundwater. This process will decompose azurite, probably with the formation of
atacamite or, in some cases, of clinoatacamite—the newly identified fourth isomer of the copper-
trihydroxychloride system ( Jambor et al. 1996). Clinoatacamite, in fact, may be the mineral
alteration that is identified as paratacamite in the earlier literature on azurite transformation in
wall paintings. Bolingtoft and Christensen (1993) examined a color change in early Gothic wall
paintings dating to around 1275 from the Danish village of Gundsomagle. They found traces of
azurite on the paintings. This information, together with the known color scheme, allowed the
authors to infer that areas of the painting that are now green were originally blue, and that the
color change represented the alteration of azurite to atacamite due to attack by high pH, mois­
ture, and chlorides.
Dei and colleagues (1998) discovered that the use of ammonium carbonate and barium
hydroxide as a treatment for gypsum-encrusted wall paintings caused potentially deleterious
chemical changes to azurite pigments that had altered to a green form—either to malachite or

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