Raman spectroscopic analysis of azurite blackening

Abstract

Azurite is a basic copper carbonate pigment largely employed in painting realization. The areas painted with azurite are easily alterable and are often less resistant than the other parts of artworks. The azurite alteration in a black pigment, the copper oxide (tenorite), has been studied by micro-Raman spectroscopy. The blackening can be due to thermal or chemical alterations: in the second case the alterations being due to the presence of alkaline conditions. Laser-induced degradation of azurite has been studied as a function of the grain size. The results show that the temperature of the grains decreases as the size increases, and azurite degrades into tenorite only below the critical value of 25 µm. To study the chemical alteration of azurite, the pigment has been applied on the plaster of terracotta samples and analyzed at different pH values by micro-Raman spectroscopy. As opposed to most part of the analytical techniques, it can detect the presence of both azurite and tenorite molecules in the same micro areas, and provides a valuable tool to determine azurite degradation.

On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments

“…It appears to be stable to light and atmosphere, and shows good performances both in oil and tempera mediums [111,112], although its poor hiding power in oil is reported in literature [113]. Degradation of azurite in frescoes seems related to pH and grain size [91,114]. Azurite degrades to green compounds: malachite (CuCO 3 ·Cu(OH) 2 ) and paratacamite/atacamite (Cu 2 Cl(OH) 3 ) are some examples [28,64,111,112,[115][116][117][118].…”

“…Humidity and chloride ions from various sources cause the formation of black copper oxides (CuO) and green chlorides (nantokite CuCl, paratacamite/atacamite or botallackite Cu 2 Cl(OH) 3 [103,116,[118][119][120][121]). Azurite degrades to black tenorite CuO when exposed to heat in presence of alkali [20,68,91,103,113,114,[121][122][123], while cold alkaline conditions might not affect it [111], or cause conversion to malachite [119], or the formation of tenorite via formation of copper hydroxide Cu(OH) 2 [35,64,114]. On the other hand, it is decomposed by acids, such as oxalic acid to form oxalates (CuC 2 O 4 ·nH 2 O, mooloite) [32,55,116].…”

“…Azurite turns bluishblack when exposed to H 2 S vapours [124], especially in frescoes [111], as covellite CuS is formed. The effect of laser irradiation on azurite is the formation of black CuO, which depends on the particle size and therefore on the temperature increase [112,114,125]. Selective biological activity is observed towards lead pigments [124].…”

Recent advances in linear and non‐linear Raman spectroscopy. Part III

“…The azurite alteration in a black pigment, the copper oxide (tenorite), has been studied by micro-Raman spectroscopy by Mattei et al [51] This technique can detect the presence of both azurite and tenorite molecules in the same micro-areas and provides a valuable tool to determine azurite degradation. A comprehensive study by Froment et al [52] reports on the Raman characterisation of about 50 natural red to yellow pigments of various origins.…”

Editorial

“…The The interested reader may also look up some recently published papers in JRS on similar subjects: a Raman microscopy study of nine variants of the green-blue pigment verdigris; 1 identification of pigments in coloured drawings from the Wyung Hall of the Imperial Palace; 2 identification of pigments in a sixteenth century Persian book of poetry; 3 a Raman microscopy and IR spectroscopy study of prehistoric paintings from Los Murciélagos cave (Zuheros, Córdoba, Spain); 4 an application of surface-enhanced Raman scattering (SERS) for the identification of anthraquinone dyes used in works of art; 5 an on-site Raman analysis of the earliest known Meissen porcelain and stoneware; 6 a Raman identification of ancient stained glasses and their degree of deterioration; 7 a study on the use of photobleaching to reduce fluorescence background in Raman spectroscopy to improve the reliability of pigment identification on painted textiles; 8 a study of the corrosion from the printing plates of 'Guan Zi' by Raman spectroscopy; 9 a Raman identification of glassy silicates used in ceramics, glass and jewellery; 10 a Raman spectroscopic analysis of selected astronomical and cartographic folios from the early 13th century 27 a microRaman spectroscopy and particle-induced X-ray emission study of pigments and degradation products present in 17 th century coloured maps; 28 micro-Raman characterizations of mortars in Pompeii; 29 a Raman spectroscopic analysis of azurite blackening; 30 a Raman spectroscopic analysis of a Roman ivory artefact (The Goodmanham plane); 31 a Raman spectroscopic and combined analytical approach to the restoration of severely damaged frescoes (the Palomino project); 32 and an application of surface-enhanced Raman spectroscopy (SERS) to the analysis of natural resins in artworks.…”