Raman spectroscopy of green minerals and reaction products with an application in Cultural Heritage research

Abstract

Raman spectroscopy is a powerful technique for the characterization of materials and is of valuable use in archaeometrical research in general. Green compounds of natural or synthetic origin are found in many research areas, ranging from mineralogy, to pigment identification, to corrosion studies. However, a detailed and comprehensive database of spectra and references is still missing in the literature. This paper provides both, a literature review and downloadable Raman spectra of reference products, to the researcher dealing with green materials in cultural heritage. Moreover, it tackles nomenclature issues. The collected spectra are discussed in relation to the preliminary/commercial identification of the material itself and to the published data. Practical aspects regarding the laser wavelength selection are also discussed with regards to the comparison to published reference spectra. The range of studied green materials is wide and encompasses Cu containing compounds (natural and synthetic, more or less known as pigments or degradation products, including polymorphs of the same formula), Fe based (green earths and synthetic organic pigments), modern Cr and Co green pigments. This approach is illustrated by analysing a cross section of a green zone of the Early Netherlandish panel painting ‘Ghent Altarpiece’ by the Van Eyck brothers. Copyright © 2016 John Wiley & Sons, Ltd.

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

“…In fact a variety of salts (organic acids salts such as copper citrate [104], silicates, phosphates, sulphates, chlorides, etc.) were as well used as pigments [105,106], and should not be regarded anymore as degradation products only. Moreover, the situation is complicated by inconsistent nomenclature use in artistic literature [1,105].…”

“…were as well used as pigments [105,106], and should not be regarded anymore as degradation products only. Moreover, the situation is complicated by inconsistent nomenclature use in artistic literature [1,105]. It is well known that malachite and azurite are not stable in fresco, and that they tend to discolour in oil [50].…”

“…Green copper sulphates are commonly found as degradation products on copper artefacts exposed to polluted environments [105]. Brochantite was identified as a pigment as well [1], but it can transform into the more stable polymorphs, antlerite, langite or posnjakite, according to the given conditions of relative humidity, inorganic pollutants (SO x ) and biological activity (affecting the pH and the type of acids) [127,129,168].…”

Micro‐Raman spectroscopy and complementary techniques (hXRF, VP‐SEM‐EDS, μ‐FTIR and Py‐GC/MS) applied to the study of beads from the Kongo Kingdom (Democratic Republic of the Congo)

“…Also, here, degradation products such as bicarbonates (NaHCO 3 ) or sulfates (MgSO 4 ∙H 2 O) (1047 cm −1 ) were identified. Moreover, the main band of a copper salt (sulfate or phosphate) is visible at 972 cm −1 . The absence of other weak bands makes the identification of the exact compound impossible; however, the presence of Cu could suggest that the red glass is coloured by means of cuprite.…”

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

“…Coccato et al described the Raman spectroscopy of green minerals and reaction products with an application in cultural heritage research. Their paper provides both a literature review and downloadable Raman spectra of reference products to the researcher dealing with green materials in cultural heritage . Tunon and co‐workers applied micro‐Raman spectroscopy to the study of Iberian archaeological materials.…”