Raman microscopy in art history and conservation science



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Wall paintings and fresco
Wall paintings and fresco are normally unlike other artistic works, due to their large size and by their having been exposed to the elements, light and pollution. As such, mural art rarely survives intact. Raman and FT-Raman spectroscopy has been used to examine the effects of lichen encrustations on fresco art [102-104] and on ecclesiastic architecture [105]. The lichen Dirina massiliensis forma sorediata has been shown to biodeteriorate frescoes aggressively through the chelating action of oxalic acid (H2C2O4) produced by its burrowing hyphae. Calcium oxalate monohydrate (CaC2O4.H2O, whewellite) was found to be present several millimetres below the surface of the frescos, and significant amounts of substratal material, normally calcite or gypsum, have been established to incorporate into the thallus of the lichen, thus adding further disruption to the artwork. The Raman spectra of some biodegraded painted areas suggest that the lichen might also be capable of neutralizing toxic heavy metals from pigments by incorporating them into other oxalate species [106]. The same Dirina mycobiont was also found to be responsible for the white crust, originally thought to be the remnants of an early conservation effort, covering the limestone exterior of an English church [105|. Here, however, the biodeterioration product was found to be calcium oxalate dihydrate (CaC2O4.2H2O, weddellite), suggesting alternative survival strategies of the lichen, possibly related to water storage, depending on its host substrate or its environment.

The identification of pigments by Raman spectroscopy has been performed on samples of wall paintings from Roman Palestine [107], Byzantine Greece [28,108], medieval England [109] and Spain [85, 110, 111] and Renaissance Italy [106]. Many of the same artistic strategies discussed for manuscript illuminations and fine art were shown to have been employed for the painting of murals and frescos. Instances of pigment economy have been suggested by the adulteration of the expensive pigment cinnabar with red lead [37, 85, 110] or red ochre [109] as revealed by Raman analysis. The hierarchical use of costly cinnabar and lapis lazuli for paintings of the Holy Family has been reported in Spanish frescoes whose ancillary figures were painted with diluted cinnabar and an arguably cheaper organic red, possibly a resin [37, 85]. In the study of another fresco bearing geometric designs [37, 110], however, different pigment mixtures ranging from pure cinnabar to

mostly red lead were used indiscriminately, suggesting in this instance that the use of pigments of varying quality was arbitrary or possibly due to numerous artistic hands.
Textiles and plant fibres
FT-Raman analysis has been performed on a number of archaeologically important natural plant fibres, leading to the construction of a library of reference spectra [29]. Attempts have been made to identify the structural changes in archaeological linen that lead to its brittleness and dark colour after long periods of interment [112, 113]. Despite its age and burial conditions, the analysis has shown that a 4000-year-old Egyptian mummy wrapping is in quite good structural condition by comparison of its Raman spectrum with that of a sample of modern linen. However, spectral changes between the mummy linen and a badly deteriorated linen sample from a burial near the Dead Sea (614 AD) are slight, and provide little information as to the nature of the colour change and friability of these textiles [112]. The presence of carotenoid-like compounds in water-soluble extracts of the linen 'crumbs', that is the physical detritus of the material, suggests tentatively that this process might result from enzymatic damage to the cellulose structure via fungal or microbial activity [113].

An analysis of contemporary 'jeans' in a textile museum collection has identified the dye used in the manufacturing process [114]. In almost all instances where the style and cut of the garments match the fashion definition of jeans, the Raman spectrum of indigotin, the main component along with indirubin of indigo dye, was observed. Interestingly, in some textile samples, Raman-inactive bu modes of indigotin gave rise to detectable bands, indicating that the dyeing process had led to the loss of planarity and centrosymmetry in the molecule. Similarly, mordants were also shown to affect the Raman spectra of haemateine dye [83], but no structural changes were detected between neat madder and the same dye applied to wool with various mordants [115].

The analyses of jeans mentioned previously [114] showed that the technique lacked the sensitivity to detect the spectrum of indirubin [60], a compound known through chromatography to exist in small quantities in the native dye. Although the technique was unable in this instance to detect dye compounds that exist in extremely small quantities, a related application using FT-Raman spectroscopy and a fibre sampling accessory identified small amounts of cobalt blue and red matador dyes (1-2% w/w) on acrylic textile fibres [116]. In this work, the spectrum of the polymer was successfully subtracted from that of the fibre and dye to yield a high quality spectrum of the dyestuff alone. An identical manipulation using diffuse reflectance FTIR spectra was less successful.




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