Exploiting reflectance FTIR spectroscopy for the in situ identification of pigments in illuminated manuscripts

The non-destructive identification of pigments used in pictorial works is of great interest in the field of the study of cultural heritage, in particular when it is applied in situ and thus in the place of conservation of the artefacts, avoiding the costs and risks related to the transport to the laboratory. As far as the in situ identification of inorganic pigments is concerned, the techniques mostly established are, undoubtedly, X-ray fluorescence (XRF), which however provides only the elemental composition of the pigment; visible-NIR reflectance spectroscopy, not always specific due to the bandwidth; and micro-Raman spectroscopy, highly specific but sometimes of difficult application for those compounds that are only weak scatterers. Therefore the recent development of portable instrumentation for reflectance FTIR spectroscopy is very appealing, allowing the application in the field of a technique highly specific for the identification of compounds. Indeed, several studies have been reported in the literature concerning the use of this technique for the study of cultural heritage.1-2 In the present work a very compact portable FTIR spectrometer which is provided with a reflectance module for contactless measurements and a DTGS detector was used. This kind of instrumentation works at room temperature and exhibits a linear response on the spectral range between 7500 and 375 cm-1, thus allowing to record also overtone and combination bands at higher wavenumbers. In this way, the reflectance FTIR spectra of several inorganic pigments spread on parchment with different binders (arabic gum and egg white) were recorded in order to build a spectral database and moreover to evaluate the influence of the binders on the spectroscopic patterns. Exploiting the specular reflection arising from the samples, the reflectance FTIR spectra acquired could be converted to the more conventional absorbance FTIR spectra applying the Kramers-Kronig transformation. Finally, reflectance FTIR spectra were acquired from some northern-Italian illuminations dating to the XVI century, and then were compared to the reference FTIR spectra, in order to identify the coloring matter. Lapis lazuli, azurite, malachite, lead tin yellow and yellow ochre were recognized in the illuminated details analyzed. In some cases, it was also possible to identify the organic matters used in the illuminations as binders. Basic copper sulfates brochantite and posnjakite as a case study As it has been recently discussed in the literature,3 among all the pigments commonly employed in the ancient paintings, those which provide the green colour are the most elusive from the point of view of identification. Even neglecting the not uncommon cases in which the green hue was obtained by a mixture of blues and yellows, indeed, most of green pigments are based on copper compounds (for example malachite CuCO3·Cu(OH)2 and verdigris Cu(CH3COO)2·2H2O) and thus they result to be indistinguishable by means of techniques such as XRF, able to identify only copper, or vis-NIR reflectance, being the chromophore the same ion Cu2+, and also hardly identifiable by means of Raman spectroscopy, being all of them rather weak scatterers. Moreover, in several pictorial works and in particular in illuminations, besides the most common and already mentioned malachite and verdigris, also basic copper sulphates were discovered, in more detail posnjakite CuSO4·3Cu(OH)2·H2O and brochantite CuSO4·3Cu(OH)2 in northern-European and Italian illuminations dating from XII to XVII century4-7. When these pigments were recognized in other contexts, such as mural paintings8 and ancient wallpapers9, their formation was supposed to be the result of the degradation of the most common copper pigments, also because recipes for the production of such basic copper sulphates were not known before the XVII century and these compounds, although present in nature as minerals, are not widespread. Nevertheless, especially in the case of illuminations, often perfectly preserved, scientists have more often hypothesized that such pigments, of natural origin or artificial origin, were used on purpose6. In the present work, as a case study, pure brochantite and posnjakite were synthesized and characterized. Then reflectance FTIR spectra of these compounds spread on parchment were used for the identification of green pigments of northern-Italian illuminations, recognizing mainly posnjakite. Notably, the reflectance FTIR spectra of the basic copper sulphates mentioned above are reported here for the first time.