Characterisation of lime-based paints by micro-Raman and external reflection FTIR spectroscopies

Simi Mangani, Gianluca Pastorelli

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Abstract

Pigment characterisation on paintings and other works of art is crucial for finding solutions to conservation, restoration and authentication issues in the context of art museum collections. A variety of non-destructive analytical techniques, specifically elemental and molecular spectroscopies, are commonly employed for this purpose, and libraries of reference spectra are used for identification. A book by Günther Tholen, 100 Farben für den Maler (100 Colours for the Painter), was selected as a tool to evaluate and test a range of spectroscopic methodologies for pigment identification as well as to expand the spectral database of the Centre for Art Technological Studies and Conservation (CATS) of the National Gallery of Denmark (SMK). The book, published in 1950, is a collection of swatches of historical and modern pigments applied in two binders (oil and glue) for house paints, which were initially examined by X-ray fluorescence (XRF) and fibre optic reflectance spectroscopies.
While most of the measurements provided a good match between the colour descriptions in the book and the analytical results, the chemical compositions of some colours were revealed to be either different from the details in their labels or difficult to interpret. The latter group is almost exclusively composed of lime-based paints, which are poorly described in the book and produced very heterogeneous results. Because the data yielded by XRF and reflectance spectroscopies was not sufficient to fully understand the complex nature of such pigments, additional analyses were carried out by external reflection-FTIR and micro-Raman spectroscopies, to obtain further information. Most of the lime-based paints could be identified by their Raman spectra, which were consistent with the expected content reported in literature and the information supplied by various paints manufacturers. Additionally, the FTIR analyses provided a knowledge base for the evaluation of the effect of different binders on the pigments’ spectral features.
This research aimed at characterising each colour in the book’s collection with a range of spectroscopic techniques and the obtained results confirmed the effectiveness of complementary analyses for non‐destructive identification of pigments in different binders. Besides the importance of having access to a comprehensive library of spectroscopic data for pigment identification purposes, this work highlights the usefulness of building a spectral database of lime-based paints from the first half of the 20th century that will be of significant value as a reference for other researchers in the field of heritage science.
Original languageEnglish
Publication date30 Nov 2019
Publication statusAccepted/In press - 30 Nov 2019
Event14th Infrared and Raman Users Group Conference - The Cultural Heritage Agency of the Netherlands, Amersfoort, Netherlands
Duration: 27 May 202029 May 2020
http://www.irug.org/

Conference

Conference14th Infrared and Raman Users Group Conference
LocationThe Cultural Heritage Agency of the Netherlands
CountryNetherlands
CityAmersfoort
Period27/05/202029/05/2020
Internet address

Keywords

  • Lime-based pigments, FTIR, Raman

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