Abstract
The use of non-invasive macroscopic imaging techniques is becoming more prevalent in the field of cultural heritage, especially to avoid invasive procedures that damage valuable artworks. For this purpose, an X-ray powder diffraction scanner (MA-XRPD) capable of visualising crystalline compounds in a highly specific manner was recently developed. Many inorganic pigments present in paintings fall into this category of materials. In this study, the 17th century oil painting Girl with a Pearl Earring (c. 1665) by Johannes Vermeer was analysed with a combination of transmission and reflection mode MA-XRPD. By employing this scanner in reflection mode, the relative sensitivity for compounds that are present at the paint surface could be increased, establishing it as a highly relevant technique for investigating the degradation processes that are ongoing at paint surfaces. Many of the original pigments employed by Vermeer could be identified, along with four secondary alteration products: gypsum (CaSO4·2H2O), anglesite (PbSO4), palmierite (K2Pb(SO4)2) and weddellite (CaC2O4·2H2O). The formation of gypsum was linked to the presence of chalk in the upper glaze layer while the formation of palmierite and weddellite is driven by the presence of lake pigments (and their substrates). In this manner, MA-XRPD can also be used to pinpoint locations relevant for sampling and synchrotron µ-XRPD analysis, which provides information on the microscopic make-up of the paint. A paint cross-section taken from an area rich in palmierite was analysed with synchrotron µ-XRPD, which confirmed the presence of this secondary compound at the interface of the upper paint layer with the ground layer as well as the presence of anglesite in the ground layer. The capacity of MA-XRPD to identify and chart secondary alteration products in a non-invasive manner has only very recently been demonstrated and makes it a highly relevant technique for the assessment of the chemical condition of works of art.
Highlights
As a master of light and colour, Johannes Vermeer’s palette was both limited and distinctive
These samples can be analysed with laboratory-based techniques, such as scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM–EDX) and Fourier-transform infrared (FTIR) spectroscopy [7, 8]
Synchrotron-based methods such as microscopic X-ray powder diffraction (μ-XRPD) and X-ray absorption near edge structure (μ-XANES) have been employed, for example to reveal the mechanism behind the conversion of red lead to plumbonacrite and to investigate the degradation of cadmium yellow pigments [4, 9]
Summary
As a master of light and colour, Johannes Vermeer’s palette was both limited and distinctive He made extensive use of the costly pigment ultramarine and used several red pigments, including vermilion and red lakes [1, 2]. Degradation phenomena can cause binding media, varnish layers and original pigments to undergo chemical and physical transformations, leading to discolouration or a deterioration of the structural integrity of the paint material. These processes can be initiated by internal factors, for example the chemical interaction of pigments with organic binders, or by external factors, which can range from environmental. Synchrotron-based methods such as microscopic X-ray powder diffraction (μ-XRPD) and X-ray absorption near edge structure (μ-XANES) have been employed, for example to reveal the mechanism behind the conversion of red lead to plumbonacrite and to investigate the degradation of cadmium yellow pigments [4, 9]
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