The examination of Van Gogh’s chrome yellow pigments in ‘Field with Irises near Arles’ using quantitative SEM–WDX

• Published in: Heritage science 2019-12, Vol.7 (1), p.1-11, Article 100
• Main Authors: Geldof, Muriel, van der Werf, Inez Dorothé, Haswell, Ralph
• Format: Article
• Language: English
• Subjects: Chemistry and Materials Science; Cross-sections; Cultural heritage; Cultural resources; EDX; Lead; Low concentrations; Materials Science; Optical microscopy; Pigments; Quantitative analysis; Raman; Raman spectroscopy; Research Article; Scanning electron microscopy; SEM; Spectrum analysis; Sulfur; Van Gogh; WDX; X ray analysis
• ISSN: 2050-7445; 2050-7445
• DOI: 10.1186/s40494-019-0341-3

In this paper we present the results of quantitative measurements on the pigment chrome yellow (PbCr 1−x S x O 4 with 0 ≤ x ≤ 0.8) using scanning electron microscopy-wavelength dispersive X-ray analysis (SEM–WDX). Traditionally, Optical Microscopy (OM) in combination with scanning electron microscopy-energy dispersive X-ray analysis (SEM–EDX) is used for the identification of many pigments in paint cross-sections based on their particle characteristics and elemental composition. However, in the case of chrome yellow, the lead (Pb) and sulphur (S) peaks overlap, which makes quantitative analysis unreliable. SEM–WDX does not suffer from this problem and we have demonstrated that this technique can distinguish different types of chrome yellow based on the quantification of the sulphur-content of the pigment. This identification can be performed on paint cross-sections, allowing for distinction between chrome yellows in different paint layers. In addition, our study showed that the different types of chrome yellow can still be identified even in low concentrations. Van Gogh made wide use of different hues of chrome yellow. Using this method, we have identified the types of chrome yellow he used in Field with Irises near Arles , which we have been able to correlate with the information in his letters. Raman spectroscopy of the same samples confirmed the SEM–WDX results, but evidenced a higher sensitivity of the latter technique in revealing small amounts of sulphur-rich PbCr 1−x S x O 4 in mixtures with PbCrO 4 . SEM–WDX is also more accurate, because it allows the lead(II) sulphate fraction to be determined within 1 mol% absolute, whereas with Raman spectroscopy only relatively broad ranges can be defined. The on-going research of Van Gogh’s paintings as part of a cataloguing project—a collaboration between the Van Gogh Museum, the Cultural Heritage Agency of the Netherlands and Shell—opens the way for a comprehensive comparison of the chrome yellows used by Van Gogh using SEM–WDX.