Abstract
In this paper we present the results of quantitative measurements on the pigment chrome yellow (PbCr1−xSxO4 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 PbCr1−xSxO4 in mixtures with PbCrO4. 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.
Highlights
Chrome yellow pigments were invented in the early nineteenth century, with hues ranging from lemon-yellow to deep orange-yellow depending on the composition and crystal structure of the pigment [1, 2]
Comparison with Raman spectroscopy showed that scanning electron microscopy-wavelength dispersive X-ray analysis (SEM–Wavelength Dispersive X-ray analysis (WDX)) is rather 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 sensitivity of scanning electron microscopy (SEM)–WDX seems to be comparable to Raman spectroscopy; a detection limit of around 5 wt% for lead chromate sulphate (PbCr0.5S0.5O4) in lead white was determined for both techniques
Summary
Chrome yellow pigments were invented in the early nineteenth century, with hues ranging from lemon-yellow to deep orange-yellow depending on the composition and crystal structure of the pigment [1, 2]. To test the detection limit of WDX, we measured a series of standards where primrose chrome yellow pigment was mixed with lead white. The pure primrose chrome yellow pigment contained around 50 mol% lead(II) sulphate ( PbCr0.5S0.5O4) as determined by WDX.
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