Combined Micro- and Macro scale X-ray powder diffraction mapping of degraded Orpiment paint in a 17th century still life painting by Martinus Nellius

Jonas Simoen,Steven De Meyer,Nouchka De Keyser,Geert Van Der Snickt,Annelies Van Loon,Frederik Vanmeert,Katrien Keune,Ermanno Avranovich,Koen Janssens

doi:10.1186/s40494-019-0324-4

Jonas Simoen, Steven De Meyer + Show 7 more

Open Access

https://doi.org/10.1186/s40494-019-0324-4

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Abstract

The spontaneous chemical alteration of artists’ pigment materials may be caused by several degradation processes. Some of these are well known while others are still in need of more detailed investigation and documentation. These changes often become apparent as color modifications, either caused by a change in the oxidation state in the original material or the formation of degradation products or salts, via simple or more complex, multistep reactions. Arsenic-based pigments such as orpiment (As2S3) or realgar (α-As4S4) are prone to such alterations and are often described as easily oxidizing upon exposure to light. Macroscopic X-ray powder diffraction (MA-XRPD) imaging on a sub area of a still life painting by the 17th century Dutch painter Martinus Nellius was employed in combination with microscopic (μ-) XRPD imaging of a paint cross section taken in the area imaged by MA-XRPD. In this way, the in situ formation of secondary metal arsenate and sulfate species and their migration through the paint layer stack they originate from could be visualized. In the areas originally painted with orpiment, it could be shown that several secondary minerals such as schultenite (PbHAsO4), mimetite (Pb5(AsO4)3Cl), palmierite (K2Pb(SO4)2) and syngenite (K2Ca(SO4)2∙H2O) have formed. Closer inspection of the cross-sectioned paint layer stack with μ-XRPD illustrates that the arsenate minerals schultenite and mimetite have precipitated at the interface between the orpiment layer and the layer below that is rich in lead white, i.e. close to the depth of formation of the arsenate ions. The sulfate palmierite has mostly precipitated at the surface and upper layers of the painting.

Highlights

And atmospherically induced alterations in artists’ pigments are the result of several degradation processes, some of which are already well known while others are still in need of more detailed investigation and documentation
We can conclude that by combining information about the lateral distribution of primary and secondary arsenate and sulfate minerals that are formed due to the degradation of the painters’ pigment orpiment, with in-depth information obtained by microscopic analysis of a paint cross section, it is possible to formulate and substantiate a working hypothesis about the multistep mechanism that gives rise to the formation of secondary minerals
A three-step mechanism appears to be consistent with all observations: (i) formation of arsenate and sulfate ions as a result ofdegradation in their parent paint layer which gives rise to soluble acidic species, (ii) migration of these ions towards other layers where dissolution of mainly C a2+ and P b2+ ions from calcite and lead white can take place and (iii) local precipitation of Pb-containing arsenate and sulphate minerals both on the interfaces between As- and Pb-rich paint layers and at the paint surface

Summary

Introduction

And atmospherically induced alterations in artists’ pigments are the result of several degradation processes, some of which are already well known while others are still in need of more detailed investigation and documentation. Of which the presence in a 17th century painting can be expected/is plausible, the rare As-containing minerals schultenite (PbHAsO4) and mimetite (Pb5(AsO4)3Cl) are found to be present in the general yellow area, but with distributions that are quite different from each other.

Results

Conclusion