Unraveling the Composition of Rembrandt's Impasto through the Identification of Unusual Plumbonacrite by Multimodal X‐ray Diffraction Analysis

Lead(II) Formate in Rembrandt's Night Watch: Detection and Distribution from the Macro- to the Micro-scale

The Night Watch, painted in 1642 and on view in the Rijksmuseum in Amsterdam, is considered Rembrandt's most famous work. X-ray powder diffraction (XRPD) mapping at multiple length scales revealed the unusual presence of lead(II) formate, Pb(HCOO)₂ , in several areas of the painting. Until now, this compound was never reported in historical oil paints. In order to get insights into this phenomenon, one possible chemical pathway was explored thanks to the preparation and micro-analysis of model oil paint media prepared by heating linseed oil and lead(II) oxide (PbO) drier as described in 17^th century recipes. Synchrotron radiation based micro-XRPD (SR-μ-XRPD) and infrared microscopy were combined to identify and map at the micro-scale various neo-formed lead-based compounds in these model samples. Both lead(II) formate and lead(II) formate hydroxide Pb(HCOO)(OH) were detected and mapped, providing new clues regarding the reactivity of lead driers in oil matrices in historical paintings.

The Ins and Outs of 14C Dating Lead White Paint for Artworks Application

Lead white is known as one of the oldest pigments in art and can be used as a dating material. Upon production following the Stack process, the ¹⁴C isotope of atmospheric carbon dioxide is fixed in the carbonate, and its radiocarbon dating can be used as a proxy for the age of a painting. The previously reported carbonate hydrolysis protocol reaches its limitation when confronted with samples presenting a mixture of carbonates, such as lead carbonate (cerussite or hydrocerussite), calcium carbonate (calcite), and/or calcium magnesium carbonate (dolomite). Thermogravimetric analyses indicate that decomposition of lead carbonate can be achieved at 350 °C in TGA diagrams, as other mineral carbonates only decompose to carbon dioxide at temperatures above 700 °C. Thus, a thermal approach is proposed to separate the various carbonates and isolate the specific ¹⁴C signature to the lead carbonate. In practice, however, discrepancies between the measured radiocarbon ages and expected ages were observed. FTIR analyses pointed to the formation of metal carboxylates, an indicator that the organic binder is not inert and plays a role in the dating strategy. Upon drying, oxidation and hydrolysis take place leading to the formation of free fatty acids, which in turn interact with the different carbonates upon heating. Their removal was achieved by introduction of a solvent extraction step prior to the thermal treatment, which was confirmed by GC-MS analyses, and thus, the collected carbon dioxide at 350 °C results can be assigned correctly to the decomposition of the lead white pigment. The proposed procedure was furthermore verified on mixed carbonate-bearing paint samples collected from a Baroque oil painting.

X-ray Diffraction Mapping for Cultural Heritage Science: a Review of Experimental Configurations and Applications

X-ray diffraction (XRD) mapping consists in the acquisition of XRD patterns at each pixel (or voxel) of an area (or volume). The spatial resolution ranges from the micrometer (μXRD) to the millimeter (MA-XRD) scale, making the technique relevant for tiny samples up to large objects. Although XRD is primarily used for the identification of different materials in (complex) mixtures, additional information regarding the crystallite size, their orientation, and their in-depth distribution can also be obtained. Through mapping, these different types of information can be located on the studied sample/object. Cultural heritage objects are usually highly heterogeneous, and contain both original and later (degradation, conservation) materials. Their structural characterization is required both to determine ancient manufacturing processes and to evaluate their conservation state. Together with other mapping techniques, XRD mapping is increasingly used for these purposes. Here, the authors review applications as well as the various configurations for XRD mapping (synchrotron/laboratory X-ray source, poly-/monochromatic beam, micro/macro beam, 2D/3D, transmission/reflection mode). On-going hardware and software developments will further establish the technique as a key tool in heritage science.