C U L T U R A L H E R I T A G E , A R C H A E O L O G Y A N D P A L A E O N T O L O G Y

S C I E N T I F I C H I G H L I G H T S

1 4 8 H I G H L I G H T S 2 0 2 3 I

Mapping an unusual lead compound in Rembrandt s The Night Watch

X-ray powder diffraction has revealed the unusual presence of lead formate in several areas of Rembrandt s masterpiece, The Night Watch. The microscale distribution of such compounds provides new clues on Rembrandt s recipes and the reactivity of lead driers in historical paintings.

The Night Watch, painted in 1642 and today displayed in Amsterdam s Rijksmuseum, is one of Rembrandt s most important paintings and his largest work of art. In 2019, a research and conservation project was initiated in order to study Rembrandt s painting technique, the condition of the painting and how best to preserve it for future generations. As part of this study, multiscale X-ray powder diffraction experiments were carried out both in- situ at the Rijksmuseum and ex-situ, on tiny fragments of the painting at beamline ID13 to investigate the chemical compounds present at the surface. These analyses revealed the presence of an unexpected organo- metallic compound in several areas of the painting: lead II formate (Pb(HCOO)2) (Figure 120). While the formation of this compound was previously reported in model paint samples [1], it had never been detected in an historical painting. Interestingly, lead formate was also detected in areas of The Night Watch that did not contain lead- based pigments such as lead white (a mixture of lead carbonates PbCO3 and Pb3(CO3)2(OH)2) nor lead-tin yellow (a lead stannate Pb2SnO4). The origin of the Pb2+ ions necessary for lead formate crystallisation was thus unknown.

It was hypothesised that Rembrandt used an organic medium (linseed oil) containing dissolved (and/or partially dissolved) lead II oxide (PbO), a metallic drier described in seventeenth-century workshop recipes for its siccative properties [2]. To investigate this, model samples simulating different historical oil paint formulations were prepared in the laboratory and structurally probed using micro-X-ray powder diffraction (XRPD) at beamline ID13. The results obtained were compared with the data collected from fragments of The Night Watch.

The experiments mapped the presence of several neo- formed crystalline compounds at the microscale, including lead II formate, and revealed their formation over time. Crystals of lead formate were detected throughout a film of drying oil preliminarily cooked with PbO. Moreover, when PbO particles were deposited in a film of raw linseed oil, it was possible to observe not only a high concentration of lead formate in the vicinity of the PbO grains, but also the apparition of the more alkaline lead hydroxyformate Pb(HCOO)(OH) at the surface of the grains (Figure 121). Based on these results, it was hypothesised that lead formate forms from the reaction of PbO on formic acid (formed during the oxidative curing of oil), and/or the substitution of this formic group with different lead carboxylate groups (originating from the preparation of the lead medium and from the radical polymerisation reactions active during the oil curing).

Research is ongoing to investigate the dynamics of the lead formate formation, as well as their stability in easel paintings. It is expected that these results will provide

Fig. 120: a-d) Macro X-ray powder diffraction (MA-XRPD) crystalline phase distribution on

a selected area (girl s light-yellow dress) of The Night Watch (1642) by Rembrandt van Rijn

(1606 1669). Yellow rectangles indicate the area scanned using MA-XRPD. The white arrow indicates the location of sample SK-C-5_070.

The intensities scale from min (white) to max (red). e-h) Synchrotron radiation

micro-XRPD (SR-μ-XRPD) crystalline phase distribution on sample SK-C-5_070.

The red rectangle indicates the scanned area. The intensities scale from min (black)

to max (white).