We recently discovered a ‘missing link’ compound [1] in the pathway from the artists’ pigment minium (red lead, Pb3O4, a semi-conductor material also used in metal anti-corrosion coatings) towards (hydro)cerrusite called plumbonacrite, 3PbCO3.Pb(OH)2.PbO. The alteration mechanism, leading to a whitening of read lead involves (a) photo-reduction of Pb4+ to Pb2+, followed by (b) CO2 capture by the resulting Pb2+ [1]. A more frequently occuring form of minium degradation is blackening, due to the formation of β-PbO2 (plattnerite). In addition, another Pb-compound giving rise to blackening is galena, PbS. A combination of Raman spectrometry, scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), synchrotron X-ray microdiffraction and X-ray absorption tomography was used to examine sample material originating from a 2000 year old Roman-Egyptian “Red Shroud” mummy (P.J.Getty Museum, Los Angeles) that showed a superficial yellow-white discoloration. Throughout the red lead paint, spherical nodules (ca 50-100 μm in diameter) were observed. Some of these contain the white compounds laurionite (Pb(OH)Cl) and (hydro)cerussite [PbCO3 and 2PbCO3.Pb(OH)2] in an outer shell, around a black, higher-density nucleus of β-PbO (litharge). XRPD line scans and distributions have revealed a strikingly large variety of Pb,Cl compounds inside and around these nodules: PbCl2, Pb(OH)Cl, PbCl2.PbCO3, NH4Pb2Cl5 next to α and β-PbO2, minium, PbO, hydrocerussite and cerussite as well as PbSO4 and other sulfates are present. The nodules may be remnants of the minium production process (based on roasting of litharge); however, some of these compounds are suspected to have formed in secondary reactions since the minium was applied on the mummy surface. Some of these were only very recently formed. In this presentation, on the basis of the chemical alteration pathways described in the literature, a reconstruction of the (sequence of) reactions that is suspected to have taken place on the long and short term in this material, will be presented. [1] F. Vanmeert, G. Van der Snickt, K. Janssens, Plumbonacrite Identified by X-ray Powder Diffraction Tomography as a Missing Link during Degradation of Red Lead in a Van Gogh Painting, Angewandte Chemie-International Edition, 54 (2015) 3607-3610.

Degradation of minium (Pb3O4) on the long and short term in material originating from a Red-Shroud Mummy

251st National Meeting (2016)

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