Micro Raman investigation on corrosion of Pb-based alloy replicas of letters from the museum Plantin-Moretus, Antwerp

To better understand corrosion behaviour of lead (Pb)-based alloys exposed to volatile organic compounds – specifically acetic and formic acid – a preliminary research has been carried out on laboratory-made samples replicating such alloys. Replicas have been manufactured by simulating specific types of Pb-based alloys, with antimony (Sb) and tin (Sn) as alloying elements, in order to characterise corrosion products and evaluate the influence of each alloying element. Replicas were cast in a refractory mould and later placed in size-specific desiccators and exposed to concentrated acetic and formic acid vapours for 15 days. Each specimen was also subjected to gravimetric measurements and periodically monitored in order to understand the corrosion behaviour. Micro Raman Spectroscopy (μRS) and Scanning Electron Microscopy together with energy Dispersive X-Ray Spectroscopy were used for the characterisation of Pb-based replicas. Results showed the presence of typical corrosion products – more or less hydrated forms of Pb and Sn acetates and formates – and also a sudden increase in the volume and brittleness of some Sb-rich specimens as well as the presence of seldom-detected metallic Sb peaks with μRS. In this perspective, the influence of Sb as an alloying element on the corrosion of Pb-based artefacts may generate serious problems when such objects have to be restored and particular attention should be given to conservation/stabilisation procedures.