Comparative assessment of mechanical, chemical and electrochemical procedures for conservation of historical lead

Lead has a good resistance to atmospheric corrosion because it forms adherent, uniform and protective layers which prevent the corrosion advancement. Nevertheless, the exposure to the acetic and formic acids - emitted by wood and other materials used in museums' showcases, cabinets and storehouses - induces a corrosion process which can damage the surface of the objects. In order to improve their conservation, restorers apply different conservation treatments which depends on the degree of corrosion, the products formed, the skills of the conservator and the access to specific equipment, if is required. With the aim of evaluating the impact of successive conservation treatments on lead surface and assessing their efficiency, four usual treatments for lead were selected and six cycles of alteration and treatment were carried out. The assessed procedures were mechanical cleaning (suspension of CaCO3), chemical cleaning (immersion on EDTA solution) and two electrochemical treatments (potentiostatic reduction, and potentiostatic reduction followed by passivation). The samples were characterized before and after each treatment with gravimetry, colorimetry, rugosimetry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The effects observed on lead coupons depended on each treatment procedure. The mechanical cleaning with CaCO3 produced a significant mass loss and it left an irregular surface due to the granulometry of the abrasive. Additionally, it experienced a fast re-alteration. Chemical cleaning with EDTA produced moderate mass and gloss losses due to the etching of the metallic surface after successive cycles. However, the color of the samples was the closest to the original one. Finally, the electrochemical reductions produced a scarce mass loss and a slow re-alteration, although they produced crystalline deposits on the surface which modified the color of the coupons towards bluish hues.