Quantitation from Tafel analysis in solid-state voltammetry. Application to the study of cobalt and copper pigments in severely damaged frescoes

A novel method, using Tafel plots, for quantifying electroactive species in solid materials when their voltammetric signals are strongly overlapped is described. This is applied to the analysis of submicrosamples from the highly damaged frescoes painted by Palomino (1707) in the ceiling vault of the Sant Joan del Mercat church in Valencia, Spain. These paintings, which were fired in 1936, contained cobalt smalt plus azurite mixtures, this last being altered to tenorite (CuO). The reported method provides a quantitation of the cobalt smalt/azurite, tenorite/(azurite + smalt) relationships in samples, thus providing direct information on pigment dosage (smalt/azurite ratio) in pristine paintings, extent of alteration, and temperature experienced by the frescoes during the gunfire episode. Distinction between Palomino paintings and other painters was clearly obtained due to the presence of malachite in these last.