Mott–Schottky Analysis of Historical and Archaeological Copper–based Objects

Abstract

The application of Mott–Schottky (M−S) analysis of impedance data to sub–micro samples extracted from the corrosion patina of copper, bronze, and brass archaeological artifacts attached to graphite electrodes is described. The corresponding theoretical approach is developed to account for the contribution of the composition and structure of the corrosion patina and the effect of the bare graphite substrate. Experimental data in contact with 0.10 M Na2SO4 aqueous solution at pH 6.28 are consistent with the p-type semiconducting nature of the main copper corrosion products, cuprite, and tenorite. The values of the apparent flat band potential and the slope of the M−S plots allow archaeological samples to be grouped according to their M−S parameters (slopes and intercepts). Assuming equivalent conditions of corrosion, the resulting grouping is judged to be dependent on the composition, and/or method of manufacture, and/or age. Studied samples include Renaissance statues from the Hofkirche in Innsbruck and a variety of objects from museums and Archaeological Heritage Office (soprintendenza) in Austria (Bad Aussee, Johanneum Graz, and the Tyrolean State Museums), and Italy (Genoa and San Remo), dating from the Bronze Age to the 18th century. Complementary data are provided by voltammetry of immobilized particles.