Linear sweep voltammetry: a cheap and powerful technique for the identification of the silver tarnish layer constituents

The potentialities of linear sweep voltammetry (LSV) for the characterization of the tarnish layer formed on pure and sterling silver samples exposed in the chapel and in the museum, at the Cathedral of Porto, in Portugal, are well demonstrated in this study. The technique allows the identification of the constituents of the thin tarnishing layers and also its relative abundance. A much more complex composition than the silver sulphide commonly associated with silver tarnish has been found, namely, silver chlorides, silver oxides and minor amounts of silver sulphide on the pure silver, plus copper oxides and a mixed copper–silver sulphide on the sterling silver samples. The tarnishing films were very thin mainly composed by silver chloride and silver oxides layers with estimated thicknesses ranging between 0.22 and 9.63 nm.