Surface characterization of tinned bronze, high-tin bronze, tinned iron and arsenical bronze

Abstract

Tin coated bronze antiquities are found in museum collections on Greek and Etruscan items dating from around the 5th century BC, and later on artefacts throughout the Roman world and Europe and less extensively in various Asian regions. Tinned items of decorative bronze, military ware etc. would resemble silver in colour and be an inexpensive substitute, and tinned bronze mirrors would have been sought for their reflective properties. Copper cooking vessels were tinned to prevent tainting of food. Iron objects protected by tinned surfaces became popular from the medieval period although the earliest tinned iron object is from Spain around 450 BC. The technological significance of tin-enriched bronze surfaces from the curatorial and conservation viewpoint requires their correct identification, apart from ensuring their distinction from silver. Ambiguity arises because ‘tin’ on a bronze surface can occur in several ways, either from the tinned surfaces of a low-tin bronze, which can show different compound surfaces corresponding to heat treatment, or the object may actually be a cast high-tin bronze. Additionally tin-sweat may appear on a low tin-bronze casting, and corrosion can enrich surfaces with tin corrosion products and cause colour changes. Silver coloured surfaces on ancient arsenical copper alloys occur on various artefacts. Their structures have been examined and explanations for their occurrence are discussed. The characterization of tin-enriched bronze surfaces and arsenic-rich surfaces on antiquities can be achieved by microscopy, microanalysis and X-ray diffraction analysis. Specific intermetallic compound structures are found to occur with different types of surface. This chapter presents the results of the surface examination of various tinned artefacts, high-tin bronzes, comparative experimental tinned material, and arsenical copper showing the microstructural and analytical characteristics that enable identification to be made of the enriched, silver coloured surfaces on antiquities.