ROMARCHITE, HYDROROMARCHITE AND ABHURITE FORMED DURING THE CORROSION OF PEWTER ARTIFACTS FROM THE QUEEN ANNE'S REVENGE (1718)

Abstract

Pewter, a tin-rich alloy, has been widely used for ornamental and utilitarian purposes for the last 400 years because it is durable, relatively easily worked, resistant to corrosion, and similar to silver in appearance. Pewter plates and implements ha ve been recovered and examined from what is believed to be the wreck site of the Queen Anne’s Revenge , flagship of the pirate Blackbeard, that sank near Beaufort, North Carolina in 1718. All of the pewter artifacts from the site display a surface veneer of corrosion products and may be viewed as experiments on tin corrosion that have been continuously in operation for more than 280 years. Mineralogical examination of the pewter samples has revealed that the corrosion products are composed of romarchite (SnO), hydroromarchite [Sn3O2(OH)2], and abhurite [Sn 21Cl16(OH)14O6]. The corrosion generally develops in crudely concentric layers, with an inner layer of abhurite in contact with the pewter; the overlying outer layers consist of romarchite and hydroromarchite. Romarchite, hydroromarchite, and abhurite occur as irregular grains and laths up to 100 micrometers in length. Abhurite also occurs as masses of equant grains with abundant small inclusions of residual pewter. Thermodynamic considerations reveal that romarchite may be a metastable phase, and is present as the result of sluggish kinetics in the process of the formation of cassiterite (SnO 2), the most stable tin oxide in most natural environments. Observation and identification of these phases will be useful in understanding the stability of tin in the weathering environment and the nature of the metal’s corrosi on products.