Buried metal artefacts gather stories on their surfaces

COINS ANDACS other metal artefacts dug from archaeological sites bear witness to their burial in the form of the patinas of chemicals which accumulate on their surfaces—and different circumstances will result in different sorts of patina. This is particularly true of objects recovered from underwater sediments, for water can contain all sorts of dissolved substances that like to react with bare metals. But, as Gabriel Ingo of the Institute for the Study of Nanostructured Materials, in Rome, describes in this month, it is now possible to go beyond that general observation. He and his colleagues are using patinas to tell stories about individual finds.The ancients knew of seven metals: gold, silver, copper, tin, lead, iron and mercury. Of these, gold is noted for its chemical inertness, so forms no patina. And mercury, by its nature, is unsuitable for making things. But objects composed of the others often turn up in excavations—generally coated with a wide array of oxides, nitrates, carbonates and sulphides. As a chemist, Dr Ingo wondered if dissecting the patinas formed by these compounds might yield information about what the relics covered by them had been through.