Conserving the Snettisham jeweller's hoard
This wonderful array of rings, bracelets and gems came out of a jeweller's pot at Snettisham, Norfolk in 1985. Although an impressive sight, the objects did not look quite as we see them now. Centuries of burial produces changes in most types of material. The causes are natural processes, such as bacterial decay in organic objects, or corrosion in metals. Soil conditions, period of burial and, most important, the composition of the material itself influence the type and extent of deterioration.
Minerals are a naturally stable material. The engraved gems from the hoard were made from the mineral cornelian and were unaltered. They were simply dirty from the soil. Metals, however, are extracted from minerals: they are smelted from ores by the application of great heat. This means metals are essentially unstable and always trying to revert to their stable state as a mineral. This is the corrosion process.
Silver combines with salts in the earth to make silver chloride. The Snettisham wire rings, chains and pendants are almost pure silver and were covered with this grey corrosion. The corrosion penetrates beneath the silver surface, so chemical treatments can cause damage. Conservation treatments have to be carefully chosen for each object and for these wire rings the corrosion was removed with hand-tools, under a microscope.
The snake bracelets and rings are also silver, but contain copper too. They were covered with copper carbonate, a green copper corrosion product called malachite. This is the corrosion we normally see covering ancient bronzes. Because the snake jewellery is mainly silver, the malachite simply formed a layer on the surface and could be removed with the application of a weak organic acid. When this was done the original silver surface shone out again.