QUERY: CLEANING COINS USING LASERS

In April 2002 three metal-detectorists had the find of their lives in a field near Abergavenny, Monmouthshire: a scattered hoard of 199 silver pennies.

The hoard included coins of the Anglo-Saxon king Edward the Confessor (1042-66) and the Norman king William the Conqueror (1066-87). The hoard probably pre-dates the founding of Abergavenny near by in the 1080s.

The hoard was heavily encrusted with iron deposits, including traces of fabric, suggesting that the coins had originally been held in a cloth bag. It is not clear whether they had been deliberately hidden, or simply lost. Either way their owner was the poorer by a significant amount: sixteen shillings and seven pence (16s 7d, or £0.83p) would for most have represented several months' wages.

The Abergavenny hoard includes 36 identifiable mints, as well as some irregular issues which cannot at present be located. Coins from mints in the region, like Hereford (34 coins) and Bristol (24), are commonest, outweighing big mints such as London (19) and Winchester (20). At the other end of the scale there are single coins from small mints such as Bridport (Dorset), or distant ones such as Thetford (Norfolk) and Derby.

The coins were found covered in iron concretions and many of them were stuck to each other. This disfigured the coins and obscured vital details. Removing this concretion with mechanical methods, such as using a scalpel, would have damaged the silver, and chemicals failed to shift the iron.

The solution to the problem was found in an unexpected, but thoroughly modern tool - the laser. A laser is a source of light providing energy in the form of a very intense single wavelength, with a narrow beam which only spreads a few millimetres.

As laser radiation is of a single colour (infrared light was used in this case) the beam will interact intensely with some materials, but hardly at all with others. This infrared source was absorbed better by the darker overlying iron corrosion than by the light silver metal.

The laser was successful at removing much of the iron crust, but initially left a very thin oxide film on the surface. When this was removed, the detail revealed on the underlying coin was excellent; it was possible to see rough out and polishing marks transferred to the coin from the original die, as well as the inscribed legend.

Bruce asked:

To read the complete article, see: Coins from Edward the Confessor and William the Conqueror found in Monmouth field (www.museumwales.ac.uk/en/rhagor/article/1942/)

Wayne Homren, Editor

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