In analysing coins from the Neftenbach hoard (see C.E. King and J.P.
Northover, 1992: The analyses, in H.-M. von Kaenel et al., Der Münzhort
aus dem Gutshof in Neftenbach, (Zürich: Zürcher Denkmalpflege,
Archäologische Monographien 16; ISBN 3-905647-54-0), 101-117, 226-235)
we were looking at third century Roman coins with down to about 18%
silver (coins of Postumus). A naturally silver rich surface often formed
in these debased coins by inverse segregation of the silver-copper
eutectic to the surface of the cast blank, but this did not always
happen. Even where this does happen the coin will be silvery if there is
still a continuous interdendritic network of copper-silver eutectic (or
silver as the eutectic is often divorced). When below 20% the surface is
often thin and the coins decidedly pinkish, and any depletion silvering
technique would have to be pretty severe.
When the distribution of silver breaks down into discrete particles then
you do need to beging to plate/coat the coin to get a silver surface.
An interesting question came up with crucible residues from Carthage
under the vandalic occupation which showed the melting of bronze with a
definite silver content of 1-3%. An obvious possible source of such
alloys was the coinage. However analysis of Vandalic coins from Carthage
showed silver to be absent (see C.E. King, D.M. Metcalf and J.P.
Northover: Copper-based alloys of the fifth century: a comparison of
Carthage under Vandalic rule, with other mints, Revue Numsimatique, 6th
series, 34, 54-76, Pls.).
Peter Northover
--
Dr Peter Northover,
Materials Science-Based Archaeology Group,
Department of Materials, University of Oxford
Tel +44 (0)1865 283721; Fax +44 (0)1865 841943 Mobile +44 (0)7785 501745
e-mail [log in to unmask]
|