I would like to echo David's view that angular cassiterite crystals in a
bronze are in general the result of oxidation rather than the use of
cassiterite in making bronze. The slight qualification is because I do not
have the phase diagram (W. Hoffmann and M. Klein, 1966: Beitrag zur
Kenntnis des Dreistoffsystems Kupfer-Zinn-Sauerstoff, Zeitschrift fur
Metallkunde Vol. 57, pp385-91) to hand. I imagine it would be possible for
natural cassiterite to dissolve in molten copper or bronze in a crucible
and on cooling new, angular cassiterite crystals could form. Where these
crystals appear, either as needles or rhombs, often hollow, they indicate
that they have grown from the melt. With sufficient oxygen in solution
bronze will actually solidify as a mixture of copper, cuprite and
cassiterite. In my experience this is a fairly common waste product on
bronze working sites in prehistoric Europe.
In the case just described the crystals are large. If they are small and
close to the surface they are more likely to be the result of internal
oxidation through the diffusion of oxygen into the solid bronze at high
temperature. I have seen one example from an intermediate case involving a
bizarre cremation ritual in Late Bronze Age Mallorca. Here the pyre was
laced with lumps of limestone and the fire run at high temperatures in
oxidising conditions. The end product was a mix of ash, calcined bone,
oxidised metal and quicklime which was then placed in a rock shelter. At
the first ingress of water this, of course, began to turn itself into a
very strong lime mortar which made excavation interesting. A bronze awl
that had been on one of these pyres was surrounded by a thick layer of
cuprite with cassiterite needles bridging between the cuprite and the
bronze. It would appear that the temperature of the pyre was such that the
surface of the bronze was just melting. (The melting of alloys is an
interesting subject for another occasion).
While it is probably always going to be rather difficult to identify the
use of a cementation process with cassiterite to make bronze unless
unreacted grains are found in a crucible, the other case, mixing tin and
copper, has been identified. A Stamford Ware crucible from Saxo-Norman
levels in London was certainly used for melting bronze but there is a
trail of fine beads of tin down the wall of the crucible above the level
of liquid metal in it. The metal had barely wetted the surface of a
refractory crucible and was only just adhering to it. A reserahc student
who was particularly skilled at preparing sections of crucibles had such a
delicate touch in polishing that these droplets were not disturbed. A
publications is in preparation.
Peter Northover
Materials Science Based Archaeology Group,
Department of Materials,
University of Oxford
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