Dear Maikel,
When the word bronze is unqualified it is a label commonly applied today
to alloys of copper and tin. (Other alloys such as manganese bronze,
aluminium bronze, etc. are effectively trade names and are not part of
this story). The two commonest modifications of the term bronze are
phosphor bronze, where phosphorus is added as a de-oxidant, and leaded
bronze, where lead is added to modify casting behaviour. Reference to
modern handbooks will show other variations on the theme.
The key to interpretation is the word alloy, i.e. something deliberately
created (phrases such as accidental or natural alloys appear in the
archaeological literature but are a contradiction in terms). The
addition is made to modify the properties of the copper in terms of
hardenability, castability, colour, etc. The addition of amounts over
about 1-2% will make changes visible to the ancient metalworker,
initially in modifying the rather poor casting behaviour of copper. To
start to have an appreciable impact on mechanical properties the
addition should probably be nearer 3-4%. Tin concentrations below this
in archaeological material most probably represent the recycling of
mixtures of copper or arsenic copper and bronze.
Above 4% bronzes start to offer reasonable properties and continue to do
so up to about 14-15% and all this range has been used by one group or
another for utilitarian functions as suited their purposes. Certainly
some used alloys around 10% tin but in fact around 12% is probably even
better and a number of industries used that, while others used 6-9%. The
ideal of a 10% tin bronze is as much an artifact of the archaeological
literature as of ancient metalworkers. What is important is to identify
the tin contents used by the industry you are studying and examine the
way the metal was used and its properties exploited, which at the least
requires metallography and hardness testing, and not to judge this
against some hypothetical external standard. Some industries thought it
important to maintain a precise control of their alloys and the way they
worked their bronzes and some, it seems, just could not be bothered.
Yours,
Peter
--
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]
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