Dear Dan,

Yes, indeed 12% tin bronze is an ideal alloy, better than 10% because 
the gain in hardenability outweighs any loss of toughness (which is 
minimal in any case). You are not correct, though, about the effects of 
some alloying additions (and whether they were alloying additions, i.e. 
deliberate). There is actually quite a lot of information about this 
available in one way and another.

For example, there was a series of papers published on the effects of 
various impurities (often up to 2-3%) on copper published in the Journal 
of the Institute of Metals in the mid 1920s. In fact JIM has a lot of 
invaluable material in it right up until it ceased publication following 
the merger of the Institute of Metals at the beginning of the 1970s. 
That is why our department's run of the journal from 1924 on lives in my 
office. Even more data, though, lies in student dissertations in many 
universities that do not get published. There are useful ones on leaded 
bronze from University of Warwick in the 1970s and from my own 
department in 1982 (summarised in the proceedings of the 1982 Bradfrod 
Archaeometry meeting. These are in English; quite a lot is in other 
languages. Gerhard Sperl, now retired from the University of Leoben in 
Austria many years ago put together a compilation of microstructures and 
some properties of a wide range of binary copper alloys but, of course, 
this is in German.

On the subject of lead, it has been shown that 1-2% lead is a most 
useful addition as it gives a significant decrease in viscosity with 
absolutley minimal impact on other properties. Indeed, it can actually 
improve them if the bronze has not been homogenised as it seems the 
segregation of the lead particles to the interdendritic eutectoid 
modifies its mechanical behaviour. Our own research here suggest that up 
to about 10% lead the effect on mechanical properties was well within 
what a Bronze Age smith could live with while enhancing castability 
through further lowering of the liquidus temperature and increase in 
freezing range. Further, in casting an article like a sword the lead 
will tend to segregate away from the cutting edge, espeically with very 
slow cooling rates. This was published in Oxford Journal of Archaeology 
in 1982.

Even if in the Bronze Age their ability to control the composition of 
copper was limited at the smelting stage, unless using clean 
chalcopyrite ores (which they did and there is plenty of copper form the 
Bronze Age which is better than 99.9%), they did both refine their 
copper, and also understand something of the behaviour of the metals 
they did have. Thus, the Early Bronze Age fahlerz copper was not alloyed 
with very much tin because it did not need to be, as it was an excellent 
material in its own right. The later Bronze Age high Sb metal that I 
mentioned was also seen for what it was so you never see large amounts 
of tin being added to it. I would go further with their empirical 
appreciation of the behaviour of their alloys. In the first half of the 
Middle Bronze Age in Wales there is a very consistently used bronze 
composition with close to 12% tin, 1-2% lead, 1% As and 0.3% Ni. Lead 
isotope analysis says this is being supplied from multiple sources 
suggesting that the metal was being produced by ore selection and then 
alloying to a well-defined recipe. The lead improves castability, as 
noted above, while the arsenic raises the recrystallisation temperature 
to a point where the tin content homogenises. Thus the smith would see 
an alloy that is very practcial to cast, and work and anneal. Finished 
axe cutting edges from this alloy have a hardness of 200-220HV and a 
grain size of about 5 microns. Really good stuff.

What is importnat in studying all these materials is to look carefully 
at the microstructures and mechanical and heat treatment properties of 
the artifacts so that you can see what the smiths were trying to achieve 
in the control of their materials.

Don't forget also that the tin and lead used in the Bronze Age were both 
remarkably pure.

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
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