Friday, May 25, 2007

'Copper Alloy" ??

A recent discussion on NORSEFOLK has centred on 'copper alloy' cauldrons:

You will see almost any contemporary book - and all excavation reports - list 'copper alloy' as the material.

We have two possibilities here. Copper is almost always mixed with either tin or zinc. The older alloy is copper and tin = Bronze. The amount of tin is not likely to exceed 10%, at that point the material gets quite brittle, almost like glass. Workable bronzes are more likely to be closer to 5% tin. The main reason to add the tin to create bronze is to improve its ability to liquefy and cast it. The tin also hardens the copper, but at the cost of brittleness.

You also will find copper and zinc = Brass. Brass remains workable with fairly high additions of zinc, up to 30 plus per cent. The main reason to add zinc to the alloy is to extend the copper. There is also some increase in hardness, but casting brass remains a problem. It looks like historically, the zinc, plus other things like lead, were added mainly because tin was often hard to get.

(In our modern world, actual bronze sheet is pretty hard to source. Most of us are substituting brass, which is produced in thin sheets commonly available. For making up cauldrons the difference is slight from a cold working standpoint. An important safety note: ZINC FUMES ARE DEADLY!)

So my understanding is that the limiting factor in creating bronze has always been the tin. This is available naturally only at a very few locations. Naturally tin is bound up in hard granite, and almost impossible to mine from rock in ancient times. There were a very rare number of places where tin weathered out of bed rock and was found in alluvial deposits. In ancient times the metal was found in northern Turkey, but those deposits were long exhausted by the Roman era. The other useful source was in England - Cornwall.

During the Viking Age, much earlier ("Roman") bronze objects were melted down and re-cast into new pieces. Any re-use of old metals always results in a loss of both volume and quality. So the Norse metalsmiths would 'cut' their bronze with a number of other metals to extend and 'improve' them. These additions included lead and to a certain extent zinc. Adding lead lowers melting points and improves flow of the liquid metal. It also happens to be toxic, but they didn't know that.

So Viking Age 'bronze' can cover a fairly wide range of potential mixtures, both in terms of metal content and per centages. Hence the modern trend to use 'copper alloy' as the descriptive term.

A side note to this is that many VA cauldrons, certainly copper alloy and sometimes wrought iron ones, are LEAD soldered at the seams. Not something we want to duplicate in our modern re-creations!

(sorry for the lack of articles lately. I am gearing up for several iron smelts in early June. and have been off gathering required materials)

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