Monday, March 14, 2011

Ancient Bronze working?

" I'm trying to process bronze (10%Sn) specimens for my master thesis. I have purchased the adequate metal and my trying to find information regarding forging and annealing (temperature and time) at the roman times."

If you don't mind some practical suggestions? Its hard to tell from your message, but it seems you might be working in a more experimental archaeology method, working modern pieces and comparing them to artifacts?

I have worked with hot hammered bronze bars a few time (not extensively, I admit). You want to bring the metal up to a bright red or maybe to a dull orange colour. Once the temperature drops below a dull red (so you can not see visible colour) you most certainly stop hammering. (Alloy not withstanding)
The key to this is working indoors, or maybe at night. The temperatures with bronze are much more restrictive than with copper or certainly iron. Indoors or night would give you constant light levels in the workshop, so you could learn the best set of visual cues. I mention this quite specifically, as the ancients would not have any other method of judging temperature ranges.
The individual alloy mixes will change your working ranges. Pure copper moves like butter in the red temperatures, but can still be hammered even cold. Bronzes get brittle very quickly (and dramatically) as the temperature drops below visual (again dependent on the exact mix).

For annealing, the copper alloys are heated to a 'dull red' (colour showing) then plunged in plain water. I don't really think you need better control than that. (not in my experience anyway)

The problem relating this to Roman technology may lie on the forges used. Charcoal fires of the time are generally fairly small, so only a smaller portion of a large object could be effectively heated at a time. If my experience with working copper sheet hot is any clue, you would be heating and working only part of a large object (imagine a sheet for a cauldron) at any one time.

Fun stuff to play with! Copper alloys move very fast under the hammer blows. Beware toxic elements (especially copper / zinc brass pretending to be bronze). Historic alloys also contain lead, also extremely un-healthy in the workshop!

I know this is a bit thin!
I'm off to Virginia for two weeks of Smeltfest tomorrow. Present will be Lee Sauder and Skip Williams (the hosts) / Mike McCarthy (I'm driving down with)/ Tim Young and Jake Keen from the UK / Jesus Hernandez and Jeff Pringle (two excellent bladesmiths) / Sheldon Browder and Steve Mankowski from Williamsburg. I am SO looking forward to this. I'll try to post from the madness...

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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