25 May 2017

Making Bronze Swords

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At Sarah Hoyt’s site, J.M. Ney-Grimm wondered exactly how the ancients manufactured swords in the Bronze Age.

Making a sword was resource intensive, both because of the valuable metals required and because of the labor from many skilled individuals that went into it. …

Bronze is made by mixing a small part of tin with a larger portion of copper. The ancients didn’t have modern strip mines or deep underground mines. Nor did they have sophisticated machinery run by deisel engines. How did they get copper and tin out of the ground?

Copper mines bore some resemblance to my expectations. The copper deposits needed to be relatively near the surface, but the ancients actually did tunnel down to a vein of ore. There, at the working face, they built a fire to heat the ore-containing rock. Once the rock reached a high enough temperature, they doused it with cold water. This process increased the brittleness of the rock and induced a preliminary degree of cracking. Blows from a hammer or pick could then break it into rubble, which could be heated in a smelting furnace to extract the copper.

Tin was another matter, one entirely new to me.

Tin was found in alluvial deposits in stream beds, usually as a very pure tin gravel well stirred with gravels of quartz, mica, and feldspar (gangue). So the trick was to separate out the tin gravel from the others.

The method of the ancients, as far back as 2,000 BC, was this:

• Dig a trench at the lowest end of the deposit.
• Dig a channel from the nearest water source to pour water over that part of the deposit
• Allow the stream of water to wash the lighter gangue into the trench
• Pick up the heavier tin gravel that remained
• When the lower portion of the deposit had yielded all its tin, dig another trench a bit higher and redirect the water channel, to allow the next section of the deposit to be harvested

The tin gravel thus obtained would be roughly smelted on site, simply roasting the gravel in a fire. The pebbles resulting from this rough smelt would then be transported to a dedicated furnace for a second smelting that yielded the purer tin needed by bladesmiths.

Modern ingots are rectangular blocks, but those of the ancients took several different forms. The earliest were so-called “biscuit” ingots, round on the bottom like a muffin, gently concave on the top. They took the shape of the earthen pit into which the molten metal dripped from the smelting furnace.

But metal is heavy, and the biscuit shape awkward to carry. Around our own Mediterranean, an “oxhide” form was developed. It weighed about 80 pounds and possessed four “legs,” one at each corner, that allowed it to be tied between pack animals or gripped and carried by men.

I became fascinated with an ingot form used much later by the Chinese in the Malay Penninsula. These were hat shaped, much smaller (weighing only a pound), and actually used as currency.

Bronze has one very peculiar property in the smithy.

Most metals, such as iron or even copper, when heated and cooled slowly to room temperature, become more ductile and more workable. They are less prone to internal stresses.

Bronze does not behave like this. When slow cooled, it becomes brittle and difficult to work. Thus it must be heated to cherry-red and then quenched in water. This quick cooling makes it so soft that it can then be hammered. The hammering condenses the metal, giving it more rigidity.

A bladesmith will hammer near the edge of a blade to harden it and help it keep its sharpness, while allowing the center rib to retain more of its resilience.

Were These Swords Any Good?If you compare a bronze sword to a steel sword, the steel is always going to win. But when the Bronze Age gave way to the Iron Age, bronze metallurgy was at its peak. Several thousand years had gone into the development of the most superb techniques. Iron metallurgy was in its infancy, and getting the iron swords to be rigid enough was a problem. The iron swords just weren’t as good as the bronze ones, which were light, strong, just rigid enough, and held an edge well.

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One Feedback on "Making Bronze Swords"

Joel

That’s very interesting. Thanks!



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