Tuesday, April 10, 2012

Crossover - Slag Rings in Archaeology?

The impact of an OAC project grant may extend well past the intended application.

My interest in bloomery iron started with the historic process of making the iron. Although there are some scattered living traditions (notably in Africa and India), these are fragmentary at best. For Europe, the technology of making iron has changed significantly since 1000 AD, with several pronounced shifts in method, equipment and type of metal produced. In attempting to re-discover what is a 'lost' tradition, modern researchers and practitioners are guided by very limited archaeological remains alone.
The best experimental archaeology may offer insights into how to interpret what may be puzzling artifact remains.


These are some shots of slag rings recovered from two of our recent smelts (at Smeltfest 2012, Lexington VA, March 2012)

Lee Sauder has been using a heavy forged copper tuyere on all his smelts for the last several years. I'm not entirely sure just why he came up with this innovation. I believe it was in attempt to find a durable solution to the problem how the high temperatures inside the iron smelting furnaces were melting off the ceramic and steel pipe tuyeres then in use.
His tuyere was forged from a solid copper plate roughly 3/8 inch thick. First the piece was cross peened along the long axis to both spread and thin the rectangle into a triangle shape. Then the resulting form was wrapped into a cone. The finished cone is roughly 2 cm ID on the furnace end, about 4 cm ID on the bellows end. The piece is maybe about 40 cm long altogether. The closing seam is just butted together (not fused or entirely air tight).
Lee's clay 'medium shaft' production furnace.
The conical forged copper tuyere can be seen to the right

In use, what happens is that the heat the tuyere end is subjected to quickly travels back to the larger end exposed outside the furnace. The combination of radiation to the outside air, and rushing cold air down the inside surface, all combines to keep the tuyere end well before the slumping or melting point of the copper material. The result is virtually no effect to the the copper tuyere, even after many firing sequences. I think Lee has used this same tuyere for something like 30 smelts, with no damage at all!

As the rings would sit against the furnace wall

Inverted, showing the slag and ore fragments on the top surface

The slag will harden to a shell around the tip of the tuyere. These rings do not solidly attach fuse to the copper, normally hand pressure will break them clear.
You can see that both the internal and external diameters are indicated in the slag rings.
You can determine the upper and lower surfaces, with the heavier accumulation on the 'up' side of the tuyere in the furnace.
You can get some estimate of the tuyere angle. The slag has formed proud of the furnace wall, so if you assume the inner wall to be vertical, the inside surface does record the tuyere angle.

Inner surface (inverted here)
The inner diameter and thickness of tuyere can be determined.

Both the rings show cracking in roughly the same place. I think this is an effect of the cooling rate of the slag and the shape of the rings. One of the collected rings had in fact separated into two pieces ( the ring on the left in the images above).

One ring was broken into two pieces

We have worked with ceramic tube tuyeres as standard here for the last while. These are uniform, cheap and fairly durable. They also are quite obvious as a physical remain. Same goes for the iron (steel pipe) tubes we have also made use of. As the iron tuyeres are consumed with every smelt, I don't think that this material likely for VA process - just from a practical standpoint. (wasting iron to make iron?)

Copper tuyeres might be another mater. They would be 'relatively expensive' as objects, but because of their proven durability would be worth the investment for repeated smelt operations. The copper would be too valuable to discard, likely just being cut up as raw material for bronze production at the end of their smelting use. Any finds of copper cut to rings as a bronze related find? It would be the easy way to re-cycle the material.

Anyway, the slag rings are quite distinctive. Lee said he gets these every time. Worth a check against remains?? (Kevin Smith had mentioned that he had recovered some semi circular slag fragments from his excavation of an 'industrial' VA iron smelting site at Hals in Iceland. It will be interesting to see if these modern pieces in any way resemble his artifacts.)

We messed with using a copper tuyere a long while back, but at the time I did not have any heavy copper bar or sheet. The copper tuyere I made up was only 1/8 thick material, and did not transmit heat fast enough to keep the end from melting back to the furnace wall. This would have certainly produced some droplets of copper into the slag someplace. Perhaps another signature to look for in the archaeology?

(Modified from the initial posting on Hammered Out Bits)


  1. Hey Darrell,

    Such rings have been documented previously archaeologically as part of Parks Canada publication "A Frontier Fur Trade Blacksmith Shop 1796-1812" by John D. Light and Henry Unglik, 1987. An example which I believe is known to you. Though in that case, the tuyere is identified via forge clinker rings (page 6 of the publication). For anyone interested, the full pdf of the publication is available here - http://www.sha.org/documents/research/Parks_Canada_Resources/A%20Frontier%20Fur%20Trade%20Blacksmith%20Shop%201769-1812%20-%20English.pdf

    So it would seem that slag/clinker rings can identify the dimensions, location, and the position of the tuyere. Unfortunately, most field excavations (now most widely done as part of salvage archaeology ahead of development) do not see slag as important and hence do not collect it.

    Your idea about copper re-use is interesting. I happen to be working on a report for a blacksmith shop at the moment and a handful of copper objects were found in association with blacksmithing remains. It might be that they were recycled from the tuyere but most of the items are small, thin objects like springs and wires. What would be the original dimensions of a copper tuyere and roughly what sort of objects would you expect if the copper tuyere was recycled?

    Anatoly Venovcev
    Staff Archaeologist and Material Culture Specialist
    Archaeological Services Inc.
    Toronto, Ontario

    1. Anatoly (et all)

      I have read the Unglik volume (actually have it here someplace). I believe what he would be referring to would be 'forge bottoms' rather than the kind of iron smelting slag ring I'm illustrating above. The formation method, composition and physical appearance is quite different. (I've got a couple of forge bottoms I've saved that I could show you next time you are up here.)

      In the Settlement Era in Canada, both side blast and bottom blast forges were in use. Also forges fired both charcoal and coal. Those variables alone are all going to produce visibly different forge slag remains. Differing working tasks at the forge also produce different types of slag deposits. (As if this could be simple!)

      Also, in a Canadian context, the tuyere itself is most likely to be made of *cast iron*. I can't imagine that forged copper tuyeres would ever have been seen. Cast iron is the material of the Victorian Age. Such tuyeres would be durable and both easy and cheap to manufacture.

      Stay tuned - Yesterday (Wednesday April 11) my work was to forge out a copper smelting tuyere from a heavy bar. I'll be describing and illustrating it at some point!)


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

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