Sunday, October 25, 2009

Clay at L'Anse aux Meadows?

This is a response to a comment by one of the regular voices on the Early Iron Group discussion.

Do you make a difference between loam and clay?
We always use loam mixed with sand and other things (when needed).
I am sure that the inside of the furnace wall participates at the proces. I dig out the loam as close as possible to the ore bank.

Loam has very less lutum and clay minerals but much iron combinations.
If you mix it with (lots of) sand it dries very quick and you can even start firing it while it is still wet.
I am curious about the situation of loam and ore near the excavation of that oven in LaM.


At the physical site of L'Anse aux Meadows itself there is not any clay material present. On the actual Viking Age occupation area, the location is a sea shore, with a narrow bank that rises up out of the shallow water. The bank is maybe 30 metres wide. This bank then falls away into a bog, so imagine a thin crescent shape that runs along the sea. So what you have is a combination of fine stones and sand at the water line (a fine gravel). The core of the bank (especially around the furnace hut where the smelter is) is mainly sand. This is topped with a layer of compressed organic material - peat. The bog side is of course peat as well.
Looking roughly north, out over the Marine Terrace at L'Anse aux Meadows NHSC.
The Norse occupation area extends bit more to the right than is seen in this image. The white area with benches seen to the right centre is roughly were the charcoal pit was found, just below it, dug into the stream bank, was the 'Furnace Hut' and its iron smelter.

There is a clay bank in the area. I personally have not seen this, so can't tell you exactly how close it is, or exactly what consistency or kind of deposit it is. A couple of the local people told me about it. One of the staff members at Norstead (an attraction in the area) had been digging the stuff out and building and firing simple hand built pots out of this clay. It looked like a pretty clean low firing grey clay. Most certainly this clay deposit will be some form of water deposited material, my guess is that it would be along one of the many streams in that region. The Norse would certainly have had to have hand carried the stuff a good distance!

From what I saw at Ribe (with Michael Nissen) and Heltborg (with Jens Olesen) the situation in Denmark is quite different. What I observed at both those locations is quite deep underlaying clay and sand layers making up the whole ground. This base material had organic material mixed with it as the top layer, often pretty thin in places. So when you stuck a shovel in, you pretty much got some combination of clay, sand and bits of vegetation. For the Heltborg symposium in 2008, Jens had a pile of clay that they had just used a front end loader to scoop out of the side of a hill. That stuff was almost pure clay, we actually had to add extra sand to it - just as it came from the earth. I was quite amazed! (Denmark, the land of iron riches.)

So I think what you are calling loam is a mixture of materials that does not naturally occur at LAM (and most certainly not around here where I live in Wareham - some 3000 km away!)

We have been adding straw (typically wheat stocks that are gathered up into bales after the crop is harvested), chopped into roughly 5 - 8 cm lengths. Our typical mix is roughly 50 % dry powered potters clay, mixed with an equal volume of the straw. Add enough water to let you manipulate the stuff by hand. Normally we only add sand to balance the water content (like if we use too much water by mistake - its hardly exact). My experiences absolutely agree with yours, the inclusion of vegetation in the form of plant stems both helps to hold the constructed walls together against cracking, but also allows any steam to vent into the hollow stems (or out through the channels) to reduce cracking as well. We also have repeatedly gone straight from building wet cobb mix walls straight to the pre-heat with split wood with no real problems.

Working with straight clay is another matter. Twice we have used boxes of prepared potters clay, which comes pre-mixed as large blocks, soft enough to hand work, maybe 20 x 15 x 25 cm. (Smelts in October 07 and the last one on October 11 09) We cut these with a dry wall saw into thick slabs, about 5 cm thick (so 'brick' shapes, about 15 x 5 x 25) These were stacked, on end, a rough octagon shape, then smoothed over at the edges. As there is nothing to help let the steam escape from inside the 5 cm thick mass, there were plenty of pops, cracks - and flying clay pieces, as the pre heat was underway. Pieces as large as a golf ball flew as far as 5 metres a couple of times! Too exciting. As I mentioned in the last short report , having the clay construction surrounded by stones and packed with sand / ash mix, both held the structure together and helped to keep (most) of the combustion gases inside the smelter.

We have had excellent results with the clay and chopped straw cobb construction. Individual furnaces have been re-used as many as five times with only minor repairs (around the tuyere area). The structures stand up to Ontario winters, by just putting a cover over the top to keep the snow out. Once the walls have gone through a smelt sequence, they pretty much sinter into a rough ceramic and are pretty durable. Built at our normal 7 - 8 cm thickness, they are strong enough to be self supporting. To construct a furnace at 25 cm interior diameter and roughly 60 cm tall requires about two and half 20 kg bags of dry clay. The cost for that around here is about $30 CDN.

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