Tuesday, June 08, 2010

Horse shit - not Bull shit?

Recently the topic of iron smelting furnace construction has been discussed on Early Iron. Skip Williams mentioned the use of animal manure in clay mixtures for building walls. The question was asked :
Why use manure?

Short answer:
Structural strength and venting steam

If you consider the mechanics of the furnace, some light is shed here.
Provided you are pushing high air volumes* the furnace has a very high energy output.

One construction style is to have a relatively thin furnace wall, and so radiate enough heat off the *outside* that the wall material does not overheat and slump or melt. Sauder & Williams' 'Flue Tyle' furnace works on that principle.

The alternative is a relatively massive furnace wall, which endures through shear thickness. DARC's Econo Norse furnace is based on this principle. Wall materials will erode, but there is enough thickness to prevent burn through on a single firing. The walls would be patched between uses. This is the historical method - pretty much across all cultures and times.

Now, balance that against the physical dynamics of clay into ceramics.
If a piece of clay is any thicker than about 3 - 4 cm, the process of heating through to extremely high temperatures causes a lot of problems. First problem is any remaining water in the matrix flash heating to steam, with incredible expansion. This causes cracking at the very least, even explosive spalling (!) at the very worst.
This is even more difficult if you are heating it from one side only (like in a furnace wall). Even if absolutely bone dry, the difference in expansion alone between the inner, hot side and the outer, relatively cold, side will result in cracking.
Careful pre-heating of the furnace can certainly limit these effects. Historically furnaces where built either extremely massive, or set into earthen banks. In either case, this allows the structure to remain supported even if the walls are cracked. Earth banked furnaces also keep any cracks from venting those important working gases from the furnace.

A partial solution to the steam venting and resulting cracking is to provide some extra internal support and a venting method within the furnace wall itself. So what we are talking about here is the addition of some organic materials into the clay mixure to create a 'cobb'.
Straw is the ideal natural material for this. Our experience has shown that copping dry straw down to roughly 5 - 10 cm lengths gives the best effect. The short lengths provide a number of desirable effects:
- The individual pieces act as reinforcing bars inside the clay. Any cracks which might develop are this prevented from splitting open.
- The straw is hollow, and so acts both to absorb and vent out steam as it develops.
- As the thick furnace wall heats over the smelt, the inner layers will get hot enough to sinter to ceramic. The straw inside the inner wall will actually burn away, resulting in a somewhat insulating zone. The outer most portions of the walls never get hot enough (over most of the furnace) to carbonize the straw. So the outer layers still retain the re-enforcing quality.

A free standing furnace, with clay / straw cobb walls, is quite durable. Protected from moisture (limiting the freeze thaw cycle) it will easily require only minimal repairs even if several years old.

So - the point of the manure?
You need to use *specific* manure! Thats horse manure. Horses are not as efficient in grinding and digesting grasses, so horse manure is composed of a lot of small pieces of material, maybe 1 cm or so in length. The resulting cobb has a very fine texture, which is certainly easier to manipulate during furnace construction than the use of chopped straw. Personally, I find manure mix has less structural strength. (However I freely admit that I have not built an entire furnace from this material.)

You want to use *old dry manure*. The 'balls' are easy to shred up by just rubbing them between your hands. Fresh manure is more difficult to break up and mix with the clay. There is also a bigger variation in moisture content that can be a bit of a problem.

You should take a look at the work of Micheal Nissen from the Ribe Viking Centre in Denmark. He uses the 'bellows plate and blow tube' system. He works with a mix of roughly 50 / 50 clay and horse manure (volume) to construct the thin plate that sits around the tuyere area of the furnace.

* See Lee and Skip's basic research on this. The working number is 1.2 - 1.5 litres per minute, for each cm2 of cross section at tuyere level. This can be produced with hand powered bellows, either a 'traditional' N European 'great bellows', or a larger than normal Norse style double bellows. Certainly no problem with electric blowers.

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