Clay & Organics - Furnace Building

 Question from Dan - on Don Fogg's Bladesmith Forum - 'Bloomers & Buttons'

Any advice on smelting furnace materials? Firebrick? Kitty Litter?

As I have mentioned, there is a dance between ore (as the lead) and furnace details. Playing the tune is construction materials on melody and charcoal type sometimes improvising.

Powered potters clay is cheap and very easy to work with - and can be purchased as known types.
Local clay is considerably more work (unless you are one of the lucky ones) and usually will have quite specific (likely unknown) properties.
You can make your own decision if digging, hauling, drying, breaking, cleaning then re-mixing a local clay is worth the roughly $10 - 15 a bag of powdered clay costs.

Working with clay obviously gives you the most flexibility in terms of design. (Making a cylinder using rectangular bricks is sometimes not the easiest.
Remember that the air blast, so the burn pattern, in the furnace will be some variation on a *spherical' volume. You make a square or rectangular furnace and there are going to be corners that are not going to burn / contribute to the reactions. (Japanese Tatara aside, but there the system uses multiple air points to get around the physics.)

Parks Canada / LAM - 2001 (first smelt attempt). You can clearly see that the two corners opposite the tuyere point are not ignited at all.

Like everything else, there is a knack to working with hand mixed clay.
Lee Sauder's  / Owen Bush's  advice of mixing up your clay balls and leaving them to relax is excellent (time / space / manpower allowing)

At this point I don't think any of the successful, multiple smelt workers are using a straight clay (??)

Sand in the mix reduces the way  wall material expands when it heats to operation temperature.
A high sand mix does require considerably more effort on the build. It also requires much more careful drying / baking before the furnace is used.
The result however, with the care needed, is most certainly a more durable furnace.
This is the mix that Lee is using, and he certainly has had an individual furnace re-used dozens of times.

One of Lee's Furnaces (a slightly older version)

Organics added will do three things :
- Pieces remaining in the outer layer of wall act to bind the whole structure together. This action tends to limit potential cracking, and hold the walls together even if cracks form.
- Organics with hollow cross section (ie - straw) allow some place for the expanding steam to go, steam produced when the water in the clay heats. The massive increase in volume as water flashes to steam is the primary source of cracking of the clay walls. (Obviously very careful / long duration drying limits this available water remaining in the walls)
- Pieces remaining in the inner layer of wall will eventually burn away with the high temperatures of an operating furnace. This in effect leaves air spaces, which are insulators. The gross effect is to help limit the loss / spread of heat into the exterior surface of the furnace. (This at least in theory, honestly I doubt anyone has actually tested for this ??)

Obviously differing organic additives will perform differently:
- We tested 'peat moss' - purchased locally as spaugam moss for gardening. The long pieces soaked up excessive water and held it, making mixing a bit of a pain (hard to get consistent mix). Then the drying became a real problem. In the end we got excessive wall cracking - there was just too much water being held by the mix.
I highly expect that what anyone would get locally as 'peat moss' might vary an extreme amount - region to region.
- We have had extremely good results with shredded, dry horse manure. Get last year's pucks, rub them between your palms. What you end up with are very dry lengths of grass, usually about 5 - 10 mm long. Added to clay, these pieces act just like fiberglass bits in car repair 'bondo'. (I learned this technique from Micheal Nissen in Denmark.)
- Our old stand by here is chopped dry straw. Cut the pieces to 5 - 10 cm long with a hatchet or machette. Straw (rather than hay) has the hollow core mentioned above. The net effect is just like adding rebar to concrete. It does make hand mixing a bit of an effort.

Learning from Lee, the normal mix used here at 'Viking Age Central' (Wareham Ontario) is a rough mix by volume :
1/3 powdered clay / 1/3 course sand / 1/3 organic material
Often we will use a higher fire temperature clay (like the EPK suggested) for the part of the furnace around the tuyere level (which suffers the highest temperatures). I have noticed any particular problem mixing different clay types in one furnace.

We are undertaking more experimental archaeology here than production smelting here. For that reason, we are constantly changing details on furnace construction (so building a *lot* of individual furnaces!). To date we have only used one furnace for a total of five smelt events, the average is closer to two smelts per furnace before we build a different model.

After seeing a whole lot of furnaces built, I would suggest the most important factor is :
Take your time and use care in the build - this pays back with less cracking and more durability
The single biggest error people make is not making sure the individual balls / blocks of mixed clay are well fused to each other as you build up the walls. Cracks are almost always along the joint lines of the individual blocks as added.

Darrell
see
www.warehamforge.ca/ironsmelting
for images of a whole lot of differing 'Short Shaft' furnaces