What appears under consideration here are thick walled furnaces, with an eye to increasing the durability of the structure.
I have worked a lot with various cobb mixes - blends of clay / sand / organics. Our (DARC) basic design here has been primarily chopped straw plus clay. I have built a lot of furnaces, as our primary investigations have been with various archaeological models.
From conversations with Lee Sauder, my understanding (??) is that the addition of sand to the clay increases the resistance to temperature of the finial wall material. Most importantly, the sand does not expand with heating near as much as the clay will, thus making the walls more stable at smelting temperatures. Careful drying of the constructed furnace is critical.
(A reminder to new readers - as water heats it expands as steam - something like 50 times the volume. If this steam does not vent slowly, cracking, even explosive spalling, is the result.)
Straw cobb works in a different manner. Our most typical mix starts with 50 % dry potters clay / 50% straw by volume. Add maybe an additional 10 - 15 % sand to stiffen the mix. The straw is chopped to roughly 10 cm (4 inch) or less pieces.
The individual pieces of straw in the mix carry out several functions:
- Being hollow, they give the steam someplace to expand into, reducing cracking effects.
- As the inner surfaces of the furnace reach smelting temperatures, the straw burns out. This leaves hollows in the clay matrix - in effect acting as an insulating layer. This actually does increase the overall resistance of the wall to high temperature.
- The outer layers of the furnace, although hot, are not high enough to burn away the straw. So the straw acts like pieces of rebar in concrete. Even if cracks will develop, the straw binds the gaps - holding the walls together.
Generally I would say that the high sand furnaces Lee builds (PDF) are more durable over the long term use. I think he had one furnace that ran over 35 individual smelts. The down side is that the construction requires considerable care and time to undertake correctly.
The use of the clay / straw cobb allows for much quicker construction. I have certainly done a build in the morning, then fired in the afternoon. I would generally say these furnaces are not as durable. The most we have used one furnace has been five smelts. This is more because of our climate in Central Ontario, working in an exposed smelting area. Clay cobb is much more susceptable to damage through the freeze / thaw cycle (mainly because of those internal voids). (Our normal pattern here is three smelts a year, June / October / November, so almost every furnace sees at least one winter cycle.)
Something Michael Nissen (from Ribe in Denmark) showed me when I visited with him in 2008 - the use of shredded horse manure as the organic mix. This results in a much finer texture to the cobb. It does still retain the heat resistance and strength of the straw mixes. Gather old horse pucks, which shred easily when rubbed between your hands. Mix 50 / 50 with the powered clay. I have not used this for a complete full sized furnace. It has become my standard mix for the smaller Aristotle re-melting furnaces, where it shows great refractory ability. Michael (at that point) was using a 'bellows plate and blow hole' system. This features a roughly 15 x 20 cm thin plate set into the front of the furnace - with a hole in it through which the air is blown. This is hottest part of the furnace wall, yet even 1 cm thick plates of horse cobb have demonstrated great durability.
I have constructed a couple of furnaces in metal shells.
One was a variation on our 'Econo Norse' brick teaching furnace. I managed to scrounge a 1/2 sized metal barrel (20 gallon) from the dump. This was used as an external shell for an arrangement of standard fire brick set in a hexagon pattern, three bricks tall. The gaps in the pattern were held in place with a mix of clay and sand - as much as a binder as anything else. Although heavy, the size is such I can lift the completed furnace by myself. The solid metal shell allows this furnace to be portable. The ideal way to use it is in combination with a concrete block plinth, packed with ash / charcoal fines (learned from Lee & Skip). See the smelt report : http://www.warehamfo...eport04-08.html
Last spring, I built a variation on Lee's successful design. This was his suggested mix of 50 clay / 50 sand. I had an old metal garbage can, which proved just the exact size required with the bottom cut out and turned upside down. At this point this furnace has only been fired once - but it remained in almost perfect condition after the smelt. You'll see in the image below it also uses a forged copper tuyere - again prototyped by Lee.
I thought I had a close up of the shrinking of the clay walls. Although I used the metal can as an exterior form, buy the time the furnace was completely dried with an internal fire, there was about 1 cm shrinking all around. Thats with an internal diameter of 25 cm and wall thickness of 5 cm. My mix was 'looser' than what Lee uses, so much of that volume might have been the extra water (??). This furnace is set on a circular brick base (again stolen from Lee). The intent is not to make the furnace portable, but to protect the walls from the effects of rain and more importantly Canadian winters (!). I have covered the entire thing with a old plastic 45 gallon drum for the season. See : http://warehamforgeb...tion-smelt.html
PS - sorry about the order of the images - I pretty much pulled them down as I thought of it.
1) Three older furnaces, set out as an experiment in aging / weathering. The two at the rear are both clay /straw cobb construction.
2) My new Sauder style 'production' furnace, before firing.
3) Michael Nissen's furnace at the Heltborg Symposium, 2008. Body of straight dug clay, inset horse manure cobb bellows plate.
4) The 'Econo Norse in a Can' furnace - before firing