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Clay Ovens and Heaters

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Thermal mass heating and cooking/baking ovens (such as Rocket Mass Ovens or Heaters, Cob ovens) require systems of duct work and heat sinks whose construction involves the use of raw clay mixed with sand and/or straw. For example, check http://dirtcraft.ca or this page with videos and technical info.

Many sources recommend using a Fireclay because these can withstand the temperatures in the oven. However this is not really correct. Red hot coals in a campfire are only about 1000-1200F, even the lowest duty clay is capable of withstanding temperatures far higher than that. In addition, fireclays come in a wide range of plasticities, water permeabilities, drying performances, etc. (like other clays). It is these properties that are more important than the fact it is a fireclay. In fact, we most often recommend a raw clay we mine called 98Mix. That clay is an earthenware. However it dries hard and strong and maintains plasticity even when blended with significant sand.

Other sources advise simply digging clay out of the ground and mixing it in a certain proportion with sand. The situation here is similar as with the fireclay. While this might have worked for the author with his clay and sand, it will not likely work with your clay. The reason is that sands vary widely in grain size and shape, that variation can multiply to orders of magnitude differences in particle surface area (which must be coated and adhered by the clay). As noted, clays have plasticity, that is, an ability to hold a shape. More plastic clays are stickier and thus form better and dry to a harder surface, but they shrink alot more and therefore crack much more on drying. Less plastic clays do not form as well, but shrink much less, crack less and have a softer dried surface. There is a 100-fold difference in particle size between the most plastic and least plastic clays, this can translate to 1000 times more surface area. Very plastic clays also dry much slower (as much ten times). Our dealers may have dry powders of our grey-colored, medium plasticity prepared clay bodies (like M340, H550), these shrink about 6.5% on drying. For people buying clay at our plant site, again, we normally recommend 98Mix (a very plastic greenish colored raw clay straight from the quarry).

To withstand the rain you need to build a roof or do something to make it able to shed water (a dried clay might seem hard, but it will turn back into mud when it comes into contact with water). The most obvious solution is to plaster it (using some sort of cement or plaster mix that sets hard). An alternative is to add a hardener/sealant to your clay mix (e.g. silicone, corn starch, polymers, gums). If you choose the hardener do plenty of testing to make sure it will work.

Develop some sort of testing regimen to determine the optimal recipe of sand and clay (plus other things e.g. straw and fiber of other types) that does not shrink too much but still hardens strong. Some people have found clays that require no sand at all. Let us know if you need help.