All Glossary

Clays for Ovens and Heaters

Thermal mass heating ovens (such as Rocket Mass ovens or heaters, Cob ovens, pizza or baking ovens) employ heat sinking mechanisms to store energy. Their construction often involves the use of heavy blocks (purchased or made) and raw clay mixed with sand and/or straw or other fibre to cement the whole thing together. Dried clay mix is obviously not as strong as cement, but it is quite practical for this application (e.g. much less expensive, adequate strength coupled with easy construction, disassembly and adjustments).

Some 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 (e.g. terra cotta) is capable of easily withstanding temperatures to 1800F+. Fireclays (or clays in general) come in a wide range of plasticities, water permeabilities, drying performances, etc (these properties are an important practical issue to constructing the oven). Plainsman Clays, for example, most often recommends their 98Mix (a very plastic greenish-colored terra cotta raw quarry clay). It dries hard and strong and maintains plasticity even when blended with significant sand.

Some sources advise simply digging clay out of the ground and mixing it in a certain proportion with sand. This might have worked for the author with his clay and sand, but will it work for you? Sands and clays vary widely in grain size and shape (that variation can multiply to orders of magnitude difference in grain surface area). Clays vary widely in plasticity (ability to form a shape), stickiness, drying hardness, permeability, drying speed and drying shrinkage. The most plastic clays (e.g. 98Mix mentioned above) are the stickiest and shrink and crack the most. They can host higher percentages of sand, which cuts the shrinkage, yet still dries hard and strong. While pottery clays might be plastic for building pottery most dry fairly fragile. Most pottery clays dry shrink about 6.5% whereas a superplastic terra cotta or ball clay might shrink 8-10%. And dry like concrete! Of course, you must experiment with varying proportions of sand (plus fiber) to find a compromise between something that will dry with minimal cracking and still be hard and strong enough.

To withstand the rain a roof is often needed to shed water and protect from direct rain (a dried clay might seem hard, but it will slake and turn back into mud when it comes into contact with water). Alternatively, a plaster or stucco finish can be employed. Or apply a sealant layer or add a hardener/sealant to your clay mix (e.g. silicone, corn starch, polymers, gums). If you choose to use an in-mix hardener, do plenty of testing to make sure it will work, hardeners can reduce plasticity significantly.

Related Information

A Thermal Mass Pizza or Baking Oven

Lower section is metal construction, dome is clay and brick with stucco surfacing. Door is cement and brick. Fire is started inside to heat it up, then ash is removed for baking. By Bruce Fochler, Prince George, B.C.

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Mass-Wood stove project at Wooly Ewe yarn shop in Smithers, BC

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