200 mesh | 325 mesh | 3D Design | 3D Printer | 3D Slicer | 3D-Printed Clay | 3D-Printing | Abrasion Ceramics | Acidic Oxides | Agglomeration | Alkali | Alkaline Earths | Amorphous | Apparent porosity | Ball milling | Bamboo Glaze | Base Glaze | Base-Coat Dipping Glaze | Basic Oxides | Batch Recipe | Bisque | Bit Image | Black Coring | Bleeding colors | Blisters | Bloating | Blunging | Bone China | Borate | Boron Blue | Boron Frit | Borosilicate | Breaking Glaze | Brushing Glaze | Calcination | Calculated Thermal Expansion | Candling | Carbon Burnout | Carbon trap glazes | CAS Numbers | Casting-Jiggering | Celadon Glaze | Ceramic | Ceramic Binder | Ceramic Decals | Ceramic Glaze | Ceramic Ink | Ceramic Material | Ceramic Oxide | Ceramic Slip | Ceramic Stain | Ceramic Tile | Ceramics | Characterization | Chemical Analysis | Chromaticity | Clay | Clay body | Clay Body Porosity | | Clay Stiffness | Co-efficient of Thermal Expansion | Code Numbering | Coil pottery | Colloid | Colorant | Cone | Cone 1 | Cone 6 | Cone plaque | Copper Red | Cordierite Ceramics | Crackle glaze | Crawling | Crazing | Cristobalite | Cristobalite Inversion | Crucible | Crystalline glazes | Crystallization | Cuerda Seca | Cutlery Marking | De-Airing Pugmill | Decomposition | Deflocculation | Deoxylidration | Digitalfire Foresight | Digitalfire Insight | Digitalfire Reference Library | Dimpled glaze | Dip Glazing | Dipping Glaze | Dishwasher Safe | Dolomite Matte | Drop-and-Soak Firing | Drying Crack | Drying Performance | Drying Shrinkage | Dunting | Dust Pressing | Earthenware | Efflorescence | Encapsulated Stain | Engobe | Eutectic | Fast Fire Glazes | Fat Glaze | Feldspar Glazes | Firebrick | Fireclay | Fired Strength | Firing Schedule | Firing Shrinkage | Flameware | Flashing | Flocculation | Fluid Melt Glazes | Flux | Food Safe | Foot Ring | Forming Method | Formula Ratios | Formula Weight | Frit | Fritware | Functional | GHS Safety Data Sheets | Glass vs. Crystalline | Glass-Ceramic Glazes | Glaze Bubbles | Glaze Chemistry | Glaze Compression | Glaze Durability | Glaze fit | Glaze Gelling | Glaze Layering | Glaze Mixing | Glaze Recipes | Glaze Shrinkage | Glaze thickness | Globally Harmonized Data Sheets | Glossy Glaze | Green Strength | Grog | Gunmetal glaze | Handles | High Temperature Glaze | Hot Pressing | Incised decoration | Industrial clay body | Ink Jet Printing | Inside-only Glazing | Insight-Live | Interface | Iron Red Glaze | Jasper Ware | Jiggering | Kaki | Kiln Controller | Kiln Firing | Kiln fumes | Kiln venting system | Kiln Wash | Kovar Metal | Laminations | Leaching | Lead in Ceramic Glazes | Leather hard | Lime Popping | Limit Formula | Limit Recipe | Liner Glaze | LOI | Low Temperature Glaze Recipes | Lustre Colors | Majolica | Marbling | Material Substitution | Matte Glaze | Maturity | Maximum Density | MDT | Mechanism | Medalta Potteries | Medium Temperature Glaze | Melt Fluidity | Melting Temperature | Metal Oxides | Metallic Glazes | Micro Organisms | Microwave Safe | Mineralogy | Mocha glazes | Mohs Hardness | Mole% | Monocottura | Mosaic Tile | Mottled | Mullite Crystals | Native Clay | Non Oxide Ceramics | Oil-spot glaze | Once fire glazing | Opacifier | Opacity | Ovenware | Overglaze | Oxidation Firing | Oxide Formula | Oxide Interaction | Oxide System | Particle orientation | Particle Size Distribution | Particle Sizes | PCE | Permeability | Phase change | Phase Diagram | Phase Separation | Physical Testing | Pinholing | Plainsman Clays | Plaster Bat | Plaster table | Plasticine | Plasticity | Plucking | Porcelain | Porcelaineous Stoneware | Pour Glazing | Precipitation | Primary Clay | Primitive Firing | Production Setup | Propane | Propeller Mixer | Pyroceramics | Quartz Inversion | Raku | Reactive Glazes | Reduction Firing | Reduction Speckle | Refiring Ceramics | Refractory | Refractory Ceramic Coatings | Representative Sample | Respirable Crystalline Silica | Restaurant Ware | Rheology | Rutile Glaze | Salt firing | Sanitary ware | Sculpture | Secondary Clay | Shino Glazes | Shivering | Sieve | Silica:Alumina Ratio | Silk screen printing | Sintering | Slaking | Slip Casting | Slip Trailing | Soaking | Soluble colors | Soluble Salts | Specific gravity | Splitting | Spray Glazing | Stain Medium | Stoneware | Stull Chart | Sulfate Scum | Sulfates | Surface Area | Surface Tension | Suspension | Tapper Clay | Tenmoku | Terra cotta | Terra Sigilatta | Test Kiln | Theoretical Material | Thermal Conductivity | Thermal shock | Thermocouple | Thixotropy | Tony Hansen | Toxicity | Trafficking | Tranlucency | Translucency | Transparent Glazes | Triaxial Glaze Blending | Ultimate Particles | Underglaze | Unity Formula | Upwork | Viscosity | Vitreous | Vitrification | Volatiles | Warping | Water in Ceramics | Water Smoking | Water Solubility | Wedging | Whiteware | Wood Ash Glaze | Wood Firing | Zero3 | Zeta Potential

Clay 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 would be, 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 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 super plastic 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.

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

Uzume thermal mass oven in operation

There's two envelopes of bricks, that contain the fire box and the circuit for the hot exhaust gases. The inside envelope is not visible, it is air-gapped 1/2 inch from the outside one, providing for hot gas circulation and heat exchange. It is made up of firebricks to withstand the 1000 C* temperature that the firebox produces (due to a venturi effect into the secondary chamber that induces complete combustion). There's no visible smoke whatsoever above the outside chimney 10 minutes after the fire is started; the exhaust pipe is barely 100 C* where it exits upward. Inside, the exhaust gases are directed towards the floor, in brick chambers for heat exchange traveling under the firebox before reaching the chimney at floor level. Unlike most mass-stoves, heat is released almost immediately upon the heavy steel cover reaching 150 C* to 200 C*. The firebox is filled twice a day, maybe three times on a cold day, leaving it to cool off during the night after hours (wood consumption is minimal). 1 1/2 hr after starting the fire and afterwards when the combustion is over, the surface temperatures of the outside clay/sand bricks is too hot to keep your hands on it. The design that was developed in France and the company sells on-a-pallet kits they claim can be assembled quickly. The key features are the two kinds of interlocking bricks, low emission levels, low kit price and ease of start-up (a few kindlings, fill the box with wood (+/- 10 kg.), and close the door).

Links

URLs https://en.wikipedia.org/wiki/Rocket_mass_heater
Rocket Mass Heater on Wikipedia
URLs http://buildnaturally.blogspot.ca/2013/06/build-clay-cob-oven-in-your-yard.html
Build a Clay Oven in Your Backyard
URLs https://www.fornobravo.com
US manufacturer of pizza ovens and fireplaces

By Tony Hansen

Monthly Tech-Tip from Tony Hansen

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