Choosing the Right Clay Body for Canadian Potters
Clay body selection shapes every subsequent decision in a pottery practice — how a form is built, how it dries, how it survives the kiln, and how it interacts with glaze. For potters working in Canada, the choice is further influenced by which bodies are available domestically, how studio temperatures affect drying rates in winter, and whether the intended firing range matches local kiln infrastructure.
The Three Main Clay Families
Clay bodies used in studio pottery fall into three broad categories defined primarily by their firing temperature and resulting density.
Earthenware
Earthenware fires between approximately cone 06 and cone 1 (around 999°C to 1137°C). At these temperatures the clay does not fully vitrify, meaning it remains porous unless sealed with a glaze. Traditional red earthenware is the oldest ceramic material used in human history and remains common for decorative work and terra cotta.
In Canadian studios, earthenware is less prevalent than stoneware, partly because many studio potters work in the mid-fire or high-fire range. That said, earthenware is widely used in educational settings due to its forgiving plasticity and lower firing costs. Plainsman Clays in Alberta lists several earthenware bodies including their H440 red earthenware, suitable for wheel throwing and hand-building.
Stoneware
Stoneware fires between approximately cone 6 and cone 10 (around 1222°C to 1305°C). At these temperatures the clay partially vitrifies — becoming dense, hard, and low-porosity even without glaze. This makes stoneware suitable for functional ware including tableware and cookware.
Stoneware is the most widely used clay body in Canadian studio pottery. Its firing range aligns with both cone 6 electric kilns (common in smaller studios and institutions) and cone 10 gas or wood kilns (used in dedicated production studios and community kiln facilities). Formulations vary considerably: some bodies are formulated for smooth throwing, others for heavily textured hand-building or slab work.
Cone 6 vs Cone 10
Cone 6 firing (approximately 1222°C) has become the dominant range in many Canadian studios because it is achievable with standard electric kilns and consumes less energy than cone 10. However, cone 10 reduction in a gas or wood kiln produces distinctive surface effects — from carbon flashing to ash deposits — that are not replicable at cone 6.
Porcelain
Porcelain is distinguished by its white or near-white colour and translucency at high fire. It fires in the same cone 6–10 range as stoneware but contains a higher proportion of kaolin, feldspar, and silica — which gives it lower plasticity and a narrower working window. Throwing porcelain on the wheel requires more skill than throwing stoneware because the body is less forgiving of uneven pressure.
Tucker's Pottery in Ontario carries several porcelain bodies including a cone 6 porcelain formulated for throwing, and a translucent casting porcelain for slip casting work. Porcelain is popular for work where glaze colour and clarity are priorities, since the white base body does not affect glaze tone the way a grey or buff stoneware does.
Plasticity and Workability
Plasticity refers to a clay body's ability to deform without cracking under pressure and to return partially to shape when that pressure is released. Higher plasticity generally makes a body easier to throw on the wheel and more forgiving during hand-building.
Plasticity is influenced by particle size (finer particles yield more plasticity), organic content (aged or reclaimed clay tends to be more plastic), and the presence of bentonite or macaloid as additives. Many commercial stoneware bodies are formulated with a small percentage of bentonite — typically 1–3% — to improve workability without significantly affecting fired characteristics.
In Canadian studio conditions, clay moisture can drop noticeably during winter heating season when indoor humidity falls. Potters in provinces with severe winters — Manitoba, Saskatchewan, Alberta — often find they need to add moisture to stored clay more frequently than those working in coastal British Columbia, where humidity remains higher year-round.
Shrinkage and Warping
All clay shrinks as it dries and again as it fires. Total shrinkage from wet to fired can range from roughly 10% to 14% depending on the body. Higher-fired bodies typically shrink more than low-fire ones because more of the glassy phase forms during firing, pulling particles together.
Uneven drying is the primary cause of cracking during the greenware stage. Covering pieces with plastic, drying slowly in sections, or compressing the clay during throwing all reduce warping risk. Flat pieces — plates, slabs — are particularly prone to warping and often benefit from being dried on a permeable surface such as a wooden board rather than a sealed shelf.
Grog — fired and ground clay particles added to a body — reduces shrinkage and increases thermal shock resistance. Bodies with higher grog content are used for sculptural work, large-format pieces, and wood firing, where uneven heating during loading or stoking can create thermal stress.
Canadian Suppliers
The two principal domestic clay body suppliers serving Canadian potters are:
- Plainsman Clays (Medicine Hat, Alberta) — formulates clay bodies specifically for Canadian conditions and offers a detailed public database of fired tests and chemistry at Digitalfire, maintained by Plainsman's Tony Hansen. Bodies range from terra cotta to high-fire stoneware and porcelain. Shipping to western provinces is generally more cost-effective than to Ontario or Quebec.
- Tucker's Pottery Supplies (Toronto, Ontario) — stocks a range of commercial stoneware and porcelain bodies suitable for cone 6 and cone 10, serving primarily Ontario and eastern Canada. Tucker's also carries prepared glazes and raw glaze materials.
Some studios import American bodies from Laguna Clay, Standard Ceramic, or Sheffield Pottery when specific formulations are not available domestically. Cross-border shipping costs and import considerations mean this is generally reserved for larger quantity orders.
Practical Selection Criteria
When evaluating a clay body for a specific use, the following properties are worth testing rather than assumed:
- Fired colour at your target cone — the same body can shift significantly between cone 6 and cone 10, or between oxidation and reduction atmospheres.
- Absorption rate — relevant for functional ware. Fired absorption above approximately 2–3% indicates under-vitrification and potential leakage for liquid-bearing vessels.
- Glaze fit — a mismatch between clay body and glaze thermal expansion (measured as COE) causes crazing or shivering. Testing glaze on a specific body is more reliable than relying on manufacturer data alone.
- Texture at the intended scale — a heavily grogged body that works well for a large vase may feel unpleasant and wear tools rapidly when throwing small cups.
Documented test tiles — fired at your kiln's actual temperature rather than manufacturer specifications — are the most reliable reference for any new clay body.