Every decision made in the pottery studio — how a piece is formed, which glazes are applied, how hot the kiln fires — is shaped in part by the clay body being used. A glaze that works perfectly on a mid-fire stoneware may crawl, pinhole, or slide off a low-fire earthenware body fired at the same cone. A throwing technique that produces thin, even walls in a plastic stoneware may tear porcelain before the form reaches any useful height.
Canadian potters have access to commercial clay bodies through suppliers concentrated in Ontario and British Columbia, as well as regional materials that vary significantly in character. This article documents the main clay body categories in terms relevant to studio selection.
How Clay Bodies Are Classified
Clay bodies are typically classified by their firing temperature range — low-fire, mid-fire, and high-fire — and by their primary mineral composition and texture. The three main categories for studio work are earthenware, stoneware, and porcelain. A fourth category, terracotta, is often treated as a subset of earthenware.
Within each category, commercial suppliers blend raw materials to achieve specific working properties: plasticity (how well the clay responds to forming), shrinkage (how much the clay contracts during drying and firing), absorption (how much water a fired piece still absorbs, indicating how fully vitrified it is), and colour (influenced by iron content and firing atmosphere).
Earthenware
Earthenware is the oldest ceramic material in use. It fires at relatively low temperatures — typically cone 06 to cone 2 (approximately 990°C to 1120°C) — and remains porous after firing unless the entire piece is covered with a vitreous glaze. Fired earthenware absorbs 5 to 15 percent of its weight in water when unglazed. Functional ware made from earthenware must be fully covered with a watertight glaze to hold liquids.
Characteristics
- Colour: Red to terracotta when iron-bearing (most common); cream to buff in low-iron formulations. Firing atmosphere has less effect than in stoneware because iron content is usually lower.
- Plasticity: Generally high. Earthenware clays often contain significant fine-particle ball clay, which contributes to workability but increases drying shrinkage.
- Shrinkage: 8 to 12 percent from wet to fired, depending on the specific body and firing temperature.
- Glaze fit: Earthenware glazes must have a thermal expansion coefficient well-matched to the body. Mismatch causes crazing (glaze contraction cracking) or shivering (glaze under compression flaking off).
Where It's Used
In Canadian studio practice, earthenware is used for decorative ware, sculptural work, and majolica — a technique where the piece is coated with an opaque white tin glaze and then decorated with metal oxide pigments. Majolica production requires consistent earthenware bodies and careful glaze formulation, and several Canadian potters who document their practice use red earthenware from Ontario suppliers as their base material.
Stoneware
Stoneware is the most widely used clay body category in Canadian studios. It fires at cone 6 to cone 10 (approximately 1222°C to 1285°C), vitrifies fully or near-fully at those temperatures (absorption below 3 percent), and is physically durable. Its mineral composition — primarily ball clay, fire clay, silica, and feldspar — is more tolerant of variation than porcelain and more forgiving of inconsistent wedging or forming technique.
Characteristics
- Colour: Grey, buff, or brown in oxidation; warmer in reduction where iron content varies.
- Plasticity: Moderate to high. Grogged stoneware (containing grit or fine fired clay particles) has reduced plasticity but better resistance to thermal shock and better structural stability in hand-building.
- Shrinkage: 10 to 14 percent from wet to fired. Highly plastic stoneware bodies tend toward the higher end.
- Glaze fit: Stoneware accepts a wider range of glaze chemistries than earthenware. At cone 10, glaze and clay body have often begun to blend at the interface, producing stronger glaze adhesion and a broader tolerance for expansion mismatches.
Mid-Fire vs. High-Fire Stoneware
Many Canadian potters work at cone 6 rather than cone 10 — this is sometimes described as mid-fire, though the distinction is not absolute. Cone 6 stoneware vitrifies adequately for functional ware, places less thermal stress on kiln elements, and allows a broader range of glaze colourants to remain stable. Some colourants — particularly commercial underglazes containing chrome or certain cadmium-based compounds — burn out or become toxic above cone 6.
Porcelain
Porcelain is a high-fire clay body composed primarily of kaolin (a fine, white-burning primary clay), feldspar, and silica. It fires to a dense, white or near-white, translucent (in thin sections) finish. Porcelain's visual properties — the whiteness, the translucency in fine-thrown bowls, the way glazes appear luminous against the pale body — make it appealing to many potters and collectors. Its working properties make it challenging, particularly for beginners.
Characteristics
- Colour: White to off-white, shifting slightly warm or cool depending on the feldspar source. Very responsive to reduction atmosphere — celadon glazes on porcelain in reduction produce the characteristic soft blue-grey associated with classical East Asian ceramics.
- Plasticity: Low to moderate. Kaolin is non-plastic. Commercial porcelain bodies include plasticising additives (bentonite, ball clay, or macaloid) that improve workability, but porcelain will never handle like a high-ball-clay stoneware.
- Shrinkage: 12 to 16 percent. High shrinkage increases the risk of cracking during drying, particularly where wall thickness is uneven or joins are improperly made.
- Glaze fit: Porcelain requires precise glaze fit. The body is less tolerant of crazing than stoneware because the vitrified matrix has less flexibility. Many porcelain potters in Canada formulate their own glazes or work closely with documented commercial recipes designed for their specific body and firing temperature.
Throwing Porcelain
Porcelain stiffens quickly during throwing, particularly as water is introduced and withdrawn. It is less forgiving of repeated pulling — each additional pull risks tearing. Skilled porcelain throwers work faster with fewer pulls than they would with stoneware, and they tend to open wider on the first pull to minimise the number of subsequent drawing passes.
Reclaim Clay
In any active studio, clay scraps accumulate. Reclaiming — drying scraps, slaking them in water, and reconstituting the resulting slip — is standard practice. Reclaimed clay should ideally come from a single clay body, as different bodies have different shrinkage rates and glaze fits. Mixing reclaimed stoneware with porcelain, for example, produces an inconsistent blend that is difficult to glaze predictably.
Several Canadian studio potters document their reclaim practices as part of their studio notes. A common method involves spreading poured slip onto plaster drying bats or canvas-covered boards, allowing it to firm to a workable consistency, and then wedging it together with fresh clay to homogenise texture and moisture.
Where to Source Clay in Canada
Major Canadian ceramic supply companies — including those serving Ontario, British Columbia, and Quebec — carry commercial clay bodies from North American and some European producers. Regional differences in available bodies exist; a body widely stocked in Toronto may not be carried by a supplier in Kelowna or Halifax. Some potters in rural areas source local clay directly, testing and modifying it with additions of silica, feldspar, or grog to achieve appropriate working properties.
For documented reference on clay body formulation and raw materials, the Digitalfire Reference Database maintained by Tony Hansen is widely cited in Canadian ceramics education. The Ceramic Arts Network also publishes regular technical articles on clay body behaviour.
Glaze Interaction by Clay Type
The interaction between a glaze and a clay body at firing temperature is chemical. Silica and alumina from the clay body dissolve slightly into the glaze at the interface, creating a boundary layer that anchors the glaze. This process works differently across clay body types:
- On earthenware, glazes remain more distinctly separate from the body unless fired to the upper range of earthenware temperatures. Low-fire bodies do not contribute much silica or alumina to glaze melt.
- On mid-to-high-fire stoneware, the body-glaze interface is more developed. Matte glazes that rely on this zone for their surface character — particularly calcium matte glazes — often perform very differently on earthenware or porcelain.
- On porcelain, the high-silica body contributes actively to glaze development. Celadons and shino glazes, both historically associated with porcelain, behave very differently on iron-bearing stoneware — less vibrancy in the case of celadon, less blush and carbon-trap depth in the case of shino.
