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Clays

Earthenware

Earthenware is made from either red or white clay and is low-fired leaving it porous. It is used in bake-ware, dinnerware and decorative ware. Baking dishes, pie plates, etc. should be place in a warm, not hot oven and the oven temperature raised to the desire setting for cooking. Because it is porous in nature, it should be hand washed and should not be used in the microwave

Stoneware

Stoneware is a white, gray or brown high-fired, usually non-porous clay body, which is used in bake-ware, dinnerware and decorative pieces. Stoneware can be washed in the dishwasher, used in the microwave and oven, but not placed on the stove top for cooking. Baking dishes, pie plates, etc. should be placed in a warm, not hot oven and the oven temperature raised to the desire setting for cooking. If dishes are taken from the refrigerator, they should be allowed to come to room temperature before heating in the oven.

Porcelain

Porcelain is a white high-fired, non-porous, sometimes translucent clay body used in making functional and non-functional pieces. Porcelain can be washed in the dishwasher, used in the microwave, and oven, but not placed on the stove top for cooking. Baking dishes, pie plates, etc. should be place in a warm, not hot oven and the oven temperature raised to the desire setting for cooking. If dishes are taken from the refrigerator, they should be allowed to come to room temperature before heating in the oven. Very fine or thin porcelain may crack or get fine craze lines if heated excessively. It is best treated as fine china.

Paper Clay

Paper clay is any clay body to which processed cellulose fiber (paper being the most common) has been added.

Earthenware, terra cotta, stoneware, porcelain and bone china clays bodies can be made into paper clay. The more fiber added to the clay body, the stronger the unfired dry paper clay, but the weaker the fired body. The less fiber added to the clay body, the weaker the unfired dry paper clay, but the stronger the fired body.

Stages of Clay to Ceramics

Bisque (often called “biscuit” refers to pottery that has been fired but not yet glazed. The porous nature of bisque earthenware means that it readily absorbs water. The temperature of bisque firing is usually at least 1000°C, although higher temperatures are common.

Greenware refers to unfired objects. Clay bodies at this stage are in their most plastic form. They are soft and malleable. Hence they can be easily deformed by handling.

Leather-hard refers to a clay body that has been dried partially. At this stage the clay object has approximately 15% moisture content. Clay bodies at this stage are very firm and only slightly pliable. Trimming and handle attachment often occurs at the leather-hard state.

Bone-dry refers to clay bodies when they reach a moisture content at or near 0%. This will occur after glaze firing, when that is done, or after biscuit firing in the case of once-fired pottery.

Firing Processes

Raku

Raku is a low-fired process which leaves the clay body porous. It is not usually suitable for use as tableware unless highly glazed with a lead-free glaze. It is also not intended for use in the oven, microwave, or stovetop and it should be hand washed or only dusted. Most raku vases, unless otherwise noted by the maker, are too porous to hold water so an alternative water container must be placed inside. Raku ware is for decorative use only and should be kept out of direct sunlight.

Smoke-Fired

Smoke-fired pottery is a low-fired process which leaves the clay body very porous. The surface may be waxed to enhance and maintain the smoke colouration on the surface. It should not be used in the oven, microwave or on the stovetop. It should be lightly cleaned with a damp cloth or just dusted. Smoke-fired pieces, unless otherwise noted by the maker, are too porous to hold water so an alternative water container must be placed inside. Smoke-fired ware is for decorative use only and should be kept out of direct sunlight.

Salt Fire

Salt glaze pottery is stoneware with a glaze of glossy, translucent and slightly orange-peel-like texture which was formed by throwing common salt into the kiln during the higher temperature part of the firing process. Sodium from the salt reacts with silica in the clay body to form a glassy coating of sodium silicate. The glaze may be colourless or may be coloured various shades of brown (from iron oxide), blue (from cobalt oxide), or purple (from manganese oxide)

Glazes

All glazes on porcelain, stoneware and earthenware pieces are intended for tableware and bake-ware use, unless otherwise specified.

High fired glazes, usually used on stoneware and porcelain, becomes part of the clay body during firing and are very durable.

Shino

Shino is a term used to describe a high-fired glaze with blushes of iron colouration which is white/gray. It is very durable on stoneware and porcelain.

Crackle

Crackle glazes can be low or high fired. Low-fired crackle glaze, usually on raku is very porous. High fire crackle glaze on oriental ware is very durable.

Matt Glaze

A matt glaze is one that has no shine. The surface may feel a bit rough.

Satin Glaze

A satin glaze has a slight sheen.

Gloss Glaze

A gloss glaze has a very high sheen.

Lusters

Lusters are usually applied over a glaze and re-fired at a low temperature. They are used in ceramic painting and raku to give a rainbow or sheen effect on the surface of the glaze. 18-24 carat gold or silver lustre may be used as decoration on the surface of low or high fire clayware. Lusters are not durable and should be handwashed or dusted. They should be treated the same as fine china.

Other Terms

Cones

Pyrometric cones are pyrometric devices that are used to gauge heatwork during the firing of ceramic materials. The cones, often used in sets of three, are positioned in a kiln with the wares to be fired and provide a visual indication of when the wares have reached a required state of maturity, a combination of time and temperature.

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