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A clay fired at low temperatures (cone 010-02) where it does not develop maturity (vitrify). The term earthenware almost alway refers to red burning terra cotta ware (although it is a somewhat more general term referring to a wider range of colors and more primitive forming and firing techniques). Earthenwares are porous, sintering is the particle bonding mechanism (therefore ware is not nearly as strong as stonewares and porcelains). If glazed, ware is usually bright colored. Although the low fire clays from which earthenware is made can be burned higher to achieve much better strength, it is intentionally fired lower (usually because of lack of resources to burn higher or the need for porosity and thermal shock resistance intrinsic with this type of ware).

The porcelain is harder, but the terra cotta has it beat for thermal shock!

The porcelain is harder, but the terra cotta has it beat for thermal shock!

This terra cotta cup (center) is glazed with G2931G clear glaze (Ulexite based) and fired at cone 03. It survives 30 seconds under direct flame against the sidewall and turns red-hot before a fracture occurs (the unglazed one also survived 30 seconds, it only cracked, it did not fracture). The porcelain mug (Plainsman M370) is glazed with G2926B clear, it survived 15 seconds (even though it is much thinner). The porcelain is much more dense and durable, but the porous nature of the earthenware clearly withstands thermal shock much better. It is actually surprisingly durable.

Terra cotta and a surprising thing about thermal shock

Terra cotta and a surprising thing about thermal shock

This terra cotta cup is glazed with G2931G clear glaze (Ulexite based) and fired at cone 03. It survives 25 seconds under direct flame against the sidewall before a crack occurs. Typical porcelains and stonewares would survive 10 seconds. Super vitreous porcelains 5 seconds. This is a key advantage of earthenware. Sudden changes in temperature cause localized thermal expansion, this produces tension and compression that easily cracks most ceramics. But the porous nature of earthenware absorbs it much better. During initial testing I found better performance for glazed earthenware (vs. unglazed), but in later testing they proved to be fairly similar.

Cone 2: Where we see the real difference between terra cottas and white bodies

Cone 2: Where we see the real difference between terra cottas and white bodies

The terra cotta (red earthenware) body on the upper left is melting, it is way past zero porosity, past vitrified. The red one below it and third one down on the right have 1% porosity (like a stoneware), they are still fairly stable at cone 2. The two at the bottom have higher iron contents and are also 1% porosity. By contrast the buff and white bodies have 10%+ porosities. Terra cotta bodies do not just have high iron content to fire them red, they also have high flux content (e.g. sodium and potassium bearing minerals) that vitrifies them at low temperatures. White burning bodies are white because they are more pure (not only lacking the iron but also the fluxes). The upper right? Barnard slip. It has really high iron but has less fluxes than the terra cottas (having about 3% porosity).

Out Bound Links

  • (Glossary) Terra cotta

    'Terra Cotta' (Italian for 'cooked earth') is red burning earthenware. It has been made for thousands of years by indigenous cultures, most often unglazed. If glazed, high lead content mixtures have been traditional. It is fired at much lower temperatures than stoneware so, not surprisingly, it is n...

  • (Glossary) Majolica

    Pottery fired to a low temperature employing a red-burning terra cotta clay covered with a soft opaque white glaze. Historically, majolica glazes (or tin glazed earthenware) were opacified using Tin Oxide, but now Zirconium silicate is most often used. Most majolica also has colored brushwork design...

  • (Glossary) Stoneware

    Most often the term stoneware refers to a high fired (about 1200C+) ceramic clay:feldspar:quartz blend that is semi-vitreous (not translucent and not zero porosity). To appreciate the scope that stoneware can encompass it is helpful to contrast it with porcelains (this description is for people who ...

  • (Glossary) Porcelain

    Traditional utilitarian porcelains are comparatively white burning and vitreous clay bodies that are made from feldspar, clay and quartz. When fired, the feldspar flows and dissolves many of the other particles into a viscous glassy melt that bonds the quartz particles and, and if temperatures are s...

In Bound Links

  • (Typecodes) 1: CLO - [label]
  • (Glossary) Secondary Clay

    Clays that have been transported by water from the site of their primary alteration and settled into layers. They are classified according to the geological period in which they were formed. Nature purifies these clay mechanically, removing gravel and sand, but contaminates them chemically or at the...

  • (Glossary) Interface

    In ceramics, the zone of adherence between glaze and the underlying body is called the interface. The integrity of body-glaze interface is important to strength and functionality of ware. In porcelains it is much more highly developed than in earthenwares. In the latter, the glaze, although melted w...

By Tony Hansen

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