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Earthenware

What is the difference between earthenware and a regular stoneware body? Earthenwares lack the glass development to fill voids and glue particles.

Details

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).

Related Information

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

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 an 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. The TSFL test on consistently size tiles can be used to log more precise results.

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).

Bisque temperature can make a big difference with fitting glaze at low fire

Two clear-glazed tiles, one crazed, the other not

This is Plainsman Buffstone with G2931L glaze fired at cone 06. A hotter bisque not only produces a stronger body but also eliminates crazing (these specimens where glaze-fired one month ago). Firing the bisque just one cone hotter has transformed the ceramic into a denser matrix having a higher thermal expansion. That has the power to put the squeeze on the glaze, preventing it from crazing. Hotter bisque temperatures can be problematic as they reduce bisque absorbency (thus lengthening dip and drying time for the glaze slurry). But for low temperature hobby ware this is not as much of a problem since glazes are gummed and dry slowly anyway. They are multi-coated for this reason (these were applied in two coats).

Links

Typecodes Clay Other
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
Glossary Interface
In ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.
Glossary Secondary Clay
Clays form by the weathering of rock deposits over long periods. Primary clays are found near the site of alteration. Secondary clays are transported by water and laid down in layers.
Glossary Porcelain
Standard porcelains used by potters and for the production of sanitary and table ware have surprisingly similar recipes. But their plasticities vary widely.
Glossary Stoneware
To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
Glossary Majolica
Majolica is white opaque glazed red earthenware clay having colored overglaze decoration. But if you know more about what it is technically you will have more control of your product.
Glossary Terra cotta
The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.

By Tony Hansen


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