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Earthenware
A clay fired at low temperatures (cone 010-02) where it does not develop maturity (vitrify). Earthenwares are porous and therefore not as strong as stonewares and porcelains (sintering is the primary particle bonding mechanism). Earthenware glazes are usually very bright colored and if the glazes are properly fitted, earthenware can be quite strong and functional, especially if fired higher than cone 04. Terra Cotta is a special type of earthenware where red burning clay is used. Majolica is done using terra cotta clays.
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Terra cotta
'Terra Cotta' (Italian for 'cooked earth') is red ... - (Glossary)
Majolica, tin glaze
earthenware
Low fired pottery employing a red-burning clay cov... - (Glossary)
Stoneware
A high fired (about 1150C+) ceramic clay that is s... - (Glossary)
Porcelain
A comparatively white burning clay body (unless st...
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Efflorescence, Soluble Salt Migration
A term describing the whitish scum left on the surface of a fired clay body (most often red earthenware). Many clays contain soluble sulphates that are left on the surface after they have been brought there by water and it has subsequently evaporated. This can happen during initial drying after forming (during manufacture) or it can happen later after the clay is fired and subjected to repeated wet-dry cycles in use (e.g. brick). This problem can be dealt with used additions of barium carbonate.
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Pictures
Various different cone fired 10R clays with soluble salts on the surface
DFAC dried disk showing soluble salts and drying performance crack typical of a plastic pottery clay body.
Soluble salts on a range of different cone 6 fired clay bodies
Soluble salts on low fire terra cotta and brown burning bodies. The salt-free specimen has barium carbonate.
Encapsulated Stains
These stains can strongly pigment a glaze in amounts as low as 10% (if everything is right, see below). Many companies that hesitated to use these stains in the past now use them in their biggest selling products.
Encapsulated stains are not, as the name suggests and some misunderstand, a zircon capsule around an otherwise unstable compound. Rather, they are manufactured by sintering to form a crystalline matrix (in a process called encapsulation). After sintering they are ground, filter pressed and dried. In cadmium encapsulated stains, for example, the matrix between the zircon and cadmium is stable to 1220C and the selenide/selenium is released during combination. The stain is further rendered safer-to-use by washing with water or weak acid to remove any soluble uncombined compounds (e.g. cadmium or soluble impurities). This washing process does produce toxic byproducts that can only be tolerated in certain countries (e.g. India, China).
Manufacturers have specific recommendations for each stain that must be followed closely. For example, cadmium stains normally work best when glazes are slow cooled and many must be used in glazes with a qualifying chemistry (e.g. glazes for cadmium stains must have no zinc (affects color), no titanium (crystallizes and makes the color fuzzy, a little TiO2 in the clay is tolerable) and have low alumina. Obviously, these stains must not be ball milled, glazes must be milled prior to adding the stain. Suppliers may hesitate to publish some specifics that might give away trade secrets of their products, even relating to the chemistry requirements in host glazes. Generally detailed testing is needed to establish a firing curve and a compatible host glaze to get the best possible color.
The crystalline compound created must not be fired above its recommended temperature or the normally toxic compound will dissolve into the glaze (normally without volatilization unless the maximum temperature is greatly exceeded) and the color will be lost and the glaze rendered toxic. Cadmium content and cadmium release are separate issues. The manufacturing process of these stains is designed to create a stable coloring zircon-compound from a parent metal that would otherwise be unstable (e.g. leachable toxicity). Metal release tests must be done to monitor metal release.
Other kinds of encapsulated stains besides cadmium are: Zircon Pr Yellow, Zircon Vanadium turquoise, Zircon Iron Coral, Zircon Vanadium Orange, Zircon Pr/Vanadium Green. The color depends on the sintering temperature.
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Engobe
A white or colored slip applied to clay as a coating or by slip trailing. In traditional ceramics application is usually done at the leather hard stage, whereas in tile it is normally applied after dust pressing or as a powder layer during pressing. In traditional ceramics, engobes contrast with slips, they are usually formulated with less clay and more flux. A slip shrinks with the ware as it dries whereas an engobe is applied to an already completely dry (or partially dry) clay. The higher percentage of flux in an engobe bonds it to the surface during firing whereas a slip bonds to the surface during drying by the interlocking of clay particles at the interface. The tile industry uses the largest volumes of engobes by far, these are used as opaque barriers and gas bubble filters between less-than-white bodies and the glaze.
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Eutectic
The lowest temperature at which a mix of oxides will melt and form a eutectic reaction (simultaneous crystallization). Only mixtures of some oxides will do this. The temperature is often an anomaly, that is, it is much lower than the melting temperatures of only slightly different mixtures. Lead-tin solder is an example. Lead melts at 327C, tin at 231C. The lowest melting combination is 67 lead, 33 tin (180C).
Non-eutectic mixtures have a melting or softening range. Such mixtures do not flow well until thoroughly heated past the softening range. This softening phenomenon is what makes glazes hang onto the ware.
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