Other Clays

An enormous variety of other grades of clay minerals have been used commercially. The names of these clays can be based on the ultimate application (stoneware clay, brick clay) or the fired properties (red firing clay, vitrifying clay) [25]. The composi­tions of these clays are highly variable, but in general they contain high amounts of alkalis and high amounts of Fe2O3, TiO2, and other impurities. A few of these clays (earthenware, stoneware, and brick clays) are mentioned here, but interested readers should consult references [3,6,14,22, 25] for information on other categorizations as well as chemical composition information. Keep in mind that the designations are not based on mineralogy, composition, or any specific property. A particular clay may fall under one or more categories depending on how it is gathered, its beneficiation, or its intended use.

Earthenware refers to products produced from unbeneficiated clays with no other additives. Earthenware clays are formed by incomplete conversion of the parent min­eral formation and they contain substantial amounts of residual feldspar and quartz, giving a composition similar to a triaxial whiteware [3]. Earthenware bodies are typi­cally formed by throwing or modeling [22]. Earthenwares are self-fluxing during firing due to the alkali content. Fired earthenware bodies typically have high absorption (10-15%) and are fired at moderate temperatures (cone 5-6) [22]. Fired earthenware bodies are usually red and find use as decorative objects, as tiles, or as tableware [26].

Stoneware clays can be used without beneficiation or additives to produce ware with low absorption (0-5%) at relatively low temperatures (cone 8-9) [22]. Fired stoneware objects usually have a buff or gray color and are used as electrical insulators, cookware, decorative items, drain pipe, tiles, and tableware [26]. Stoneware can be formed by casting, throwing, or pressing. The major difference between stoneware clays and earthenware clays is Fe2O3 content, with stoneware clays usually having lower Fe2O3 than earthenware.

Brick clays tend to be high in alkalis and iron, but low in alumina [14]. The clays usually have moderate to high plasticity, which facilitates forming [25]. Often, brick clays are actually shales [14]. These clays fire at moderate temperatures (cone 1-5) and the resulting fired bodies are dark red. Clays with similar properties but different colors upon firing can be used to produce other products such as sewer tile and roofing tile [6]. Nearly any red burning clay can be classified as brick clay.