What's in a Clay? Finding the right minerals for Salt Glaze Pottery
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Clay minerals found naturally in soils around the U.S. and world form the three main types of clay used in pottery: porcelain, earthenware, and stoneware. A unique combination of the minerals kaolin, illite, chlorite, sepiolite, and smectite are collected into each ball of clay to determine the type, glaze, structure and color used in a single piece of pottery.
Potters base their mixture of clay on the plasticity of each material. Plasticity is “the ability to undergo deformation without cracking." The easiest way to determine a clay’s plasticity is using the “coil” test where a sample of clay is taken, rolled into a coil, and tested for its deformability. The chemical make-up in each clay mineral determines the particle size, permeability, and firing temperature for a piece of pottery. Between the five main minerals found in clay, kaolinite is the most common.
Kaolinite holds the chemical composition Al2Si2O5(OH)4 and is an aluminum silicate material with a low “shrink-swell” capacity. It is a soft, white mineral but is often colored orange or red by iron oxide found in the soil. Kaolinite is found all over the country and is the most common clay in the Mountain and Piedmont regions of North Carolina. It is the most prominent mineral found in kaolin, or porcelain, pottery.
Illite is a white clay mineral that was first discovered in Calhoun County, Illinois. Its composition is (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]. Illite has an absence of swelling due to its tightly interlaced molecular spacing. It is a common mineral found in sediments, soils, and metamorphic rock.
Chlorite, found in large quantities in Wales, is actually a group of silicate minerals with similar properties. Chlorite, typically found as a pale green to grey color, is composed of (Mg,Fe)3(Si,Al)4O10(OH)2.(Mg,Fe)3(OH)6. The combination of minerals allows chlorite to hold a great range in composition, temperature and pressure conditions. It is most commonly found in hydrothermal ore deposits and has a soft, flexible consistency.
Sepiolite is a clay mineral found throughout in high quantities in New England, Arizona, and California. Its chemical makeup Mg4Si6O15(OH)2·6H2O contains a hydrous magnesium silicate, causing a non-swelling, lightweight, porous clay. Sepiolite based clays are stable in high salt environments and are typically found as dull, white colored deposits.
Smectite clay minerals are unique in their ability to interaction with liquids and their finite crystal shape. The composition (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O causes a high plasticity allowing Smecitite clays to absorb and swell easily. These can be found as white, pale green, pink and yellow deposits.
Watch Science on the SPOT: Salt Glaze Pottery to learn how ceramicist Ben Owen III creates unique works of art.