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STONEWARE CLAY

Richard Willis

Natural stoneware clay is a finely and uniformly grained fabric (not rarely of the “300 mesh” variety) comprised for the most part of pegmatite. see compact clay and petuntze
As with earthenware and porcelain, stoneware was named for its material aspect rather than for its cultural aspects or location of original production, as many other wares have been named. The material (“fabric”) is principally of a finely grained feldspathic earth of uniformly sized particles that permit a very high degree of compactness, so much so that even when only kneaded and sun-baked it has a sedimentary stone-like hardness, and when fired to maturity the hardness and aspect becomes granite-like.
In practice, whether of natural (see Thiviers, below, for example) or composed (see recipes below) clays, stoneware ceramic is made of feldspathic clays (predominately pegmatic) that have been fired at a sufficiently high temperature (about 1200°C [2200°F] for most clay compositions) to vitrify the clay, that is to fuse its elemental silica to a glasslike stability such that when cooled is impervious to liquid in both plasticity and porosity: when wettened it will neither change its form nor will it seep.
Bisqued stoneware fired to below its maturity (below the point of vitrifying), and though stable (it is no longer plastic), is porous (it will seep water). Matured (vitrified) stoneware is not porous, and thus does not require a glaze; when a glaze is used, it serves a purely decorative function and/or the useful function of making the piece easier to clean.
There are three kinds of glaze traditional to stoneware ceramics: lead glaze, salt glaze, and feldspar glaze (each listed elsewhere, individually, under respective titles). “Alkaline” glaze, though occasionally referred to as though a fourth type, is, technically, integral to and a generalization over the other three; that is, alkalines — sodium, potassium, etc. (see alkali) — are inherent to the common "table" salts (see refs to < B>halite and sylvite) used in salt glazing and to feldspars, and are almost always included in lead glaze compositions. The nomenclature 'alkaline glaze', therefore, is usually intended as indicating that the principal flux(es) is(are) alkali(s) (rather than lead, zinc, boron, etc.).
high-fire base compositions for “composed” stonewares
cone 10: Na2O 0.15, K2O 0.3, CaO 0.7, Al2O3 0.5, SiO2 5.0
cone 11: Na2O 0.05, K2O 0.05, CaO 0.9, Al2O3 0.3, SiO2 5.0
cone 14: Na2O 0.25, K2O 0.25, CaO 0.5, Al2O3 1.05, SiO2 10.0
cone 17: Na2O 0.05, K2O 0.05, CaO 0.9, Al2O3 1.1, SiO2 14.5
clay recipes for white stoneware (1070-1330ºC)
(“fire-clay” = any high-refractory clay rich in calcium and alumina but poor in iron and silica that will fire to a white; “feldspar” = any alkali feldspar of near equal parts of Na and K, such as nepheline or a mix of albite and sanidine; “sand” = finely filtered morphous silica, usually as sand or quartz; and “pegmatite” = any feldspathoid with near equal parts of Na, K and Ca, such as Cornwall- or Carolina-stone)
— fire clay 70, sand 30
— fire clay 70, feldspar 20, sand 10
— fire clay 70, pegmatite 20, sand 10
(handling and maturing temperatures can be regulated by simply adjusting and/or interchanging these four base materials)
recipes for glaze bases to which colorants can be added in proportions of 1-10% (1250-1300ºC)
— pegmatite 85, crete 15
— pegmatite 80, crete 20, tin 4
— pegmatite 80, crete 15, rutile 15
— pegmatite 80, crete 15, silica 5
— pegmatite 70, crete 25, alumina 10
— feldspar 60, kaolin 10, crete 12, silica 10

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By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="STONEWARE CLAY" descrip="" searchkey="" loi="0.00" casnumber="70694-09-6"> </material>


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