Also known as natural stoneware clay, a high feldspar-content clay, owing its compactness to the uniformity of grain/particle size which thereby permit tightly compressed deposits. Compact clays are characterized by their coarse-grained, plastic, and elastic toughness in handling and long high-fire temperature range usually from 900ºC for a medium-low bisqueing to 1400ºC for a high-fire maturing and for their receptiveness to most any glaze type, even, and e
specially, salt-glazes. Compacts are essentially sedimentary and/or metamorphic feldspathic earths; and often as not may have a high sand-content together with abundant organic materials (as found naturally, or when mined without further refining). Compact clays are often referred to as stonewares because when fired to maturity they resemble granite stone, and because their stone-like durability lends well to use as -ware: cooking-ware, table-ware, decorative-ware, etc. Many, if not most, stonew
are clays so labeled are in fact not natural compact clays but rather synthetic blends intended to resemble such in compactness, toughness and plasticity while offering greater varieties in fired colors and finished textures.
Earthenware blends traditional to Europe (before and during the onset of stoneware) were (and still are) normally mixtures of compact clays and red-brick clays, firing to 1100ºC which is, as well, a minimal stoneware temperature. see clay
typical compact clay analysis: Na2O 1.37, K2O 1.64, CaO 0.34, MgO 0.37, Al2O3 25.1, Fe2O3 3.75, SiO2 62.4, TiO2 0.98, MnO, FeO traces
typical stoneware batch for 1300ºC: compact clay 50, quartz 10, cal/ferrous clay 35-45
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