Common nomenclatures are adobe brick clay, red brick clay, white brick clay, fire (or, refractory) brick clay, insulating brick clay, earthenware brick clay, and so forth: names corresponding, for the most part to virtually all the natural and man-made clays. Typically, brick clays are the least pure, least plastic, and most refractory hydrous alumina-silicates among the varieties of sedimentaries, glacials, compacts, etc., within any g
iven temperature range: adobe in (sun-baking temperatures of) 30-50ºC; red in 500-900ºC; fire in 1000-1500ºC; and so on.
To qualify clays as brick clays is normally to indicate those clays which, owing to their impurities, short elasticity, and high resistance to melting (relative to similar clays at given temperatures), are deemed better suited as materials for constructional rather than artistic products which is to say, better for bricks than for pots. In general, the bricks and other constructional forms made from brick clays are classified by hardness (adhesion/cohesion, weight compression resistance, Mohs Scale, etc.) and refractoriness (fusibility, melting point, thermal shock resistance, etc.). For practical purposes, hardness is rated from soft to hard as measured by the relative ease of imprinting with ones thumbnail, and refractoriness is rated from low-fire to high-fire as measured by the relative resistance to fusing; and, for the most part, the two classifications are in direct proportion: the softer the brick, the lower is its fusion point.
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