Al2O3×5SiO2×7H2O —Montmorillonitic clay formed from decomposition of airborne volcanic ash.  Finest particles of all clay, extremely plastic (3-times as powerful as ball clay), but shrinkage is too high to be used alone, add 2% to 3% to clay to increase plasticity. When added to glaze, will help keep in suspension and improve raw glaze adhesion. Source: Clay: A Studio Handbook

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