Natural magnesium carbonate – the mineral Magnesite, MgCO3 – is formed by precipitation from sea water, or by hydrothermal alteration of dolomite. Light magnesium carbonate – Hydromagnesite, 3MgCO3 Mg(OH)2 3H2O – is produced by boiling magnesium sulfate and sodium carbonate. This latter form is the type generally used by potters. (The difference in MgO between the two is about 4% – Magnesite is 47.8% MgO, Hydromagnesite is 44.1% MgO.) Slightly soluble, and tends to flocculate the glaze.
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