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Magnesium Carbonate - 1 lb.
MgCO3. (47.8% MgO) Also known as Magnasite. Magnesium Carbonate is used in pottery bodies, glazes, and glass. Magnesium Carbonate is used in low fire glazes to produce opacity and matteness. In low temperature glazes magnesium carbonate in amounts to 15% acts as a refractory, remaining in suspension in the glaze melt to produce a white opaque matte glaze. Magnesium carbonate is commonly added to glazes, especially at low fire, to make them crawl (it shrinks and cracks the glaze layer at the low end of firing and then its high melt viscosity pulls the glaze melt into islands). This often produces dramatic visual effects, especially if the crawling glaze color contrasts with the underlying body or slip. Additions vary from 10-30% depending on the host glaze. Dolomite and talc are more practical sources of MgO for most higher temperature applications. In glaze melts it is an active flux at higher temperatures (2150C+), and it produces elasticity, a lower expansion coefficient and smooth buttery surfaces. At lower temperatures it is a refractory and will matte glazes and make them crawl due to its high shrinkage and contributions to the surface tension of the melt. Source: Digital Fire.
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