MACAL
Magnesia with very little else along with it · A smooth buttery matte, with pastel colour · In large amounts it crawls, and the cracking starts as the glaze dries · More than half its weight leaves as gas in the firing
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Magnesium carbonate is the way to get magnesia into a glaze without bringing silica or calcium along with it. More than half its weight leaves as gas during the firing.
Magnesium carbonate adds no silica and no calcium of its own, which is what recommends it where a recipe needs magnesia and nothing more. In small amounts it mattes a glaze: magnesia grows fine crystals as the kiln cools, and those give the smooth, buttery surface it is known for, with colour running to pastels and, in a stoneware glaze carrying enough of it, cobalt going lavender rather than blue. In large amounts it crawls, and that starts long before the kiln. The powder is very fine and very bulky, it holds a great deal of water, and the layer shrinks and cracks as it dries, so the crawl is visible in the dried glaze before anything is fired.
