Magnesium reaches a glaze through two common materials: magnesium carbonate, which is nearly pure magnesia, and magnesium silicate, which is talc and brings silica with it. Both give the smooth, buttery, slightly opaque surfaces that are among the most attractive matts in ceramics.
What magnesium does
At stoneware temperature magnesium is a strong flux that produces a characteristic smooth semi-matt with a soft, fatty quality quite unlike a barium matt's dryness. It also pushes some colourants in unusual directions, most famously turning cobalt towards purple and pink rather than blue, which is the basis of several well-known glaze families.
Carbonate against talc
Magnesium carbonate is the concentrated route and is used in small percentages. Talc, the silicate, brings magnesium and silica together and is used in larger amounts, often as a body ingredient as well as a glaze one. Recipes name which they mean, and substituting one for the other without adjusting the silica in the recipe changes the glaze substantially.
In a clay body
Talc is widely used in low-fire bodies, where it lowers the maturing temperature and, importantly, adjusts thermal expansion so that a glaze fits. Many commercial earthenware bodies rely on it, which is part of why an earthenware glaze fitted to one body may craze on another.
Handling and buying
Both are fine powders with the usual precautions: ventilation, a fitted respirator, wet cleaning. Neither is expensive, and because carbonate is used in small percentages a modest bag lasts a long time. Talc is bought in larger quantities where it is being used in bodies as well as glazes.
Questions people ask about magnesium carbonate
What is magnesium silicate in pottery?
Talc. It brings magnesium and silica together and is used in glazes and, widely, in low-fire clay bodies.
Why did my cobalt glaze go purple?
A high-magnesium base pushes cobalt away from blue towards purple and pink. It is a well-known interaction rather than a fault.
Can I swap talc for magnesium carbonate?
Not directly. Talc carries a lot of silica with it, so the rest of the recipe must be adjusted.