Raw Materials › MAGNESIUM CARBONATE –HEAVY Image 1 of 1 MAGNESIUM CARBONATE –HEAVY from $6.60 sold out Magnesium carbonate heavy (magnesite) is a refractory material used in glazes to produce opacity and matte surfaces. Weight: Select Weight 1 kg 5 kg 25 kg Sold Out Added! Magnesium carbonate heavy (magnesite) is a refractory material used in glazes to produce opacity and matte surfaces.