In ceramics, epsom salts (magnesium sulfate heptahydrate, MgSO4 7H2O) is used as a flocculant. It thickens glazes and engobes and it increases plasticity in clay bodies.
Epsom salts flocculates ceramic slurries - it thickens glazes and engobes without changing the water content. Glazes low in clay (or even with no clay!) will suspend better and can be applied in a thin even layer. Epsom salts are very powerful and should be added slowly while vigorously mixing. Less than 1% of the slurry should be epsom salts by weight, which means adding grams to liters of glaze. Dissolving epsom salts in water to make a concentrated solution is also an option - the liquid will disperse easier and quicker than crystals.
Due to the small amount used, epsom salts are not a meaningful source of magnesium. For the purpose of glaze chemistry, it can be ignored.
See Slurry Rheology for more detailed information on how to adjust glazes and engobes.
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