It is a melter. It is a chemical that helps other chemicals melt better.

Pottery glazes are made of silica, alumina and fluxes. Both silica and alumina have very, very high melting points. By adding a flux, the melting point can be lowered to a more practical range.

The type of flux that you add will depend on how hot you want to fire a glaze. The lowest melting flux is lead, but that is generally not used any more because of the dangers of lead.

Boron is next on the list. If you want to make low temperature glazes for use on raku, for example, then boron is your pal.

At higher temperatures, sodium, potassium, calcium, magnesium and a few others work best. BTW, when I say calcium, I'm really talking about calcium oxide or (more likely) calcium carbonate. Since almost all pottery glaze ingredients are oxides or carbonates, the oxide or carbonate part of the name is usually dropped.

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