A ceramic flux functions by promoting partial or complete liquefaction.
[1][2] The most commonly used fluxing oxides in a ceramic glaze contain lead, sodium, potassium, lithium, calcium, magnesium, barium, zinc, strontium, and manganese.
Lead oxide is the traditional low temperature flux used for crystal glass, but it is now avoided because it is toxic even in small quantities.
[3] In clay bodies a flux creates a limited and controlled amount of glass, which works to cement crystalline phases together.
Fluxes play a key role in the vitrification of clay bodies by lowering the overall melting point.