The glaze is nothing but a coating of glass, which is fused to the clay surface by fire.
Theoretical Glaze Formula: Silica (Glass) + Flux (Fusion) + A Refractory Material (Toughness)
Silica (Flint) is the essential ingredient of glaze which achieves the glassy crystal surface. Most clay contains silica so that most glazes contain at least 50% of clay. Silica has a high mature temperature, which is commonly used to increase the melting point, strengthen the costing of glaze, and improves the expansion of glaze.
Flux is used to lower the melting point of a glaze. Lead oxides and alkaline (borax, soda ash, and boric acid) are common fluxes in low-fire glazes.
A refractory material is to strengthen the hardness and toughness of a glaze. Such as alumina will increase the bonding of a glaze on the clay surface, which have more resistant to abrasion.