Glaze is nothing but a coating of glass, which is fused to the clay surface by fire.
Theoretica Glaze Formula: Silica (Glass) + Flux (Fusion) + A Refractory Material (Toughness)
Silica (Flint) is the essential ingredient of glaze which achieve 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, strength the costing of glaze and improve 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 strength 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.