Silica is the most important ingredient in ceramics! The principle, and often only glass forming oxide in glaze. Normally comprises more than 60% of most glazes and 70% of clays. Special purpose formulations which lack silica often compromise structural stability and strength.
Silica is adjusted in relation to fluxes to regulate melting temperature and gloss.
Increase it at the expense of B2O3 to make glaze harder, more durable, and brilliant. Boric oxide and silica can be interchanged to glaze melting temperature.
Decreasing silica increases the melt fluidity; increasing it raises the melting temperature, increases acid resistance, lowers expansion, increases hardness and gloss, and increases devitrification.
It is normal to use as much as possible in any glaze to keep expansion low, to prevent crazing, and enhance body/glaze fired strength. Note, however, that in certain boracic and feldspathic compositions it can increase crazing so that other low expansion oxides may be needed to reduce glaze expansion.
With boron and alumina, it has the lowest expansion of all oxides.
Thus the particle size of the parent material is often important in determining whether contributed silica affects the chemistry and/or mineralogy or acts as an aggregate.