Although we are not ceramic chemists, we need a good idea of what makes glazes work as they do. Even if we are working with commercial glazes, we still need a base of understanding in order to apply and fire them with consistent and desirable results.
Glazes consist of four key components, each with their own function.
Silica, the Glass Former
Silica is both the base material for glass and for ceramic glazes. Indeed, if you get it hot enough, silica forms glass all by itself. However, silica melts at about 3100⁰ F (1710⁰ C or cone 32), which is much too hot for ceramic kilns. As such, it cannot be used on its own.
Silica is a major component of raw clay, and can be introduced in the glaze that way. It can also be introduced into glazes as silica oxide, flint, and silica sand.
Alumina, the Refractory
Alumina, or aluminum oxide, is used in nearly all glazes as a stiffening agent, allowing glazes to stick to a pot's vertical surface without running off when it has melted. Alumina is often introduced into a glaze through adding clay, which also contributes some silica.
Fluxes lower the melting point of the silica, making it usable to create ceramic glazes. They can be divided into alkaline fluxes, alkaline earths, and miscellaneous fluxes which are often metallic oxides.
Various fluxes work in the glaze in their own peculiar way; they are not interchangeable. Some are very active fluxes, allowing glazes to mature at earthenware temperatures. Others are less active, and are useful in mid-range and high-fire temperature.
It is important to note that many of the metallic oxides are also toxic. Care must be used whenever handling them
Colorants and Glaze Modifiers
Silica, when melted, is transparent. Colorants are added to glazes to produce a wide range of hues. Ceramic colorants must be able to withstand high temperature without burning off; most colorants are metallic oxides, which can also affect melt point. This must be taken into account in developing a glaze formula.
In addition to colorants, glazes can also have other modifiers added. These may modify the glazes opacity, iridescence, or working qualities when the glaze is still raw (unfired).