Zirconium oxide is extremely refractory, even more so than alumina. Like alumina, it maintains its refractory character even within a mix of other oxides. It is primarily used as an opacifier in glazes, it does not easily go into solution in the glaze and so does not participate in the chemistry. It is normally used in the form of zirconium silicate.

It is beneficial to add zircon to transparent glazes in amounts up to 3% to improve hardness and duravility. This is because small amounts can be taken into solution and will therefore not opacify.

ZrO2 reduces the thermal expansion of a glaze (although not by the same mechanism as other oxides that impose their expansion by going into solution in the glaze melt). Thus its contribution to the calculated thermal expansion of a glaze may differ unexpectedly from what it actually does in the kiln.

ZrO2 reduces glaze melt fluidity, both because of its high melting point and high surface tension.

It is commonly used in stains to stabilize colors and as a mechanism to encapsulate otherwise volatile oxides.