Refractory materials, like all solids, expand on heating, the degree of expansion being related to the chemical composition of the refractory. Magnesium oxide has the highest coefficient of expansion of the refractory materials commonly used. Silicon carbide has a comparatively low coefficient of expansion and fused or vitreous silica has the lowest expansion of all the refractory materials commonly used. Materials with high coefficients of expansion typically have poor resistance to thermal shock.