The mullite content of an alumina-silica refractory material has an important influence on volume stability, load bearing properties, and its satisfactory use in refractory applications. This classification is considered useful for purchase specifications and quality control.

1.1 This classification covers refractory products consisting predominantly of mullite (3 Al₂O₃·2 SiO₂) crystals that are formed by either converting any of the sillimanite group of minerals, or synthesizing from appropriate materials in a melt or sinter process.

1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.