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 Al₂O₃2 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.