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.