This classification establishes an orderly grouping of insulating firebrick to provide group identifications for use by those producing or purchasing these materials. The group identification number is not meant to specifically designate the maximum service temperature although the number times 100 does approximate the temperature (in degrees Fahrenheit) to be used to check the reheat change and can be used as a guide for relative temperature stability. The bulk density limits the weight per unit volume for any group as sold, but is not meant to be used for detailed engineering calculations.