1.1 This specification covers brick intended for use as the lining in the fireboxes of residential fireplaces.1.2 Firebox brick are manufactured from clay, fire clay, shale, or similar naturally occurring earthy substances and subjected to a hea This specification covers brick intended for use as the lining in the fireboxes of residential fireplaces. Firebox brick are manufactured from clay, fire clay, shale, or similar naturally occurring earthy substances and subjected to a heat treatment at