This AW Perkins Castable Refractory Cement – Case of 24 Half-pound Tubs is the ideal sealant for masonry-to-masonry joints under 1/4 in. thick. It is a ready mixed mortar that can be used for setting, coating or resurfacing fire brick, but used extensively as the mortar between fire bricks when building fireplace fireboxes. AW Perkins Castable Refractory Cement – Case of 24 Half-pound Tubs must be heat cured, after which it can withstand 2000 degree F temperatures. It cures to a buff color.