It is an excellent product to tuck-point between firebricks and repair broken firebricks. (Maximum application thickness is 1/8") Contains no asbestos. Designed especially to patch fireplace fireboxes, bonding firebrick to firebrick. A silicate product that when fire cured, sets rock hard - as strong as firebrick itself. Withstands 2000*F. Comes in three colors: Gray, Buff, and Black. Not for exterior use.