The house that I purchased recently has an "Englander Fireplace" freestanding wood stove. The firebrick that lines the firebox has cracked and some pieces are missing. I have tried to pull the old bricks out to replace them but they seem to be attached somehow. Do I need to break out a hammer and chisel? Since these are attached, do I need to reattach the new ones with some type of mortar or adhesive?