I am remodeling my home and are planning to install a fireplace. I recently ran across a large supply of insulating bricks which are available at a great price. Although the bricks are in excellent condition, the boxes look fairly old. The bricks are John-Manville SIL-O-CEL* NATURAL INSULATING BRICKS (25 nine inch straight) per box. The guy that's selling them says they will work fine for the purpose. What do you thing ?