I know that when we think about bricks in a forge we think about fire bricks.I had a small engine shop that I heated with wood for several years that I heated with a double barrel stove.I didn't have any fire bricks , so I lined the fire box with regular glazed building bricks.I burned several cords of wood a winter, and after approximately three years I tore the stove apart.The bricks were almost perfect.I had one or two cracked bricks, and the others displayed no signs of overheating.I am of the opinion that you don't need fire bricks.My Grandfather was a hundred years old at his death, and he always said that the brick was the best building material that man ever created.He often stated that bricks made that when he was a boy, were just as good as the day that they were made, only the mortor wore out.