The appearance of impurities depends on the content of impurities on the surface of refractory bricks. In refractory bricks, we often see that there are some black spots on the surface. These are the impurities in the raw materials. Theoretically, the fewer impurities, the better, because these impurities are mostly iron oxidation. In the high temperature environment of the kiln, it is simply reduced to molten iron flowing out, resulting in damage to the refractory brick structure, and then affecting the service life of the refractory brick.