... manufactured as per JM standard with lower Fe2O3,excellent thermal shock resistance and conductivity. JM SERIES INSULATING BRICKSJM SERIES INSULATING BRICKS ... ARE MANUFACTURED ACCORDING TO JM INTERNATIONAL STANDARD.THEIR MAIN FEATURES ARE LOWER Fe2O3 CONTENT, EXCELLENT ... 0.360.370.44Thermal shock resistance (times)≥ (1000 air )--203030 JM Insulating brick