Firing plates are made of cordierite or cordierite/mullite, materials of a high thermal shock resistance. Their maximum temperature of use is 1280 °C, and they are recommended for long term use up to 1250 °C. The firing plates must be propped up at 3 or 4 points in the furnace – those stacked on top of each other should be propped up at the same points, to the extent possible. The thickness of the plates should be selected to match the weight of the objects to be supported. The plates should be engobe-coated on one side.