Sintering agent

Boric acid is a typical sinter agent for quartzitic castables which is added to the furnace lining to reduce the sintering point.

In this process, the sintering point of pure quartzite of approx. 1700 °C is reduced to such low temperature ranges that sintering already starts at effective furnace operating temperatures (approximately 1500 to 1550 °C).

Additional references:
Refractory materials, Lining
Fused-silica sand
Quartz sand