Fire clay should contain a higher percentage of 'non-detrimental' silica and alumina. They should have a very small quantity of detrimental lime, magnesia, iron oxide, and alkalis.
2) becomes soft at about 2800℉ and finally fuses and becomes a glassy substance at about 3200℉. It melts around 3300℉. This high softening and melting point has established it as the principal material for the production of refractory bricks.
2O3) has even higher softening and fusion temperature than silica. It melts at about 3800℉. It is, therefore, used in combination with silica.
