Refractory materials used to protect the walls of the furnace from reaction with its molten contents (abrasive, melting, or chemical). Three divisions are (1) acid refractories rich in silica (flint, ganister, fireclay), which react with basic oxides; (2) neutral refractories (chromite, graphite), and (3) basic refractories, rich in oxides of calcium and magnesium and low in silica.