Classification of refractory materials. Typically, refractory materials are based on one or more of six high melting-point, crystalline oxides (Al 2 O 3, CaO, Cr 2 O 3, MgO, SiO 2, ZrO 2), as well as carbon, or a combination of these (Ewais, 2004; Carniglia and Barna, 1992; Routschka and Barthel, 2008).Web