A refractory vitreous material contains at least 75 per cent of silica, at least 3 per cent of beryllia and ceria and thoria together (of which not all is ceria and not all is thoria) and not more than 7 per cent of other oxides. Alumina if present must be less than 4 per cent. The ceria used is the commercial product containing about 50 per cent CeO2, 30 per cent La2O3, and 20 per cent other oxides. A number of suitable compositions are given in the Specification. The materials may be founded in a crucible of zirconium silicate prepared as described in Specification 492,394, or they may be formed into a coherent rod and then melted as described in Specification 486,444. Specification 488,883 also is referred to.