Thermal insulation is provided for use in applications requiring continuous resistance to temperatures of 1260° C. without reaction with alumino-silicate firebricks, the insulation comprises fibers having a composition in wt %
- 65%<SiO2<86%
- MgO<10%
- 14%<CaO<28%
- Al2O3<2%
- ZrO2<3%
- B2O3<5%
- P2O5<5%
- 72%<SiO2+ZrO2+B2O3+5*P2O5
- 95%<SiO2+CaO+MgO+Al2O3+ZrO2+B2O3+P2O5
Addition of elements selected from the group Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y or mixtures thereof improves fiber quality and the strength of blankets made from the fibers.