A formation of internally nucleated glass ceramics articles that can be heated in the 1350-1450° C. range for extended periods of time without significant deformation or change in shape is disclosed. The predominant crystal phase of these glass ceramics is celsian (BaAl2Si2O8) or its strontium equivalent (SrAl2Si2O8), or solid solutions or mixtures of these compositions, all belonging to the feldspar mineral group.