Abstract: Thermally crystallizable glasses and glass-ceramics having a narrow critical range of compositions coming within the SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O and SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O-ZnO systems and glass-ceramic products made therefrom including heat exchangers, such as regenerators and recuperators used with gas turbine engines, thermal reactors and catalytic reactors for the gaseous exhausts of combustion engines, and other products which are dimensionally stable at temperatures of at least 1500.degree.F. and above, have very low coefficients of thermal expansion, and have a modulus of rupture of at least about 10,000 psi which is retained when operating at temperatures of at least 1500.degree.F., over an extended period of time.