Abstract Glasses with the composition 41ZnO·49SiO2·4La2O3·6Al2O3 were crystallized 100 K above glass transition temperature. This resulted in the surface crystallization of an unknown phase chemically close to willemite. After 20 h, a 85 µm thick surface layer was formed which was composed of dendrite-like structures and an interpenetrating glass phase. The bulk glass has a coefficient of thermal expansion much larger than that of the surface layer. This leads to compressive stresses at the surface and a notable increase in the mechanical strength. After crystallization for 20 h at 800 °C, a four point bending strength of 219 MPa was reached while the glass shows a strength of 73 MPa. This may help to improve the mechanical properties of glasses and might also be important for ceramic glazes.