Chemically hardened spectacle crown glass

Claim 1

A chemically hardened optical crown glass having a sharp U-form tension profile, a refractive index n d of 1.5225-1.5235, a thermal expansion coefficient in the temperature range between 20° C. and 300° C. of 90 × 10.sup. -7 to 100 × 10.sup. -7 /° C., a dispersion γd of 54-60, a softening temperature of 690°-750° C., a transformation temperature of 480°-550° C., and a flexural tensile strength of at least 3000 kp/cm 2 produced by melting, shaping and cooling a batch consisting essentially of the following composition in parts by weight based on 100 parts of total composition wherein the sum of the alkali oxides in the composition amounts to 14.5- 18.0% by weight, and wherein the sum of CaO, BaO and ZnO is larger than 6.3 and smaller than 10% by weight: SiO.sub.2 50.5 - 61.0 Al.sub.2 O.sub.3 7.0 - 17.5 B.sub.2 O.sub.3 2.0 - 6.1 Li.sub.2 O 0 - 1.0 Na.sub.2 O 12.0 - 15.0 K.sub.2 O 2.5 - 3.5 CaO 0 - 4.1 BaO 0 - 0.3 ZnO 5.0 - 10.0 PbO 0 - 0.5 TiO.sub.2 0.1 - 0.9 As.sub.2 O.sub.3 0 - 0.5 Sb.sub.2 O.sub.3 0 - 1.0 and thereafter subjecting it to an ion exchange in which potassium, rubidium or cesium ions are diffused into the glass in exchange for smaller alkali metal ions at a temperature below the upper cooling temperature which corresponds to a viscosity of 10 13 poises.