Abstract: A glass composition capable of being chemically strengthened by ion-exchange within 100 hours to provide a glass with a surface compressive stress of greater then 400 MPa and an ion-exchange depth greater then 200 microns comprising: SiO2 58% to 70% (by weight), Al2O3 5% to 15%, Na2O 12% to 18%, K2O 0.1% to 5%, MgO 4% to 10%, CaO 0% to 1% with the provisos that the total of the Al2O3 and MgO is in excess of 13%, the total of the amounts of Al2O3 plus MgO divided by the amount of K2O is at least 3 and that the sum of the amounts of Na2O, K2O and MgO is at least 22%.

Abstract: A glass melting tank provides heat in a melting chamber 11 from pairs of electrodes 26, 27 and 28. A three-phase electrical power supply 34 provides power to the electrode pairs and the voltage supplied for the different phase is independently controlled by a power control 33.

Abstract: The glass melting tank for supplying a flat glass forming process has a melting zone for melting solid batch material and a refining zone with reduced return flow of glass. The base of the refining zone comprises a fusion cast refractory material comprising alumina, silica and between 31% and 43% by weight of zirconia. The refractory is conditioned by oxidation at at least 1450.degree. C. for 24 hours.

Abstract: In the refining zone of a glass melting tank the molten glass is supported on a heat conducting base, such as molten tin, heat is removed from the glass through the base and the depth of glass, length of refining zone and temperature difference between the inlet and outlet of the zone are controlled so that the ratio of return flow to forward flow through the zone is between 1/6th and 1/2.