GB1026770A - Glass article and method of treatment - Google Patents

Abstract

A glass article is strengthened by thermal tempering followed by chemical tempering. Thermal tempering produces a compressive layer (Fig. 2) extending relatively deeply below the surface, while chemical tempering (Fig. 3) produces a compressive layer which is shallower but has higher modulus of rupture (M.O.R.); the combination of the invention is shown in Fig. 4. Thermal tempering is performed by heating the glass to between the annealing and softening points, and cooling rapidly, by applying a chilling medium until the surface is rigid. <PICT:1026770/C1/1> <PICT:1026770/C1/2> <PICT:1026770/C1/3> Chilling means include liquid sprays, air baths, and immersion in oil or fused salt baths. By the chilling operation, a thermal gradient is established, so that in the interior, but not in the surface, viscous flow and strain release proceeds, with consequent release of interior stress. The glass is then cooled to ambient temperature, thereby eliminating the temperature gradient and developing compressive stresses in the surface layer. The chemical tempering is performed by substitution of a relatively large ion for a smaller ion in the glass at below the glass strain point; e.g. K+ may be substituted for Na+, or Na+ for Li+. In examples two glasses were used having the following compositions in per cent by weight: SiO2, 66.1/64.5; Al2O3, 20.0/20.9; Na2O, 8.7/5.0; Li2O, 3.6/3.9; K2O, 0.2/0; MgO, 0.9/4.0; As2O3 + Sb2O3, 0.5/0.8. Test pieces of one glass were then heated in air to 8000 C.; held 4-5 minutes; quenched in a KNO3 bath at 450 DEG C. for 20 seconds; cooled to room temperature; and transferred to a molten NaNO3 bath at 400 DEG C. for four hours. Test pieces of the other glass were heated to 850 DEG C.; held 2 1/2 minutes; and immersed in a bath of 85% NaNO3, 15% Na2SO4 for 2 hours at 450 DEG C. Other glasses that may be treated are comprised of exchangeable alkali metal oxide, 5-25%; Al2O3 and/or ZrO2, 5-25%; balance, essentially SiO2; other glass-making oxides-optionally, up to 20%. Drinking glasses are exemplified.