Subsurface-fortified glass laminates

In a subsurface-fortified glass laminate having a thickness in the range of about 0.100-0.150 inches comprising a tensilely-stressed central core portion, a pair of compressively-stressed subsurface fortifying layers fused to and substantially enveloping said central core portion, a pair of tensilely-stressed outer core layers fused to and substantially enveloping said subsurface fortifying layers and central core portion, and a pair of compressively-stressed surface layers fused to and substantially enveloping said outer core layers, subsurface fortifying layers, and central core portion, wherein

• a. the tensilely-stressed central core portion and outer core layers are composed of a glass having a composition consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 50-75% SiO.sub.2, 3-20% Al.sub.2 O.sub.3, 3-8% F, and 3-20% total of alkali metal oxides selected from the group consisting of Na.sub.2 O, K.sub.2 O and Li.sub.2 O, said Na.sub.2 O comprising at least about one third of said total of alkali metal oxides and said K.sub.2 O not exceeding about 8% of said composition;
• b. the compressively-stressed subsurface fortifying layers and surface layers are composed of a glass having a coefficient of thermal expansion at least about 15 .times. 10.sup.-.sup.7 /.degree.C. less than that of the selected core glass and a composition consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 50-65% SiO.sub.2, 10-20% Al.sub.2 O.sub.3, 5-25% CaO, 0-12% MgO, 0-10% B.sub.2 O.sub.3, and 0-12% total of oxides selected from the group consisting of Li.sub.2 O, Na.sub.2 O, K.sub.2 O, TiO.sub.2 and ZrO.sub.2 ;
• and wherein
• c. the tensilely-stressed central core portion and outer core layers have a sodium oxide content equal to or exceeding on a weight percent basis the sodium oxide content of the subsurface fortifying layers;
• the improvements which comprise:
• i. maintaining the compressively-stressed subsurface fortifying layers at a thickness in the range of about 0.0015-0.0025 inches;
• and
• ii. spacing the compressively-stressed subsurface fortifying layers a distance of at least about 0.017 inches from the outer surfaces of the glass laminate.