Chemically strengthened opal glass

A chemically strengthened, spontaneously opalized glass article exhibiting a surface compressive stress after being subjected to surface abrasion composed of an interior portion which is in tension and surface compressive layer, said interior portion comprised of by weight on the oxide basis:

• 34 to 60 percent SiO.sub.2
• 6 to 23 percent Na.sub.2 O
• 0 to 15 percent K.sub.2 O
• 18 to 26 percent (Na.sub.2 O + K.sub.2 O)
• 0 to 18 percent Al.sub.2 O.sub.3
• 0 to 20 percent ZrO.sub.2
• 4 to 21 percent (Al.sub.2 O.sub.3 + ZrO.sub.3)
• 7 to 20 percent P.sub.2 O.sub.5
• 0 to 9 percent CaO
• 0 to 9 percent SrO
• 2 to 9 percent (CaO + SrO)
• less than 0.1 percent Li.sub.2 O
• said constituents comprising at least 90 percent by weight of said interior portion and proportioned relative to one another so as to provide the glass with a viscosity of 100 poises at a temperature less than 2800.degree. F. (1538.degree. C.), and said surface layer having a higher content of K.sub.2 O and a lower content of Na.sub.2 O than the interior portion.

A method of strengthening an opal glass article comprising the steps of:

• a. contacting a source of potassium ions with the surface of a spontaneously opalized glass article comprising by weight on the oxide basis:
• 34to 60 percent SiO.sub.2
• 6 to 23 percent Na.sub.2 O
• 0 to 15 percent K.sub.2 O
• 18 to 26 percent (Na.sub.2 O + K.sub.2 O)
• 0 to 18 percent Al.sub.2 O.sub.3
• 0 to 20 percent ZrO.sub.2
• 4 to 21 percent (Al.sub.2 O.sub.3 + ZrO.sub.2)
• 7 to 20 percent P.sub.2 O.sub.5
• 0 to 9 percent CaO
• 0 to 9 percent SrO
• 2 to 9 percent (CaO + SrO)
• less than 0.1 percent Li.sub.2 O
• said constituents comprising at least 90 percent by weight of the glass article and proportioned relative to one another so as to provide the glass with a viscosity of 100 poises at a temperature less than 2800.degree. F. (1538.degree. C.);
• b. heating said source of potassium ions while in contact with the glass to a temperature at which potassium ions from said source replace sodium ions in the glass, but essentially below the strain point of the glass, thereby creating a zone of compression at the surface of the glass which extends at least 5 microns into the glass;
• and
• c. removing the glass from contact with the potassium ion source.