Method of making hardened ophthalmic lenses by ion exchange

Claim 1

An ion replacement method for strengthening an ophthalmic crown type of glass wherein the oxide weight percent content of said glass consists essentially of oxides falling within the limits defined by the following table: SiO 2 : 60-75 Na 2 O: 5-10 K 2 o: 5-10 caO: 7-15 LiO: 0-5 MgO: 0-2 ZnO: 2-8 Al 2 O 3 : 0-7 ZrO 2 : 0-2 TiO 2 : 0-2 Sb 2 O 3 : 0-2 CeO 2 : 0-4.5 As 2 O 3 : 0-1.5 to enable said glass when suitably supported for drop-ball test purposes to withstand without fracture the impact produced by a 5/8 steel ball dropped freely from a height of 50 inches onto the outer face of the glass near the center thereof, said method consisting essentially of subjecting said glass to a heat treatment period, said treatment period consisting of an initial preheating to a temperature sufficiently high to prevent damage to said glass when it is subsequently placed in a heated potassium nitrate salt bath, said salt bath being at a temperature above the strain point of said glass, submersing said heated glass in a liquid bath of potassium nitrate having a soaking temperature above the strain point of the glass for a time of from approximately 2 to 63 hours, withdrawing said glass article from said bath and lowering the temperature of the glass article.