Patent ID: 7490486

Claim:
A process of producing a refined borosilicate glass having a hydrolytic class 1, said process comprising the steps of: a) preparing a glass batch with a composition, in wt. % on the basis of oxide content, consisting of: SiO 2 65-82 Al 2 O 3 2-8 B 2 O 3 5-13 MgO + CaO + SrO + BaO + ZnO 0-7 ZrO 2 0-2 Li 2 O + Na 2 O + K 2 O 3-10 b) adding a refining agent to said glass batch, said refining agent consisting of from 0.01 wt. % to 0.8 wt. % of at least one sulfate, expressed as an equivalent amount of SO 3 , and from 0.005 wt. % to 1.0 wt. % of fluoride; c) melting said glass batch including the refining agent added in step b) to form melted glass; and then d) hot-shaping said glass.