Patent ID: 8746010

Claim:
A method for producing glass sheets from molten glass using a forming structure that has an outer surface at least a part of which is composed of zircon and comes into contact with the molten glass during use of the forming structure, said method comprising in order: (A) melting batch materials to produce molten glass having a treatment composition, said batch materials comprising silica; (B) delivering the molten glass having the treatment composition to the forming structure at a delivery temperature T delivery/treatment ; (C) melting batch materials to produce molten glass having a production composition; (D) delivering the molten glass having the production composition to the forming structure; (E) using the forming structure to produce a glass ribbon; and (F) separating glass sheets from the ribbon produced in step (E), said glass sheets having the production composition; wherein: (i) at T delivery/treatment , the treatment composition promotes the reaction: ZrO 2 +SiO 2 →ZrSiO 4 ; (ii) the treatment composition has a zircon breakdown temperature T brkdwn/treatment ; (iii) the treatment composition has a ΔT treatment =T brkdwn/treatment −T delivery/treatment ; (iv) the production composition has a delivery temperature T delivery/production ; (v) the production composition has a zircon breakdown temperature T brkdwn/production ; (vi) the production composition has a ΔT production =T brkdwn/production −T delivery/production ; (vii) ΔT treatment and ΔT production satisfy the relationship: Δ T treatment >ΔT production ; (viii) the performance of steps (A) and (B) reduces the level of zirconia defects in glass sheets produced by steps (C) to (F); and (ix) step (B) is no longer being performed when step (D) is performed.