Method of making a float glass convertible into a glass ceramic and float glass made thereby

A flat glass that is subsequently convertible into a glass ceramic;

• wherein said flat glass has a thickness that is below an equilibrium thickness of a glass melt on a metal bath, said flat glass has a net width of more than one meter, and said flat glass contains no more than 50 crystals per kilogram with a size of greater than 50 μm within the net width, and wherein said flat glass is made from said glass melt;
• and
• wherein said flat glass has a composition, in percent by weight on an oxide basis, of Li2O, 3 to 5;
• Al2O3, 15 to 25;
• SiO2, 50 to 75;
• TiO2, 1 to 5;
• ZrO2, 1 to 2.5;
• SnO2, 0 to 1;
• MgO, 0.1 to 2.5;
• Na2O, 0 to 1.5;
• K2O, 0 to 1.5;
• CaO, 0 to 2;
• SrO, 0 to 2;
• BaO, 0 to 3;
• wherein ΣTiO2+ZrO2+SnO2 is from 2.5 to 5 percent by weight;
• ΣNa2O+K2O amounts to from 0.2 to 2 percent by weight;
• and including color-imparting compounds containing elements, said elements being selected from the group consisting of V, Cr, Mn, Fe, Co, Cu, Ni, Se and U, or mixtures thereof;
• and wherein said flat glass is made by a method comprising the steps of:
• a) feeding melted crystallizable glass of a predetermined composition onto a metal bath;
• b) forming a continuous glass sheet of a predetermined thickness on the metal bath from the melted crystallizable glass of the predetermined composition on the metal bath by drawing the glass sheet, so that said predetermined thickness is less than an equilibrium thickness of the melted crystallizable glass on the metal bath;
• c) conducting the glass sheet through different zones, said different zones having different temperatures;
• d) cooling the glass sheet during passage through said zones from an upper temperature to a lower devitrification point (UEG) of the crystallizable glass with a first cooling rate according to formula (I):
• KR
• UEG
• KG
• ⁢
• ⁢
• max
• ≥
• Δ
• ⁢
• ⁢
• T
• UEG
• KGmax
• 100
• ·
• KG
• ⁢
• ⁢
• max
• ,
• (
• I
• )
• wherein said upper temperature is in a temperature range from 15° C. above a temperature (TKGmax) at which a crystal growth rate in the glass sheet is maximum to 15° C. below said temperature (TKGmax) and wherein KRUEGKGmax denotes the first cooling rate in ° C. min−1 for said temperature range between said upper temperature and said lower devitrification point, ΔTUEGKGmax denotes a temperature difference between said upper temperature and said lower devitrification point, and KGmax denotes a maximum value of said crystal growth rate in μm min−1 at said temperature (TKGmax) at which said crystal growth rate is maximum;
• e) during the forming of the glass sheet by the drawing, cooling the melted crystallizable glass on the metal bath at temperatures above said upper temperature with a second cooling rate that is smaller than said first cooling rate;
• and
• f) subsequently continuously drawing the glass sheet off the metal bath,
• whereby said flat glass is formed.