A glass ceramic comprises (in wt.-% on oxide basis): SiO.sub.2 35 to 60, B.sub.2O.sub.3>4 to 10, P.sub.2O.sub.5 0 to 10, Al.sub.2O.sub.3 16.5 to 40, TiO.sub.2 1 to 10, Ta.sub.2O.sub.5 0 to 8, Y.sub.2O.sub.3 0 to 6, ZrO.sub.2 1 to 10, MgO 6 to 20, CaO 0 to 10, SrO 0 to 4, BaO 0 to 8, ZnO 0 to 4, SnO.sub.2+CeO.sub.2 0 to 4, SO.sub.4.sup.2-+Cl.sup.- 0 to 4, wherein the total content (SnO.sub.2+CeO.sub.2+SO.sub.4.sup.2-+Cl.sup.-) is between 0.01 and 4 wt.-%. The glass ceramic may be processed by the float glass method, may be transparent and is, inter alia, suitable as a substrate for thin film semiconductors, in particular for display applications, solar cells etc.