Transparent colored glass ceramic with good thermal stability and variable transmission in the ir region

Claim 1

A transparent, colored glass ceramic consisting essentially of high quartz solid solution crystals, especially suitable for the manufacture of heatable plates, wherein: a) the change in the linear thermal expansion coefficient has a positive value of not greater than 1.8×10 -7 /°K. over one to four heat treatment cycles of i) heating from room temperature at 4° C./min to 720° C.; ii) maintaining the temperature at 720° C. for one hour, iii) heating at 2° C./min to 880° C., iv) maintaining the temperature at 880° C. for 90 min, and v) cooling to room temperature; b) the change in the linear thermal expansion coefficient after one heat treatment cycle and tempering at 825° C. for 35 hours has a positive value of not greater than 1.6×10 -7 /°K.; c) the transmission of the glass ceramic in the IR region between 800 nm and 2.6 microns can be variably adjusted; and d) the glass ceramic consists essentially of the following base glass composition (in weight percent): ______________________________________ SiO.sub.2 62-68 Al.sub.2 O.sub.3 19.5-22.5 Li.sub.2 O 3.0-4.0 Na.sub.2 O 0-1.0 K.sub.2 O 0-1.0 Na.sub.2 O + K.sub.2 O 0.5-1.5 BaO 1.5-3.5 CaO 0-1.0 BaO + CaO 1.5-4.0 MgO 0-0.5 ZnO 0.5-2.5 TiO.sub.2 2.0-2.8 ZrO.sub.2 0.0-3.0 TiO.sub.2 + ZrO.sub.2 3.5-5.5 MnO.sub.2 0.0-0.40 Fe.sub.2 O.sub.3 0.0-0.20 CoO 0.0-0.30 NiO 0.0-0.30 V.sub.2 O.sub.5 0.0-0.80 Cr.sub.2 O.sub.3 0.0-0.20 F 0.0-0.20 Sb.sub.2 O.sub.3 0.5-2.0 ______________________________________ and wherein it was transformed from this glass-ceramic base glass by a heat treatment between 680° C. and 920° C.