Glass-ceramics

Claim 1

A glass-ceramic obtained by melting and heat treating a glass being substantially free of an alkali ingredient and comprising the following ingredients in weight percent: ______________________________________ SiO.sub.2 30-65%, Al.sub.2 O.sub.3 5-25%, ZnO 8-40%, MgO 2-20%, PbO 0.5-15%, CaO + SrO + BaO 0-15%, TiO.sub.2 2-15%, B.sub.2 O.sub.3 0-6%, La.sub.2 O.sub.3 + Y.sub.2 O.sub.3 + Gd.sub.2 O.sub.3 + Ta.sub.2 O.sub.5 Nb.sub.2 O.sub.5 + WO.sub.3 0-10%, ZrO.sub.2 + P.sub.2 O.sub.5 + SnO.sub.2 0-5%, in which ZrO.sub.2 0-2.5%, P.sub.2 O.sub.5 0-5%, SnO.sub.2 0-2%, As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0-2%, MnO.sub.2 + Ni.sub.2 O.sub.3 + CO.sub.2 O.sub.3 + Fe.sub.2 O.sub.3 + Cu.sub.2 O + GeO.sub.2 + Bi.sub.2 O.sub.3 0-10%, Li.sub.2 O + Na.sub.2 O 0-1%, and a total amount of F 0-5%, contained in a fluoride or fluorides of one or more metal elements in said metal oxide or oxides ______________________________________ to form a glass-ceramic containing gahnite as predominant crystals, said glass-ceramic being substantially free of α-cristobalite and willemite, being free of flexion in its thermal expansion curve, having diameters of crystals grown ranging between 0.02 and 5 μ, having coefficients of thermal expansion in the range of about 41 -200×10 -7 /°C., and exhibiting an opaque appearance.