Abstract: A low alpha-ray level glass which emits only an extremely small amount of alpha-ray while maintaining excellent chemical durability, coefficient of thermal expansion and hardness is obtained by introducing fluorine into a SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --R.sub.2 O (R being an alkali metal) system glass of a specific content range. The amount of alpha-ray emitted from this glass is below 0.02 count/cm.sup.2.hr.

Abstract: A low expansion crystallized glass-ceramic which has a coefficient of thermal expansion (.alpha..times.10.sup.-7 /.degree. C.) within the range of from -10 to +10, has a remarkably reduced variation in the coefficient of thermal expansion accompanied by variation in heat treatment conditions, has an excellent optical homogeniety owing to reduced melting temperature in the base glass and has an improved transparency is obtained by restricting the ratio in weight of Al.sub.2 O.sub.3 to (SiO.sub.2 +P.sub.2 O.sub.5) within the range of 0.38 and 0.50 and adding MgO, BaO and ZnO of specific content ranges as essential ingredients in a base glass of a SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O system containing P.sub.2 O.sub.5 as an optional ingredient and TiO.sub.2 and ZrO.sub.2 as nucleating agents.

Abstract: An optical glass which is a SiO.sub.2 -Nb.sub.2 O.sub.5 -R'.sub.2 O (where R' is an alkali metal element)-F system of a specific composition range exhibits optical constants of a refractive index (Nd) of about 1.52-1.67 and an Abbe number (.upsilon. d) of about 32-50, is free of PbO and has an improved light transmissivity.

Abstract: According to one aspect of the invention, there is provided glass for a photomask consisting, in weight percent, of 45 to 62% SiO.sub.2, 3 to 15% B.sub.2 O.sub.3, 12 to 25% Al.sub.2 O.sub.3, 5 to 15% MgO, 0 to 5% CaO, 0 to 5% SrO, 0 to 5% BaO, 0 to 5% ZnO, 0.5 to 5% PbO, 0 to 20% La.sub.2 O.sub.3 +Gd.sub.2 O.sub.3 +Y.sub.2 O.sub.3, 0 to 1.5% Na.sub.2 O, 0 to 1.5% K.sub.2 O, 0 to 0.5% As.sub.2 O.sub.3 and 0 to 6% Sb.sub.2 O.sub.3. This glass is easy to melt and homogenize and has good resistivity to phase separation and excellent heat resisting and refractory properties, chemical durability, ultraviolet transmittance and metal vapor deposition property. According to another aspect of the invention, there is provided glass for a photomask consisting, in weight %, 45 to 60% SiO.sub.2, 0.5 to 12% B.sub.2 O.sub.3, 10 to 22% Al.sub.2 O.sub.3, 5 to 17% MgO, 0 to 5% CaO, 0 to 8% SrO, 1.5 to 15% BaO, 1.5 to 17% ZnO, 0.5 to 10% PbO, the total content of ZnO and PbO being at least 6%, 0 to 7% Ta.sub.2 O.sub.5 +Nb.sub.