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 method for manufacturing a magnetic disk substrate includes a step of polishing a glass-ceramic having a crystal phase consisting of crystal grains having an average diameter of less than 3 .mu.m with a polishing material having a grain diameter smaller than the diameter of the crystal grain. In one aspect of the invention, the glass-ceramic includes lithium disilicate (Li.sub.2 O--2SiO.sub.2) and alpha-quartz (alpha-SiO.sub.2) as predominant crystal phases and grown crystal grains of the alpha-quartz each have a globular grain structure consisting of aggregated particles and have a diameter within a range of 0.3 .mu.m-3.0 .mu.m.

Abstract: A magnetic disk substrate is made of a glass-ceramic having crystal phases of lithium disilicate (Li.sub.2 O.2SiO.sub.2) and alpha-quartz (SiO.sub.2). The crystal phase of alpha-quartz is formed of grown crystal grains each having a globular grain structure made of aggregated particles. Each globular grain has a size within a range from 0.3 .mu.m to 3 .mu.m. Surface roughness (Ra) of a polished surface of the magnetic disk substrate is within a range from 15 .ANG. to 50 .ANG. which is an optimum range of surface roughness for the magnetic disk.

Abstract: An optical glass having optical constants of a refractive index (Nd) from 1.50 to 1.60 and an Abbe number (.nu.d) ranging from 62 to 73, a transformation temperature (Tg) of 550.degree. C. or below, and barely cracks or sticks to metal molds when the glass is subjected to mold pressing, maintains excellent chemical durability, and having softening capability at a low temperature which can be obtained by restricting the composition of the glass in weight percents within a specific range of a SiO.sub.2 --Al.sub.2 O.sub.3 --P.sub.2 O.sub.5 --R.sub.2 O system.

Abstract: A low expansion glass-ceramic which has a coefficient of thermal expansion (.alpha..times.10.sup.-7 /.degree.C.) within the range of from -5 to +5 within a temperature range of from -60.degree. C. to +160.degree. C., has a remarkably reduced variation of 10 ppm or below in relative length of .DELTA.1/1, has an excellent optical homogeneity owing to reduced melting temperature in the base glass, and has an improved transparency is obtained by restricting the ratio in weight of P.sub.2 O.sub.5 to SiO.sub.2 within the range of from 0.08 to 0.20, and adding Li.sub.2 O, MgO, ZnO, CaO and BaO of specific content ranges as essential ingredients in a base glass of a SiO.sub.2 -P.sub.2 O.sub.5 -Al.sub.2 O.sub.3 -Li.sub.2 O system containing TiO.sub.2 and ZrO.sub.2 as nucleating agents, and As.sub.2 O.sub.3 as an optional ingredient.

Abstract: An apparatus for continuously forming a glass-ceramic which melts raw materials and forms melted glass to a predetermined shape without cutting the melted glass and thereafter continuously crystallizes the formed glass to the glass-ceramic is provided.

Abstract: A method for detecting a defect in a glass-ceramic for magnetic disks includes steps of forming a colored glass-ceramic having Munsell value within the range of 0 to 7 in the CIE color system by subjecting to heat treatment a base glass comprising in weight percent:______________________________________ SiO.sub.2 65-83% Li.sub.2 O 8-13% K.sub.2 O 0-7% MgO + ZnO + PbO 0.5-5.5% in which MgO 0.5-5.5% ZnO 0-5% PbO 0-5% P.sub.2 O.sub.5 1-4% Al.sub.2 O.sub.3 0-7% As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0-2% a coloring agent of at least one ingredient 0.5-5% selected from the group consisting of metal oxides of V, Cu, Mn, Cr, Co, Mo, Ni, Fe, Te, Ce, Pr, Nd and Er ______________________________________and checking the glass-ceramic visually or by means of an automatic checking device for detecting a defect in the glass-ceramic.

Abstract: A magnetic disk substrate is made of a glass-ceramic having crystal phases of lithium disilicate (Li.sub.2 O.2SiO.sub.2) and alpha-quartz (SiO.sub.2). The crystal phase of alpha-quartz is formed of grown crystal grains each having a globular grain structure made of aggregated particles. Each globular grain has a size within a range from 0.3 .mu.m to 3 .mu.m. Surface roughness (Ra) of a polished surface of the magnetic disk substrate is within a range from 15 .ANG. to 50 .ANG. which is an optimum range of surface roughness for the magnetic disk.

Abstract: Glass-ceramic for information recording disks have a crystalline structure suitably used for substrate materials and the like for magnetic disks and magneto-optic disks as well as have improved surface characteristics obtained by polishing the glass, and besides the base glass thereof is excellent in melting properties and moldability thereof. The base glass composition consists essentially of ingredients of SiO.sub.2 -Al.sub.2 O.sub.3 -ZnO-MgO-CaO-SrO-BaO-TiO.sub.2 system. The main crystalline phase of the glass-ceramic produced by heat-treating the base glass having a viscosity of log .eta.=3.0 poise or less at 1250.degree. C. is gahnite, a size of the crystal grain is within a range of from 50 to 1000 Angstrom, and a degree of surface roughness (Ra) of the polished glass-ceramic is within a range of from 0.5 to 9.0 Angstrom.

Abstract: A cordierite glass-ceramic having a cordierite crystal phase as a predominant crystal phase is obtained by melting a base glass consisting in weight percent of:______________________________________ SiO.sub.2 38-50% Al.sub.2 O.sub.3 18-30% MgO 10-20% ______________________________________the ratio in weight of Al.sub.2 O.sub.3 to MgO being 1.2 to 2.3 ______________________________________ B.sub.2 O.sub.3 0-5% CaO 0-5% BaO 0-5% SrO 0-5% ZnO 0.5-7.5% TiO.sub.2 4-15% ZrO.sub.2 0-5% As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0-2% ______________________________________and forming the glass and subjecting the glass to heat treatment. This glass-ceramic is suitable for use as a high strength material reqiring high resistivity to heat and impact and as materials requiring a fine processing or a precision processing such as electronic parts, magnetic disk substrates and machine parts.

Abstract: A glass-ceramic for a magnetic disk substrate in which the grain size can be controlled and the surface roughness after polishing is within the range from 15 .ANG. to 50 .ANG. is provided by heat treating a base glass including in weight percent:______________________________________ SiO.sub.2 65-83% Li.sub.2 O 8-13% K.sub.2 O 0-7% Mgo + ZnO + PbO 0.5-5.5% in which MgO 0.5-5.5% ZnO 0-5% PbO 0-5% P.sub.2 O.sub.5 1-4% Al.sub.2 O.sub.3 0-7% As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0-2% ______________________________________The glass-ceramic contains lithium disilicate (Li.sub.2 O. 2SiO.sub.2) and alpha-quartz (SiO.sub.2) as the main crystal phases.