TWI780292B

To provide an opaque quartz glass having excellent heat insulating property, mechanical strength and surface smoothness, a silica powder water slurry of concentration of 45 to 75 wt % is subjected to wet pulverization with silicon nitride beads having a mean diameter of 0.1 mm to 3 mm. The silica powder and silicon nitride beads are subjected to abrasion and the silicon nitride powder works as foaming agent and independent spherical bubbles are formed for manufacturing opaque quartz glass which has air cells having a mean diameter of 2 to 30 μm and are independent spherical, having a density of 1.90 to 2.20 g/cm3, a whiteness of 80 or more, a reflectance of 80% or more for light of a wavelength of 0.2 to 3 μm at thickness of 3 mm, a bending strength of 70 MPa or more, a surface roughness Ra of the baked surface of 0.7 μm or less.