A method for producing synthetic quartz glass by fusion of SiO.sub.2 granulate involves synthesizing amorphous SiO.sub.2 primary particles, granulating the amorphous SiO.sub.2 primary particles to form an open-pore SiO.sub.2 granulate, sintering the open-pore SiO.sub.2 granulate by heating in a sintering atmosphere at a sintering temperature and for a sintering period to form a densified SiO.sub.2 granulate, and melting the densified SiO.sub.2 granulate at a melting temperature to form the synthetic quartz glass. To provide an inexpensive production of low-bubble transparent components of quartz glass despite the use of still open-pore SiO.sub.2 granulate, the sintering atmosphere, sintering temperature and sintering duration are adjusted such that the densified SiO.sub.2 granulate still comprises open pores but manifests a material-specific infrared transmission T.sub.1700 at a wavelength of 1700 nm. This transmission is in the range of 50-95% of the infrared transmission T.sub.1700 of quartz glass granules of the same material.

The synthetic amorphous silica powder of the present invention is characterized in that it comprises a synthetic amorphous silica powder obtained by applying a spheroidizing treatment to a silica powder, and by subsequently cleaning and drying it so that the synthetic amorphous silica powder has an average particle diameter D.sub.50 of 10 to 2,000 m; wherein the synthetic amorphous silica powder has: a quotient of 1.00 to 1.35 obtained by dividing a BET specific surface area of the powder by a theoretical specific surface area calculated from the average particle diameter D.sub.50; a real density of 2.10 to 2.20 g/cm.sup.3; an intra-particulate porosity of 0 to 0.05; a circularity of 0.75 to 1.00; and an unmolten ratio of 0.00 to 0.25. This synthetic amorphous silica powder is less in amount of gas components adsorbed to surfaces of particles of the powder and in amount of gas components within the particles, so that a synthetic silica glass product manufactured by using the powder is remarkably decreased in amount of generation or degree of expansion of gas bubbles even upon usage of the product in a high temperature and reduced pressure environment.

It has been found that conventional cheap waterglass qualities in a strongly acidic medium react to give high-purity silica grades, the treatment of which with a base leads to products which can be processed further to give glass bodies with low silanol group contents.

The production of a quartz glass grit comprises the granulation of pyrogenetically produced silicic acid, and the formation of a SiO.sub.2 granulate and the vitrification of the SiO.sub.2 granulate using a treatment gas, which contains at least 30% by volume of helium and/or hydrogen. Said process is time consuming and cost intensive. In order to provide a method which makes it possible, starting from a porous SiO.sub.2 granulate, to manufacture, in a cost effective manner, a dense, synthetic quartz glass grit, which is suitable for melting bubble-free components made of quartz glass, according to the invention the vitrification of the SiO.sub.2 granulate occurs in a rotary kiln having a mullite-containing ceramic rotary kiln, for the manufacture of which a starting powder, which contains a molar proportion of at least 45% SiO.sub.2 and Al.sub.2O.sub.3 is applied by means of a thermal powder spraying method, forming a mullite-containing layer on a mold core, and the mold core is subsequently removed, and wherein the ceramic rotary kiln is flooded with a treatment gas or rinsed with a treatment gas, and wherein the ceramic rotary kiln is flooded with a treatment gas or rinsed with a treatment gas, which contains at least 30% by volume of helium and/or hydrogen.

A low-density insulating material for use in a vacuum insulated structure for an appliance includes a plurality of microspheres that includes a plurality of leached microspheres. Each leached microsphere has an outer wall and an interior volume. The outer wall has a hole that extends through the outer wall and to the interior volume. A binder engages outer surfaces of the plurality of leached microspheres, wherein the binder cooperates with the plurality of leached microspheres to form at least one microsphere aggregate. The interior volume of each leached microsphere defines an insulating space that includes an insulating gas. The insulating space of each leached microsphere is at least partially defined by the binder.

A low-density insulating material for use in a vacuum insulated structure for an appliance includes a plurality of microspheres that includes a plurality of leached microspheres. Each leached microsphere has an outer wall and an interior volume. The outer wall has a hole that extends through the outer wall and to the interior volume. A binder engages outer surfaces of the plurality of leached microspheres, wherein the binder cooperates with the plurality of leached microspheres to form at least one microsphere aggregate. The interior volume of each leached microsphere defines an insulating space that includes an insulating gas. The insulating space of each leached microsphere is at least partially defined by the binder.

Process for the preparation of hollow spherical glass particles comprising at least SiO.sub.2, Al.sub.2O.sub.3, and an alkali metal oxide, wherein the process comprises the preparation of precursor particles comprising at least SiO.sub.2, Al.sub.2O.sub.3, and an alkali metal oxide by mixing the starting materials, slurrying the starting materials with water followed by spry-drying and heat-treating the obtained precursor-particles at a temperature from 1000 C. to 1800 C., preferably from 1300 C. to 1600 C. by contacting the precursor particles with at least one naked flame.

A method of forming a glass bead includes a step of hot working a first layer of molten glass on a mandrel. The method includes another step of fusing a first molten glass structure to the first layer of molten glass to form a fused workable piece. The first molten glass structure includes a first color. The method includes yet another step of forming the fused workable piece into the glass bead including the first color dispersed through the glass bead.