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.

The present disclosure concerns expandable silica particles having a coating comprising talc powder and kaolin powder provided on the outer surface of the expandable silica particle and expandable and expanded silica particles comprising silica fume and/or ultrafine quartz silica sand beneath the surface of the particles. Methods for producing expandable and expanded silica particles are disclosed, including a method using a vibration plate and a furnace having a vibration plate for carrying out that method. The expanded silica particles have high compressive strength, substantially uniform cell size and distribution, low water absorption, and low porosity on the outer surface. They are useful as a filler in matrix materials, like concrete or epoxy, as insulation material with various binder materials, and as water filtration medium.

A plurality of glass bubbles having, in combination, a D50 size of no greater than 13 micrometers, an average true density of no greater than 0.42 grams per cubic centimeter, and a 90% crush strength of at least 55 megapascals are described. Methods of making such glass bubbles and compositions and articles such as sheet molding compounds and dielectric layers prepared from such glass bubbles are also described.

A glass filler powder includes a bubble therein, a volume fraction of the bubble being from 0.2% to 2%.