In general, a multistep process is used to produce high-temperature hollow glass microspheres. Glass is initially produced at high temperatures from soda-lime-borosilicate, after which it is milled to a fine particle size. Trace amounts of a sulfur-containing compound, such as sodium sulfate, are then mixed with the glass powder. The particles are run through a high-temperature heat transfer process, during which the viscosity of the glass drops and surface tension causes the particles to form perfect spheres. Continued heating activates the blowing agent, which releases minute amounts of sulfur gas that form bubbles within the molten glass droplets. The result is a rigid, hollow sphere manufactured with an eye to increasing crush resistance (that is, the ability to withstand external pressure and avoid fracture of the bubbles) without sacrificing low density.
Hollow glass microspheres also can be produced by processing perlite — common volcanic glass. Typically, the process involves an acid-leaching treatment, using hydrochloric or sulfuric acid at temperatures from 150°C to 200°C (302°F to 392°F), which is followed by a heat treatment process for finishing. Unlike engineered hollow glass microspheres, which consist of a single closed cell, those produced from perlites are multicellular.