Cenosphere

A cenosphere or kenosphere is a lightweight, inert, hollow sphere made largely of silica and alumina[1] and filled with air or inert gas, typically produced as a coal combustion byproduct at thermal power plants.

The color of cenospheres varies from gray to almost white and their density is about 0.4–0.8 g/cm3 (0.014–0.029 lb/cu in), which gives them a great buoyancy.

"[2] The process of burning coal in thermal power plants produces fly ash containing ceramic particles made largely of alumina and silica.

Up until the 1990s it was limited to a largely carbonaceous sphere caused by the oxygen-deficient combustion of a liquid fuel droplet that was cooled below 200 °C (392 °F) before it was consumed.

[citation needed] Silver-coated cenospheres are used in conductive coatings, tiles and fabrics.