Formed by extruding and attenuating molten glass. Glass fiber is brittle, which limits its use to a small number of markets. The fiber has the characteristics of withstanding relatively high temperatures of 280-300°C as well as poor heat conductivity and therefore major markets are heat insulation and high temperature filtration. Its characteristics of resistance to mildew, moisture and many oxidizing agents, solvents, alkalis and acids heightens its importance in those end-uses. The fiber also has good electrical resistance properties