Fibrous form of glass, developed in the 1930s. Liquid glass issues in fine streams through hundreds of fine nozzles, and the solidifying streams are gathered into a single strand and wound onto a spool. Strands can be twisted into yarns, woven into fabrics, or chopped into short pieces and then bonded into mats. Glass filaments and yarns add strength and electrical resistivity to molded plastic products. Glass fabrics are used as electrical insulators and as reinforcing belts in automobile tires. Discontinuous fibres are formed into wools, mats, or boards, commonly used in buildings, appliances, and plumbing.

Glass fibres were little more than a novelty until the 1930s, when their thermal and electrical insulating properties were appreciated and methods for producing continuous glass filaments were developed. Modern manufacture begins with liquid glass obtained directly from a glass-melting furnace or from the remelting of preformed glass marbles. For producing continuous fibre, the liquid is fed into a bushing, a receptacle that is pierced with hundreds of fine nozzles through which the liquid issues in fine streams. The solidifying streams are gathered into a single strand, which is wound onto a spool. Strands can be twisted or plied into yarns, woven into fabrics, or chopped into short pieces and then bonded into mats. Discontinuous fibres are most often made in a rotary process, in which fine streams of glass are flung outward through holes in a spinning dish and are then broken and blown downward by a blast of air or steam. The fibres collect on a moving conveyor and are formed into wools, mats, or boards.

Fibreglass wool, an excellent sound and thermal insulator, is commonly used in buildings, appliances, and plumbing. Glass filaments and yarns add strength and electrical resistivity to molded plastic products, such as pleasure boat hulls, automobile body parts, and housings for a variety of electronic consumer products. Glass fabrics are used as electrical insulators and as reinforcing belts in automobile tires.