composition board, wood product produced in the form of a board or sheet, formed of cellulose fibers or particles derived from wood or other sources, and used principally as a building material. The oldest type of composition board is a relatively dense material known as hardboard, discovered accidentally in 1924 by the American scientist William Mason. After obtaining wood fibers by using high-pressure steam, Mason attempted to dry a matlike mass of them in a steam press. Because of a faulty valve, the press remained hot longer than had been planned and thus the first piece of hardboard was formed. In other forms of composition board the fibers are not as closely packed, and the density is correspondingly lower. Some of these boards find application as insulating and soundproofing materials. Other similar types are treated with waterproofing material, e.g., asphalt applied under pressure, and are usable as the sheathing of buildings. Such materials typically have a resistance to shearing forces exceeding that of plywood. Particle board, another form of composition board, is made by binding wood particles ranging in size from flakes to sawdust together with a suitable adhesive, such as a plastic resin, and pressing or extruding them to form sheets. Particle board is used as a cheaper substitute for plywood in some applications; but even though it has a higher density, it is less resistant to puncture and the effects of weather. When properly veneered it is suitable for making furniture. In its raw form it makes an excellent subflooring for dry locations.