Most of these materials (e.g., wood, concrete, hard plastics) are subjected to tensile, bending, hardness, compression, shearing, abrasion, etc., tests, by means of machines and apparatus similar in principle to those used for metal testing (by ball-imprint, impact, etc.). The heading also includes a large number of instruments, usually small in size, designed to determine the tensile strength, resistance to bending, compression, etc., of test-pieces moulded from foundry moulding sand. It also covers instruments designed for measuring the surface hardness of finished foundry moulds or mould cores.