Tin was known and used by ancient civilisations. Tin is a silvery white metal which is soft and pliable, and which emits the characteristic sound of "tin cry" when bent. It is a relatively common element, its abundance being 2.2 ppm in the earth's crust. Its principal ore is cassiterite, SnO₂, from which the metal is obtained by reduction. Tin forms a stable oxide coating on its surface which makes it unreactive in water; however, it is soluble in both acids and alkalis, and reacts readily with halogens. As tin has good chemical resistance, it is used as a coating of other metals to prevent corrosion, the coating of steel to produce tin plate being an important example of this application. Tin is widely used in the manufacture of soft solders where it is alloyed with other elements to produce a wide range of alloys with different characteristics. Tin is also a constituent of bronzes, pewter, certain bearing materials and fusible alloys.