Minerals containing Alumina represent some 15% of the earth's crust. It is therefore an abundant material and virtually inexhaustable, unlike raw materials for many alloys developed for special applications. The combination of high thermal conductivity, low thermal expansion and high compressive strength leads to good thermal shock resistance, so Alumina is suited to furnace use as crucibles, tubes and thermocouple sheaths. High purity Alumina can be used up to 1700C and is gas tight up to 1300C. Few chemicals attack Alumina. Alumina also shows good electrical insulation at high temperatures, good wear resistance and high hardness, making it suitable for components such as ball valves, piston pumps and deep drawing tools. Diamond tools are needed to machine or grind Alumina. Remarkably, continuous filament yarns have been made from alumina with reasonable but not complete success and two, similar, versions are available from Goodfellow. They are both much less flexible than normal continuous filament yarns and are rather "hairy" i.e. have broken filaments - especially the FP version.