A pressure containment element comprising mullite

A containment element 10, 18 for a pressure containment assembly is disclosed which comprises mullite or a polymorph of mullite. It may also contain kaolinite, kyanite, talc, silica and a binder material. The element may be suitable for use as a pressure vessel in an ultra-high pressure, high temperature furnace such as used for the formation of superhard materials. The method of forming the element may comprise heat treating a compacted granular green body comprising mullite and talc such that the talc undergoes a phase change.