Only non-alloyed, very pure materials such as platinum, iridium, gold and rhenium can be used for growing single crystals from oxide melts. The crucible shape is generally cylindrical. The melting temperature, the atmosphere and the constituents of the melt determine the choice of material. For this reason iridium crucibles are used at temperatures up to approx. 2300° C for growing crystals of high melting oxides (e.g. sapphire, spinell) for laser technology and the optical industry. The use of precious metals is limited to the Czochralski and Bridgman-Stockbarger processes and the crystallisation from hydrothermal solutions. While gold and platinum are suitable for oxidising atmospheres, iridium should, if possible, only be used under inert conditions because the iridium oxides evaporate and can lead to considerable loss in weight. Under reducing conditions harmful constituents of the melt can result in damage to the iridium crucible.