“…164, 65]), and to keep away any sources of contamination, e.g. by water as a polishing agent for alkali halide crystals [204], or by heavy metals like Cu and Fe which may be transferred from hot container walls to the crystal through the gas phase [66]. All work prior to about 1960 (when zone melting was introduced in this field) is suspect of uncontrolled impurities (mainly Mf; and (OH),).…”