A method is described for the controlled sublimation of tungsten wire at a temperature just below the melting point. The variance in light reflectivity with the thickness of the tungsten film condensed on a flat polished glass surface provides a sensitive indicator for the accurate control and maintenance of the filament temperature during evaporation and thus for the controlled growth rate of the deposit. This latter is tungsten and not a tungsten oxide, as has been verified by X-ray and transmission electron diffraction analyses. As many vacuum evaporators are equipped with an optical film thickness measuring device, it is possible to use them, slightly modified, for the controlled evaporation of tungsten.