Data originally published at 10.1038/159305a0. Tabulated data extracted from Y. S. Touloukian and D. P. DeWitt, *Thermal Radiative Properties: Metallic Elements and Alloys*, Thermophysical Properties of Matter - The TPRC Data Series, Volume 7, IFI/Plenum, New York, 1970; table 280, curve 1. Digital version served at EKHI (https://thermomat.ehu.eus/ekhi).