May 27, 2006 The SPEX 8000 Mixer Mill was tested with both flat-ended and round-ended vials, both made of hardened tool steel. Steel balls, 12.7, 9.5, and 6.4 mm in diameter were used. This mill was cooled only by natural air convection. The temperature of the milling vial was measured by a K-type thermocouple attached to the surface of the vial