The objective of the University of Missouri - Rolla program under the Jet Propulsion Laboratory Low Cost Silicon Solar Array contract is to investigate the interaction of molten silicon with various die and container candidate materials under varying oxygen partial pressures. This has been done by making silicon sessile drop contact angle measurements on the candidate materials to determine the degree to which silicon wets these substances, and subsequently sectioning the post-sessile drop experiment samples and taking photomicrographs of the silicon-substrate interface to observe the degree of surface dissolution and degradation. Several different materials supplied by JPL have been investigated in this manner, i.e., hot pressed silicon nitride (from both Kawecki Berylco, Inc. (KBI) and AVCO) , CNTD silicon nitride coated on hot pressed silicon nitride (Chemetal-Eagle Picher), CVD silicon carbide coated on graphite (Ultracarbon), and ? Sialon (Battelle). Of these materials, silicon did not form a true sessile drop on the Ultracarbon SiC on graphite due to infiltration of the silicon through the SiC coating or on the ? Sialon due to the formation of a more-or-less rigid coating on the liquid silicon. The lowest sessile drop contact angle (i.e., the most wetting) was obtained on the CNTD coated Si3N4 with a value of 42o. The AVCO and CNTD coated materials showed the least amount of interaction with molten silicon. The importance of oxygen partial pressure on the interaction of molten silicon with refractory materials having been established*, the oxygen concentrations in the EFG silicon ribbon furnace at Mobil Tyco Solar Energy Corp., Waltham, Massachusetts, and in the JPL silicon sessile drop furnace at Pasadena, California, were measured using the portable thoria-yttria solid solution electrolyte oxygen sensor constructed at UMR for this purpose. Oxygen partial pressures of 10-7 and 10-8 were obtained for the Mobil Tyco and JPL facilities, respectively. The measurements made at these two facilities are believed to represent non-equilibrium conditions.