Earlier work, which reported relationships between compression properties and elevated temperature aging times and weight losses, also pointed out the apparent influence of surface layer formation and growth on the retention of compression properties during extended aging times. Since that time, studies have been directed toward evaluating the growth of the surface layer. This layer was found to change in its composition and features as the aging temperature changed. Microcracks and small voids initiated and advanced inward at all temperatures. Visible oxidation at the surface occurred only at temperatures above 260 deg. C. Relationships between layer thickness and aging time and temperature were evaluated and empirically formulated. Then, the compression properties were graphically related to the surface layer thickness with excellent correlation. The surface layer was observed to influence the compression strength of thin samples only.