Finally, we tested to see if oxidation activity might be restored by increasing PO2, starting at 20 hours (orange regression lines and symbols -note change in mass scale). Doubling PO2 had essentially no effect on mass loss rate, but when PO2 was doubled again ("1"), the mass began to decrease slowly (EEO2 ≈ 2.1 x 10 -5 Da/O2 collision). Doubling again, and again ("2" and "3") had no effect on mass loss rate, i.e., EEO2 decreased, then at the two highest pressures the mass loss rate increased, such that EEO2 was constant at ~7 x 10 -6 Da/O2 collision. Thus, increasing PO2 by nearly two orders of magnitude did result in a small amount of etching, but with EEO2 ~100 times smaller that the initial EEO2.