At the start of the record shown, the NP was re-heated to 1900 K to desorb contaminants that might have adsorbed during Q stepping, then TNP was set to 1800 K, and five etching periods at increasing PN2O ("a" -"e") were used to test the reactivity of the NP and to allow Rbaseline (≈Rsublimation) to be measured. The four initial etching periods were kept short to minimize the etching mass losses and resulting changes to the NP structure, but the final etching period "e" was continued for ~12 hours at constant PN2O (=2.2 mTorr), to allow the evolution of the etching rate to be observed without complications from varying pressure or temperature. Note the 10x change in the horizontal axis scale at the start of the long etching period. To allow comparison of the etching rate and baseline rates during period "e", Rbaseline measured just before the start of period "e" is shown as a sloping dashed line.