Low energy secondary ion mass spectrometry (LE-SIMS) was used to measure Na and other elemental concentrations as a function of depth in a replica glass aged in humidities of 30 & 39% RH at 25 deg C. The ageing times varied from one up to 45 days. The replica glass was based on the composition of a severely corroded museum artefact, and had a high soda and low lime content. The Na depth profiles displayed a characteristic S-shape and the depth of the Na depletion increased with increasing ageing time. It was found that the increased in Na depletion followed the time relationship, indicating a diffusion controlled process. After an ageing period of 45 days the rate of formation of the depletion layer was measured to be 0.06 and 0.26nmh-1 for 30 and 39% respectively.
