We previously observed diffusion of sodium from the borate glass to the oxide scale by post-mortem TEM-EDS . However, these experiments were conducted at or above the glass transition temperature (~520 °C under vacuum conditions, Fig. ) of borate glasses. Here, we show that sodium diffusion and the partial dissolution of the iron oxide layer occurs within a much lower temperature range (260-420 °C, Video 1), providing crucial validation for indirect observations and calculations of sodium diffusion in borate glasses at temperatures below their glass transitions .