With M the metallic ion. We suggest that two mechanisms are responsible
for this depolymerization. During ionic exchange in this
thermal condition, an oxygen flux is possible at the surface and an excess
of metal is also detected by the infrared measurement at the extreme
glass surface, the probe depth is of micrometer order for silica.
This effect is confirmed by the negative slop of the Eq. (2). We assume
that an additional mechanism occurs at the glass surface: