The Decay of Medieval Window Glass

The basis for the currently accepted mechanism for the attack of glass by aqueous solutions was proposed by Charles (1958), after observing that vitreous silica is not attacked by water vapour even over long periods, whereas 

Equation (5.1) represents the hydrolysis of the salt of a weak acid. The sodium ion migrates into the solution, and the hydroxyl ion released in the glass can then attack the otherwise stable silica network, converting a bridging oxygen into a non-bridging site, thus disrupting the network  

The non-bridging oxygen ion so produced is now capable of dissociating another water molecule  

These equations may be summarized as follows, but the intermediate stages involving the non-bridging oxygen sites are of crucial importance, as demonstrated by the observed resistance of vitreous silica to aqueous attack  

Charles also made another important observation, namely that corrosion of an expanded glass structure (i.e., that resulting from a high Active temperature) proceeds faster than corrosion of a compacted glass structure (annealed) even if conditions and glass composition are the same. Thus the weathering characteristics of a piece of glass also depend on its thermal history. 

---