Fig. 5: Atmospheric alteration experiments of three model glass compositions representative of one or more productions of glass objects since Antiquity, MAS NMR spectra.

¹H and ²⁹Si MAS NMR spectra of the A, P, and SL glass powders altered at 40 °C, 85% RH for 6 months (see the caption of Fig. 4 for the compositions of glasses A, P, and SL). a, c, e ¹H NMR spectra acquired by varying the echo delay (TE) allow to filter the contribution of hydrated species with strong homonuclear dipolar coupling such as water. Si–OH contributions with very high chemical shift (about 14 ppm) exist in the A and P altered powders and are assigned to silanols groups H-bonded to non-bridging oxygens, as in crystalline alkali silicate hydrates. Their presence is consistent with the partial retention of alkalies in the hydrated layer. b, d, f the black and pink lines are the ²⁹Si MAS NMR spectra of the pristine and altered glass respectively. These spectra show the depolymerization of the silicate network for the glasses A and P (loss of Q⁴ and growth of Q², Q³ contributions), while the SL glass network has repolymerized. ¹H → ²⁹Si CP MAS spectra are also given in blue. ¹H MAS NMR spectra are reproduced with permission from ref. ¹⁰⁹, Editions Hermann, 2019.