Glass and Ceramics 165
●Glass dissolution leads to a dull appearance of the glass with small dots
or short cracks that are detectable only under the microscope. The thick-
ness of the glass plate or the grains reduces with time, up to complete
dissolution in extreme cases.The reactions can be quantified even before visible damage occurs by analysing
the components dissolved in the solution (preferably by ICP–AES) or by
characterising the altered glass surface (with surface-sensitive techniquessuch
as X-ray photoelectron spectroscopy (XPS), and Infrared-spectroscopy (IR)).
The rate of degradation heavily depends on the glass composition. As a
general rule, the higher the percentage of silica, the more stable is the glass.
Since sodium ions have stronger bonds within the network than potassium
ions, the durability of sodium silicate glasses is significantly higher in com-
parison to potassium-rich glasses. However, sodium and potassium interact
with each other in mixed-alkali glasses, which further complicates the pre-
diction if both are present. In principle, the durability of glasses can be esti-
mated with several theoretical approaches. These models provide good
predictions for modern glasses. Historic glasses, however, have a rather com-
plex composition. Furthermore, apart from the chemical composition, other
factors such as the surface roughness, the thermal history and the production
process, as well as the presence of inhomogeneities have an influence on the
chemical durability. Once again, for historic glasses, these parameters com-
plicate the prediction of their degradation with time. Consequently, model
glasses with medieval compositions or with an even higher content of potas-
sium oxide have been created to facilitate systematic research on the degra-
dation of historic glasses.
Figure 4Scheme of a potash-lime silicate glass after acidic attack on the surface: a
chemically altered hydrated layer has formed, covered by crystalline corrosion
products (Note: the fourth bond of silicon is out of the plane and not represented in
the two-dimensional scheme)