Hydrolytic resistance of K₂O–PbO–SiO₂ glasses in aqueous and high‐humidity environments

Abstract

Some historical glasses (lead-wood ash glasses, lead-crystal glasses…) are silicate glasses with high content of lead and potassium. This work presents the evaluation of the chemical stability of high-lead glasses in a high relative humidity atmosphere and as result of aqueous immersion.In both situations, the alteration mechanism begins with the lixiviation of alkali metal and lead ions, followed by the hydrolytic attack of the silica glass network. According to the results, the glasses with a higher content of lead show the fastest degradation due to their higher hygroscopicity. Environmental CO2 can be dissolved in the adsorbed water and favor the formation of intermediate degradation compounds.

Influence of volatile organic compounds (VOCs) in the alteration of historical unstable glasses

“…This tendency is not clear in the lead glass, where higher leaching is observed at acidic pH. This may be due to the precipitation of Pb(OH) 2 in basic media (pH > 7.8) 22 (Reaction ), which decreases the concentration of OH − and Pb 2+ ions from the medium and, therefore, protects the glass. Pb(OH) 2 has an amphoteric character since at pH > 12.4 the water‐soluble HPbO 2 − species are formed (Reaction ).…”

“…They even have a low content of alkali. These glasses are more stable to the alteration because of the stabilizer role of lead, unless they have a higher content of lead increasing its instability 15,22 …”

Influence of humidity in the alteration of unstable glasses

“…The quick ion exchange between the protons from water (H + ) and the cations Na + and K + from glass (Equation ) favored the high concentration of these ions in the aqueous solutions 49 . Interestingly, the ionic concentration in V‐Na and V‐M aqueous solutions was much lower than in V‐K one.…”

“…These bands are characteristic of silicate glasses and have been previously identified in many articles. 19,49,77 Figures 6-8 also show the spectra of the glass surface before and after the test. The glass surfaces showed the formation of new bands of different intensities, that appear between 1630 and 1645 cm −1 , 2120 and 2130 cm −1 and 3300-3400 cm −1 due to the adsorption of water (Table 4).…”

“…71 In addition, some spectra of soda and mixed-alkali glasses (Figure 6) show bands associated with carbonate groups (∼1463 cm −1 ), which could come from the reaction of environmental carbon dioxide with the adsorbed layer of water 40 . In the basic pH of this layer, the following equilibria take place (Equations 7-9) 49 :…”