Investigation of Weeping Glass Deterioration Under Controlled Relative Humidity Conditions

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

“…Water favored more the hygroscopicity than formaldehyde (Figure S2). This differs from the theoretically proposed because formaldehyde could be oxidized to formic acid (Reaction or ), 8,12 accelerating the ion exchange. However, not enough formic acid was formed, or maybe the lower humidity detected also decreased the hydration rate.…”

“…What is not yet known for sure is whether the aldehyde oxidizes to its corresponding organic acid in the air or on the glass surface 8,11 . A Cannizzaro reaction could also occur (Reaction ), which involves the base‐induced disproportionation of two molecules of a non‐enolizable aldehyde to give a primary alcohol and a carboxylic acid 12 …”

“…8,11 A Cannizzaro reaction could also occur (Reaction 5), which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid. 12 (Reaction 4) (Reaction 5) The VOCs are particularly dangerous to unstable glasses, which are glasses with low former and/or stabilizer oxides. These glasses have a higher content of alkaline ions and a lattice more open that favor the ion-exchange reactions, being therefore vulnerable to degradation even in non-contaminated atmospheres.…”

Influence of humidity in the alteration of unstable glasses

“…7,8,10 This limit is fixed by the deliquescence relative humidity (DRH) of sodium formate at 25 • C (56% RH), 35 which is the commonest alteration product in unstable glasses. 14,23,[36][37][38][39] However, potassium formate has a DRH of 19%, which means that potash glasses are prone to weeping in a museum atmosphere. 35 This research aims to determine the alteration mechanism of three types of glasses (soda, potash, and mixedalkali silicate glasses) exposed to controlled environments with different relative humidity (∼30%, ∼70%, ∼100% RH), and compare the reactivity between them.…”

“…The pieces affected by crizzling should be stored at 40%−45% RH because a higher environmental humidity can accelerate the alteration, but a lower one can dry the surface inducing the formation of fissures and detachments 7,8,10 . This limit is fixed by the deliquescence relative humidity (DRH) of sodium formate at 25°C (56% RH), 35 which is the commonest alteration product in unstable glasses 14,23,36–39 . However, potassium formate has a DRH of 19%, which means that potash glasses are prone to weeping in a museum atmosphere 35…”