Contributors: Tess Hamilton
A deterioration process by which alkali is pulled from the glass structure by moisture in the air, and deposited on the surface of an object. “Weeping” glass will have an oily or slippery surface feel, or in more severe instances, moisture will build up on the surface of an object in the form of droplets. Weeping is considered a symptom or precursor to crizzling.
Crizzling, Glass Disease, Glass Decay, Glass Deterioration, Glass Sickness
Synonyms in English
Sweating, Tearing, Oily glass, Slippery glass
Glass is a non crystalline solid composed of a mixture of former, flux, and stabilizer. The former, which makes up the bulk of the glass composition, is silica (silicon dioxide). The melting point of the silica is lowered by adding an alkaline flux, either soda (sodium compound) or potash (potassium compound). The network stabilizer, calcium oxide, balances the charges of the glass network by creating an ionic bond with the dangling oxygens on the silica molecules and preventing the crumbling of formed glass.
Glass “weeps” when polar water molecules on the surface of the glass attract the positively charged sodium or potassium ions from within the glass network. As these metal cations are displaced, it initiates an ion exchange between the alkaline and earth alkaline metals, and hydronium and water molecules within the surface film, causing the formation of a silicon rich layer of glass (gel-layer) and the deposition of alkaline ions on the surface of the glass. Leached ions then form salts as they react with pollutants in the air such as formaldehyde, carbon dioxide, or sulfur dioxide.
If the relative humidity is below the deliquescence point of the salt formed, it will result in a crystalline deposit on the surface of the object that can affect color or translucency. If the relative humidity is above the deliquescence point, the salt will attract moisture and dissolve, resulting in an oily surface film or droplets.
As the potassium and sodium fluxes migrate out of the glass network, and smaller hydrogen ions replace them, it results in the formation of an altered gel-layer which is chemically destabilized, and has a lower volume than the underlying mass glass. If this gel layer becomes too thick, or undergoes changes in relative humidity, the layer may shrink due to dehydration of the glass, and result in the formation of fine, interconnected cracks known as crizzling, and, in extreme cases, may lead to the spalling of the gel-layer or the collapse of the object (Verhaar, 2018; Koob, 2006).
As the moisture within the oily film continues to draw out alkaline ions, raising the pH of the solution, it can begin to dissolve the silica structure of the glass by breaking the Si-O bonds. This occurs at a pH > 9, and can result in surface loss, flaking, pitting, changes in translucency, or a cloudy, uneven appearance of the glass (Goldfinger 2008).
The damage that results from weeping glass is irreversible. It is recommended that glass showing an oily or crystalline surface deposit be washed in water and ethanol to remove the alkaline, hydrophilic salts, and to prevent further deterioration through both the attraction of moisture and alkaline attack of the silica structure. Weeping glass is best prevented by maintaining an appropriately low relative humidity for the object, and avoiding fluctuations in RH. (Koob, 2004). The susceptibility of a glass object to weep is based on its chemical composition. Any glass with too little calcium stabilizer (lime) or too much alkaline flux is vulnerable to alkaline leaching and weeping. Generally, potassium (potash) glasses are more susceptible to this form of glass deterioration than sodium (soda ash glasses). Due to this greater vulnerability, it has been generally recommended that glass objects be kept in stable relative humidity below 42%, which is the deliquescence point of potassium carbonate; although, many different salts can form on the glass surface depending on air quality and glass composition (Oakley, 2001). For glass exhibiting different weathering phenomena, such as crusts or iridescence, a different range for relative humidity may be recommended (Bellendorf et al. 2010).
Deliquescence Point of Salts Related to Weeping at 20° C
|Potassium Carbonate||44% RH|
|Potassium Acetate||23.3% RH|
|Potassium Sulfate||97.2% RH|
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