Glass disease
Encyclopedia
Glass disease, also known as sick glass, is a degradation process encountered in art conservation.
Glass disease is caused by an inherent fault in the chemical composition of the original glass
formula. Usually, inadequate calcium oxide
causes the alkalis in the glass to remain water soluble at a low level. Exposure to higher levels of relative humidity
during storage or display causes these salts to hydrate and leach out of the glass. Upon a reduction of relative humidity, these hydrated salts can then form a crust on the surface. This process causes a complex disintegration of the glass which can be identified through a variety of symptoms. One such sign is the aforementioned crusty deposits which can form a visible hard alkaline coating on the surface. Another symptom is a distinctive network of fine cracking, also called crizzling, which can reduce the transparency of the glass or even threaten the integrity of the structure.
Glass disease is caused by an inherent fault in the chemical composition of the original glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
formula. Usually, inadequate calcium oxide
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
causes the alkalis in the glass to remain water soluble at a low level. Exposure to higher levels of relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
during storage or display causes these salts to hydrate and leach out of the glass. Upon a reduction of relative humidity, these hydrated salts can then form a crust on the surface. This process causes a complex disintegration of the glass which can be identified through a variety of symptoms. One such sign is the aforementioned crusty deposits which can form a visible hard alkaline coating on the surface. Another symptom is a distinctive network of fine cracking, also called crizzling, which can reduce the transparency of the glass or even threaten the integrity of the structure.