The phenomenon wherein crystallization occurs in (previously fully amorphous) glass. Devitrification typically occurs during kiln firing and often presents as a whitish haze on the glass surface.
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Devitrification results in a loss of translucency, and devitrified glass is often described as having a white or grey, “hazy,” “scummy,” “chalky,” or “misty” appearance combined with a roughened surface texture (nicknamed “devit” by glass artists). Devitrification can occur both on the surface of glass or internally, though it is most commonly seen on the surface (Anderson 1998, 21). The crystal formation causes contraction of the glass, and a crinkled surface appearance or cracking can occur as a result (Newton and Davison 1989, 4); this crystallization-induced cracking is separate from deterioration that produces crizzling (from an unstable glass composition or strain cracks) made by poor annealing or thermal shock (made by poor annealing or thermal shock or due to glass weathering. Devitrification is often accidental, but can also be purposefully induced for artistic effect; it cannot be removed by washing the glass.
Devitrification can occur in a number of different ways. When glass that is kiln fired is kept at the liquidus temperature (the temperature above which a material is completely liquid) or in the “devitrification zone” for a sufficient amount of time, and/or cooled too slowly, crystal structures can have time to grow within the glass (Davison 2003, 11). The “devitrification zone” or “devitrification range” is the temperature range at which devitrification becomes more likely and typically occurs in most glasses at approximately 1300 to 1550°C (Shotwell 2002, 135), although glass with different compositions may have different devitrification zones. Glass that has not been properly cleaned before firing is also more prone to devitrification, as fingerprints, dust, and other contaminants on the glass surface can act as nucleation centers for crystalline growth (Vogel 1994, 287).
The composition of the glass affects the crystallization probability. Devitrification may be less common in ancient glass, which typically has complex compositions, than in modern glasses, with simpler compositions (Newton and Davison 1989, 4). Glass that contains boron is more resistant to devitrification (Anderson 1998, 21), while glass with an excess of lime (calcium) cools more slowly, resulting in an increased opportunity for the development of devitrification ("Mystery Slab" 2011). Glass with opalizing agents, opaque glass, and colored glass all are also more likely to devitrify (Walker 2006). These additives create “microcrystalline areas” in the glass where devitrification can more easily occur (Davison 2003, 9).
Though devitrification can appear similar to certain types of glass degradation, like weathering, or can be mistaken as a type of glass degradation itself, devitrification is not a deterioration process. Glass deterioration occurs because of a chemical interaction between glass and its environment ("Devitrification" n.d.); devitrification, however, occurs during the creation of the glass, and therefore should not be used in a general sense to mean “loss of vitreous nature” (Newton and Davison 1989, 5). Devitrification does not result in a change to the overall chemical composition of the glass. Because it is a feature of the “original” state of the glass and is not in itself degradation, devitrification is not typically seen as needing to be treated or removed in the conservation of glass objects.
References[edit | edit source]
Anderson, Harriette. 1998. “Devitrification of Glass: Its Simple Prevention.” The Firing Line 98-1.
Davison, Sandra. 2003. Conservation and Restoration of Glass. 2nd ed. Oxford, UK: Butterworth-Heinemann.
“Devitrification.” n.d. Corning Museum of Glass - Glass Dictionary. Accessed April 2, 2019. https://www.cmog.org/glass-dictionary/devitrification.
“The Mystery Slab of Beth She'arim.” 2011. Corning Museum of Glass. December 8. https://www.cmog.org/article/mystery-slab-beth-shearim.
Newton, Roy, and Sandra Davison. 1989. Conservation of Glass. Oxford, UK: Butterworth-Heinemann.
Shotwell, David J. 2002. Glass A to Z. Iola, WI: Krause Publications.
Vogel, Werner. 1994. Glass Chemistry. 2nd ed. New York, NY: Springer-Verlag. DOl: 10.1007/978-3-642-78723-2.
Walker, Brad. 2006. “Causes of Devitrification.” WarmTIPS. http://www.warmtips.com/20060423.htm.