Contributor: Natasha Kung
Strain cracks form when glass has not been sufficiently annealed or has been subjected to thermal shock. When glass is heated and cooled rapidly, the cooling rate differential between the exterior and internal portions of the glass matrix causes internal stress to build within the object. In order to relieve this stress, the object develops a series of tiny twisted cracks, which makes the structure weak and may even lead to spontaneous breakage. These cracks may not be visible without magnification, but can make the object structurally at risk.
Anneal, Coefficient of thermal expansion, Internal stress, Spontaneous breakage
Synonyms in English
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Strain cracking in glass
Glass objects can be worked into desired shapes from molten glass using a variety of tools and techniques. If the glass object cools rapidly at room temperature, the outer surface will cool at a rate significantly faster than the interior, putting the outer surface in compression, while the interior is in tension due to thermal expansion. (Newton and Davison, 1989). The thicker the glass, the larger the cooling rate differential and the more stress that builds as there is insufficient time for the molecules to occupy their lowest energy states (Vogel, 1985). If the stress is not removed by annealing, numerous strain cracks will form in the object. These small cracks can propagate throughout the object, reduce the overall strength of the object, and may even lead to failure of the object. Therefore, annealing is critical to the glass object’s durability.
Annealing not only effectively reduces the internal stress, but encourages structural consistency, and increases the object’s strength. During this step, the object is gradually re-heated and cooled under controlled conditions in a designated kiln, known as a lehr. During the “soaking period,” or the time in which glass can stabilize at a given temperature, the oven will slowly and evenly re-heat the object, and then slowly return to room temperature. This allows the various thicknesses of glass to cool at relatively uniform rates.
The annealing process occurs between the annealing point and the strain point, and since the glass is heated through this range at a relatively low rate, it should be in equilibrium upon reaching each temperature in the range (Tool, 1946). The annealing point is the temperature at which the internal stress can be relieved, and is unique to the glass composition, coefficient of thermal expansion, and wall thickness. It is higher than the material’s glass transition temperature, thereby allowing the atoms to rearrange and to reach structural consistency while maintaining the object’s three-dimensional shape. The object cools through its annealing point to its strain point, at which the molecules of the glass matrix can no longer reorient themselves and will become rigid in the structure. Following this, the object is allowed to cool to room temperature without cracking or shattering (Mehlman, 1982).
Identifying stresses and caring for glass
When glass is subjected to any strain, such as bending or stretching, it exhibits birefringence. A polariscope can be used to identify this internal stress, in which glass with increased uneven stress will appear highly contrasted with light and dark areas, whereas one with controlled or reduced stress will have uniform shading (Corning, 2011).
Objects that have substantial strain cracking or that were improperly annealed are rarely found in museum collections. One example is described by Wittstadt, referring to a case study of strain cracks found in stained glass (Widdstadt and Mottner, 2009). In general, if small amounts of stress exist in a glass object, it can be exacerbated by sudden thermal shock, causing larger cracks to propagate and the object to fail. Therefore, it is critical to maintain proper environmental conditions surrounding glass objects and to not subject them to fluctuations in temperatures.
2011. "Annealing Glass." The Corning Museum of Glass. https://www.cmog.org/article/annealing-glass.
Grossman, Richard A. 2002. “Ancient Glass: A Guide to the Yale Collection.” Yale University Art Gallery, Connecticut.
Mehlman, Felice. 1982. Phaidon Guide to Glass. Phaidon, Oxford.
Michalske, Terry A. and Bruce C. Bunker. 1987. “The Fracturing of Glass.” Scientific American, Vol. 257, No. 6. Springer Nature.
Newton, Roy and Sandra Davison. 1989. "Conservation of Glass." Butterworth-Heinmann Ltd., Oxford.
Tool, Arthur Q. 1946. “Viscosity and the Extraordinary Heat Effects in Glass.” Journal of Research of the National Bureau of Standards 37.
Vogel, Werner. 1985. “Glass Chemistry, Second Edition.” Wiley-American Ceramic Society.
Wittstadt, Katrin and Peter Mottner. 2009. “Internal fractures on stained glass windows: a conservation study,” from Holding it All Together: Ancient and Modern Approaches to Joining, Repair and Consolidation, edited by Janet Ambers, Catherine Higgitt, Lynne Harrison, and David Saunders. Archetype Publications in association with The British Museum, London. (120-126)