Glass can crack when exposed to cold conditions, but the low temperature itself is rarely the singular cause of failure. Breakage results from thermal stress created by an uneven temperature distribution across the material. This stress is caused by differential contraction, which occurs when different parts of the glass attempt to shrink at varying rates. Understanding this mechanism reveals that the rate of change and the temperature difference are the true culprits, not the absolute coldness.
The Primary Cause: Thermal Shock
The physical process responsible for cold-related glass breakage is thermal shock, a rapid temperature change that induces a sharp temperature gradient within the material. Glass is a poor thermal conductor, meaning heat does not move quickly or evenly through its structure. When one surface is suddenly exposed to cold air or liquid, it cools and attempts to contract rapidly. The inner or unexposed portion, however, remains at its original, warmer temperature for a longer period.
This temperature difference creates the gradient, causing glass molecules to contract more in cooler areas than in warmer areas. The warmer, uncontracted interior acts as an internal constraint, resisting the shrinkage of the cooler outer layer. This resistance generates significant internal forces, particularly high tensile stress in the cooling surface.
Glass is strong under compression but weak when subjected to tensile stress. When the pulling force of differential contraction exceeds the material’s tensile strength, a crack initiates. For common soda-lime glass, a temperature differential of around 40°C (72°F) across the material can cause a fracture. The crack typically starts at the surface experiencing maximum tension and propagates quickly through the glass to relieve the stress.
Factors That Increase Susceptibility
The likelihood of thermal shock failure depends on the glass’s physical condition and manufacturing process. Pre-existing microscopic flaws, scratches, or chips on the surface act as stress concentrators. These imperfections serve as weak points where high tensile stress concentrates, allowing a crack to start and propagate under a smaller temperature differential than required for pristine glass.
The type of glass determines its resistance to thermal stress. Standard annealed glass, or float glass, is more susceptible because it is cooled slowly during manufacture to relieve internal stresses. It possesses a relatively high coefficient of thermal expansion, meaning it expands and contracts significantly with temperature changes, increasing the severity of differential contraction during thermal shock.
Conversely, materials like tempered glass or borosilicate glass are more resistant. Tempered glass is rapidly cooled during production, which locks the outer surface into compression. This effectively increases its tensile strength, allowing it to withstand temperature differences up to 200°C (360°F) or more. Borosilicate glass, used in laboratory equipment and certain cookware, has a much lower coefficient of thermal expansion. This means it changes size very little with temperature variations, significantly reducing internal stress during rapid cooling.
Preventing Cold-Related Cracks
Preventing cold-related cracks centers on minimizing the temperature gradient and avoiding rapid temperature change. In household environments, this involves practical adjustments, especially during colder months. For windows, direct, localized heat sources should be kept away from the cold glass surface. Placing space heaters or aiming heating vents too close to a windowpane can create a severe temperature gradient between the heated center and the cold edges or frame, initiating a fracture.
For glassware, condition the material by avoiding sudden temperature extremes. Never pour boiling water into a glass container that has just come out of a refrigerator or freezer, or vice versa. Allowing cold glassware to warm slightly to room temperature before introducing hot liquids reduces the initial temperature difference and the resulting tensile stress.
Regular inspection of glass items for nicks or scratches is a practical preventative measure, as even a small surface defect can be the starting point for a thermal crack. For exterior applications, such as window installations in climates with extreme temperature swings, choose materials with a low thermal expansion coefficient or high thermal shock resistance, like heat-strengthened or tempered glass, which offers a significant safety margin. Focusing on gradual temperature transitions and maintaining structural integrity remains the most effective strategy for preventing cold-induced cracking.