Cold can cause glass to break, but the failure is not due to extreme low temperature alone. The true cause of breakage is the rate at which the temperature changes, creating immense internal forces within the material. This rapid change initiates a mechanical breakdown originating from the material’s fundamental properties.
The Unique Structure of Glass
Glass is fundamentally different from most other solids, such as metals, because it is an amorphous material. Unlike crystalline solids, the atoms in glass are arranged randomly, much like those in a liquid. This disordered structure is often described as a “frozen liquid” that lacks long-range order.
This arrangement makes glass vulnerable to rapid temperature change in two ways. First, the lack of uniform structure means glass is brittle, offering little resistance to pulling forces. Second, glass is a poor conductor of thermal energy, meaning heat moves through it slowly.
This low thermal conductivity prevents the glass from maintaining a uniform temperature throughout its thickness. If one side is suddenly cooled, heat cannot transfer quickly enough from the warmer interior to the cooler exterior, setting the stage for mechanical failure.
Thermal Expansion and Contraction
All materials change size in response to temperature variations, known as thermal expansion and contraction. When a material is heated, its atoms vibrate more vigorously, requiring more space and causing the material to expand in volume. Conversely, cooling decreases atomic vibration, causing the material to contract.
The degree of size change is quantified by the Coefficient of Thermal Expansion (CTE). Standard glass, such as that used in windows and bottles, has a relatively high CTE, meaning it expands and contracts significantly. For example, Pyrex, which contains boron oxide, has a much lower CTE, making it less susceptible to these size changes.
If glass is uniformly cooled over a long period, the entire object contracts smoothly, generating no significant internal forces. However, when the temperature change is sudden and uneven, the resulting differential expansion or contraction creates powerful forces the material cannot withstand.
The Mechanics of Thermal Shock
The failure mechanism that causes glass to break from sudden cooling is known as thermal shock, which results from a severe temperature gradient. This occurs when a rapid temperature change is imposed on the glass surface, such as pouring cold liquid into a hot glass or exposing a warm window to frigid air. Because glass is a poor heat conductor, the outer surface quickly adjusts while the material just beneath the surface remains at the original temperature.
Consider a hot glass container suddenly exposed to cold water: the exterior layer contracts rapidly, but the inner layer remains expanded because it is still warm. This mismatch creates internal stress: the contracting outer layer pulls on the still-expanded inner layer, generating a tensile force on the outer surface.
Glass is strong when compressed, but weak when subjected to tension, or pulling forces. When the tensile stress on the cooled surface exceeds the material’s strength, a microscopic flaw instantly propagates into a large crack. This crack travels rapidly through the glass along the path of highest stress, leading to failure.
Why Some Glass Breaks and Others Do Not
The probability of glass breaking depends on the magnitude of the temperature difference. Standard household glass, known as annealed glass, can fracture when the temperature difference between its surface and interior reaches only about 40 degrees Celsius. This small thermal tolerance explains why a simple kitchen scenario, like running a warm glass under cold water, can lead to immediate breakage.
The thickness of the glass also plays a role, as thicker glass is more susceptible to thermal shock. A thicker material exacerbates the low thermal conductivity by creating a steeper temperature gradient between the interior and exterior, leading to higher internal stresses.
Furthermore, scratches or micro-cracks on the glass surface lower its effective strength. These flaws act as stress concentration points where the tensile force of thermal shock is focused, initiating the crack at a lower overall stress level.
In contrast, certain types of glass, such as tempered or toughened glass, are manufactured to resist thermal shock. The tempering process involves heating the glass and then rapidly cooling the exterior surface with air jets, which locks the outer layer into a state of high compression. This compressive pre-stress must be overcome by the tensile forces of thermal shock before a crack can begin, allowing tempered glass to safely withstand temperature differentials up to 200 degrees Celsius.