Yes, heat can crack glass. The primary cause is not simply the presence of heat, but rather the rapid and uneven change in temperature that the glass experiences. This sudden thermal shift creates significant internal stresses within the material, which can lead to cracking.
Understanding Thermal Shock
Glass expands when heated and contracts when cooled. When subjected to sudden temperature changes, different parts heat or cool at varying rates. For example, if a cold glass is filled with hot liquid, the inner surface heats rapidly and expands, while the outer surface remains cool.
This differential expansion creates stress within the glass structure. The hotter, expanding regions pull against the cooler, unexpanded regions, leading to tension and compression. If these internal stresses exceed the inherent strength of the glass, a crack will form, often propagating quickly. This phenomenon, where rapid temperature changes induce stress and cracking, is called thermal shock.
Glass Types and Vulnerability
Different types of glass exhibit varying resistances to thermal shock, due to their unique compositions and manufacturing processes. Soda-lime glass, commonly used for drinking glasses and standard window panes, has a high coefficient of thermal expansion. This means it expands and contracts significantly with temperature fluctuations, making it more susceptible to cracking under sudden thermal changes. A rapid temperature shift of around 50°C (90°F) can induce thermal shock in soda-lime glass.
Borosilicate glass, known by brand names like Pyrex, is engineered for greater thermal shock resistance. Its composition includes boron trioxide, which results in a much lower coefficient of thermal expansion compared to soda-lime glass. This allows borosilicate glass to undergo larger temperature differentials, exceeding 150°C (270°F), without developing stress. Tempered glass, produced by controlled thermal or chemical treatments, is another type designed for enhanced durability. This process creates a pre-stressed surface layer that improves its resistance to impact and thermal shock, allowing it to withstand temperature changes of 100°C (180°F).
Preventing Heat-Related Cracks
Preventing heat-related cracks in glass primarily involves managing temperature transitions by avoiding sudden and extreme temperature changes. For example, never place hot glass directly onto a cold surface, such as a granite countertop or a metal sink, as this can induce immediate thermal shock. Similarly, avoid pouring very hot liquids into a cold glass container.
Allowing glass to adjust gradually to temperature changes significantly reduces the risk of cracking. When using glass bakeware, preheat the oven with the glass inside, rather than placing cold glass into a hot oven. Inspecting glass items for existing chips, cracks, or scratches before use is also important. These imperfections can act as stress points, making the glass more vulnerable to thermal shock even under less extreme conditions.