Glass is a common material found in many households, from drinking glasses to food storage containers. Its safety in a freezer is a frequent question, and the answer lies in understanding its fundamental properties. Glass is an amorphous solid, lacking the ordered, crystalline structure of other solids. This non-crystalline nature influences its reaction to temperature changes.
Glass and Temperature Fluctuations
Glass, as an amorphous solid, does not have a precise melting point like crystalline materials; instead, it gradually softens when heated. When subjected to temperature changes, glass undergoes thermal expansion and contraction, meaning it expands when heated and contracts when cooled. The extent to which glass expands or contracts with temperature is measured by its coefficient of thermal expansion (CTE).
Different types of glass possess varying CTEs, which dictates their response to temperature shifts. For instance, common soda-lime glass, used in most windows and bottles, has a relatively high CTE, around 9 x 10⁻⁶/K. In contrast, borosilicate glass, often found in laboratory glassware and some bakeware, has a significantly lower CTE, approximately 3.3 x 10⁻⁶/K. This difference in CTE is important because materials with lower CTEs are less prone to stress from temperature variations.
Common Reasons for Breakage During Freezing
Two primary factors contribute to glass breaking when exposed to freezing temperatures: the expansion of frozen liquids and thermal shock. When liquids, particularly water-based ones, freeze, they expand significantly. Water, uniquely, expands by about 9% in volume as it transitions from liquid to ice. If a glass container is filled to the brim with liquid and then frozen, the expanding ice exerts considerable pressure on the rigid glass walls, which can cause the glass to crack or shatter.
Thermal shock is another significant cause of breakage. This phenomenon occurs when glass experiences a rapid or sudden change in temperature. For example, placing a warm glass container directly into a very cold freezer can induce thermal shock. Different parts of the glass cool and contract at varying rates, creating uneven stress within the material. If these internal stresses exceed the glass’s strength, it can lead to cracking or shattering.
Best Practices for Freezing with Glass
To minimize the risk of glass breakage in the freezer, several practices can be followed. When freezing liquids, leave adequate headspace in the container. Leaving about 1 to 2 inches of space at the top allows room for the liquid to expand as it freezes without putting excessive pressure on the glass. For narrower containers, more headspace might be beneficial.
Selecting the appropriate type of glass is also important. Borosilicate glass, often labeled as “oven-safe” or “freezer-safe,” is designed to withstand greater temperature fluctuations due to its low thermal expansion coefficient. Tempered glass, which has been specially treated for increased strength and resistance to thermal stress, is another suitable option. Common soda-lime glass, typically used for drinking glasses, is more susceptible to thermal shock and should generally be avoided for freezer storage.
Gradual temperature changes can help prevent thermal shock. Allow hot food to cool to room temperature before refrigerating it, then chill it further for several hours before transferring to the freezer. When placing multiple glass containers in the freezer, leave some space between them to help with even cooling and prevent contact breakage. It is also advisable to loosen lids until the contents are fully frozen to relieve any pressure buildup from expanding food.