By: Berlin Packaging Specialist
Date: January 26, 2020
At what temperature does glass "break"? Glass bottles and jars are usually not affected by ambient, refrigeration, or warm temperatures. However, high heat (>300°F) and excessive thermal variations can cause glass to shatter or break. Glass is a poor thermal conductor and rapid changes in temperature (roughly 60°F and greater) may create stress fractures in the glass that may eventually crack. When heated, thin glass begins to crack and typically breaks at 302–392°F.
If a glass container is placed on a very hot source of heat (e.g., 500°C), it can gradually lose its shape and change from a permanent solid form to a plastic state.
In general, glass jars should not be heated in a microwave or oven. Glass jars made of ordinary glass may crack or explode in a microwave. Some glass containers are made of heat-resistant materials and can withstand microwaving. Consumers can find the “Microwave Safe” sign at the bottom of the glass products that are designed to be used in microwaves. However, secured lids should never be used when microwaving glass containers.
Glass may break when subjected to temperatures below freezing. This may occur because the contents freeze and their expansion cause the glass to crack (if the cap does not come off).
Why does hot water cause glass to break? It is not the hot water that breaks the glass, but the sudden change in temperature, causing internal stress to be exerted on the material. If these changes occur suddenly, they create internal tension that leads to the breakage of the container. Glass is a bad heat conductor and therefore it does not tolerate excessive changes in temperature. Normally, it can tolerate about 45°C, so if, for example, the application requires the temperature to reach 90°C for pasteurization, it is necessary to gradually increase the temperature of the environment where the container is situated. This method will prevent any problems even with the sterilization process, which reaches 130°C. It is necessary to use an even more gradual procedure for containers of a particular shape (jars with handles, sharp edges, extra large containers, etc.).
Why do some bottles bounce off the ground without breaking after falling from a height, while others shatter? Whether a glass container breaks or not depends not only on the type of impact but also on the thickness and distribution of the glass as well as the level of annealing.
What does “shatterproof glass” mean if the glass then breaks? This is a “current” term, which is not completely accurate. It refers to glass that has undergone a particular process called “tempering”, which brings the object to a temperature of about 600°C and then cools it suddenly (with cold air distributed according to the thickness of the glass), causing controlled raised tensions inside the object. In practice, a layer of tensile stress is created, bounded by two layers of compressive stress, giving the object increased resistance. The molecular structure which is created allows the glass to shatter into very small particles in the event of a sharp impact, and these cannot cause damage.