Last Updated: Dec 13, 2022     Views: 16

"Heat expands-cold contracts. This is why sudden change of temperature breaks any ordinary glass. Pyrex contracts so slightly that sudden heat cannot break it."  Image:  From freezing to boiling without breaking. Corning Glass Works. CMGL 133945.

Temperature and Tension

Glass does not break solely because of severely high or low temperatures. It is extreme temperature changes that cause glass to break. When the temperature of glass rapidly changes, some spots become cold or hot before others.  This causes instability in areas with cracks or inclusions. 

Like most materials, glass expands slightly when heated up and shrinks the same amount when it cools down. If one part shrinks or expands faster than the part next to it, then the two regions tug against each other. This pulling, called “tension” in engineering, can be strong enough to shatter glass.  

Small Imperfections

Glass will often begin to break along pre-existing cracks and imperfections. Even glass that looks perfect and has been gently handled will have minor invisible defects.  The tension caused by uneven heating and cooling will affect these minor defects first.  These slight imperfections turn into cracks that become larger, breaking the piece apart.  

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