‘How to Break Glass’ Explains Thermodynamics

Oswaldo Diaz

Time bombs exploded, glass was broken and a blow torch blazed as myths and misconceptions about glass were shattered at the “How to Break Glass” lecture on April 27.

Gary Coyne, a scientific glass blower from the department of chemistry and biochemistry at UCLA, lectured in SB243 about the properties of glass and the circumstances that cause it to break.

The set up was simple, a blow torch that could heat up to 2,000 degrees Celsius, a projector and a wide variety of glass rods, tubes and beakers. Coyne began with heating the end of a glass tube and then submerging it in cool water. After making a crunching sound the test tube was pulled out and beaten several times.

For the audience members who were eager to see the lecture live up to its name, disappointment struck, and the glass remained unharmed. This is where the lecture took many preconceived notions that the public has about breaking glass and turned them around.

Coyne explained how glass is a poor heat conductor and that most if the heat was present in the surface layer of the glass. When glass is heated, it expands, and when it’s cooled it compresses. That is why when he placed the glass in cool water, the heated outer layers contracted rapidly, creating a harder outer shell that not even a metal rod could shatter.

The audience was still trying to figure out why the glass had not broken when Coyne pulled out a tiny fragment of tungsten carbide, a dense metal used in industrial machinery.

Glendale student Kathy Burrutica reluctantly volunteered to drop the piece of metal in the test tube and in the blink of and eye its bottom shattered. “I was shocked that that little piece of metal did the job,” said Burrutica.

One of the key points in the lecture and one of the most misunderstood facts about glass is that to break glass a small flaw or a light scratch can be more effective than brute force.

“Glass breaks by tension not compression,” explained Coyne. The inside of the test tube had so much pressure built up from the hardening of its outer layers, that a small flaw made by the light scratch of a tungsten carbide shard was enough to shatter what a metal rod could not.

Coyne also experimented breaking glass with thermal strain, where glass expands or contracts in response to changes in temperature. Making a slight scratch on a large test tube is not enough break it, but when heat is applied near the flaw it expands and the glass breaks clean off where the scratch was made. Coyne explained how the flaw expands in reaction to the thermal strain applied to the section of glass next to it. The pressure changes that result form the expansion and contraction find a release in the flaw and unleash by stretching out across the glass and breaking a section of it off.

This experiment took Coyne back as he told the audience about the time his wife cracked the window of their car. The front window had a light scratch on the top corner when his wife, who was watering the plants, decided to hose down the car. The temperature of the car was so high that when the cold water touched the glass it created a thermal strain in the window that made the light scratch stretch across the entire window.

Although a flaw makes it easier for thermal strain to break glass, glass can also break on its own when certain sections are heated and left to cool down. Coyne called these types of reactions “time bombs.” When certain parts of glass are heated to high temperatures the strain it sustains from the expansion and later contraction causes the heated section to shatter. The same happened when Coyne heated up a test tube that cracked five minutes after a section of it was exposed to the flaming torch.

“Without glass, science would be blind,” Coyne said while he quoted one of his favorite lines from an unknown source. He described how the transparent nature of glass allows scientists to see through beakers and test tubes into their experiments and mixtures.

The lecture ended with a final demonstration where Coyne fused two different types of glass using his torch. The glass remained together, but when it cooled down the fused parts came apart and broke. Every type of glass has a different expansion rate, which means the glass compresses and expands at different speeds causing their fused parts to come apart as the uneven expansion rates pushed the different types of glass apart.
Everyone left the lecture with new knowledge on a subject that is part of our every day lives but many know little about.

“Its a good feeling, to really understand something that is so mundane yet so fascinating,” said Leah Gonzales, a Glendale college alumni.

Glass was not the only thing that shattered when people’s indifference and misconceptions on the subject stayed behind with the rest of the broken glass in this entertaining and highly informative science lecture.

The next lecture in the series is on May 25. “The Use of Molecular Tools in Conservation Biology” features biology instructor Maria Kretzmann in SB243 at noon.