Hot Glass

Some criminal put a rock through the drivers window of our van Wednesday evening between about 8 and 11pm. I imagine it was some neighborhood kid. Must have a had a pretty good arm, the rock wasn't that big, and automobile window glass is pretty tough stuff. I was surprised at what a mess it made. Big pile of broken glass on the ground outside, big pile on the drivers seat. Glass all over the front insides of the van. Glass in the pocket in the arm rest on the passenger's side. Even some glass on the floor behind the front seats. Most of the glass was in little crumbles, roughly cube shaped, maybe an eighth of inch across. But there were also tiny little flakes and crystals I could barely see. I imagine there are probably even some microscopic flakes that I cannot see. There were also some small pieces that had cracked but had not come apart (see photo). I wonder what is holding them together? Van der Waals force maybe?

Scuff marks on the dash from the rock, which ended up on the top of the dash all the way to the right. Rock has many little whitish scuff marks from where it impacted the glass (you can sort of see them in the picture). It took me an hour to clean up the mess, and I still have to sweep up what was left under the van. $180 to replace the window at the local glass shop.

Spent some time on the internet looking for information on tempered glass. Found a bunch of videos of guys breaking car windows by supposedly throwing bits of spark plug ceramic at them. Unfortunately, they all follow the same pattern. They show the bit of spark plug, which is generally not very big, maybe as big as your thumbnail. Then they show the window shattering, but they never show anyone actually throwing the bit of ceramic, so you have no idea of how hard they are throwing it. I did see a TV news clip a while back when carjackers in Miami first developed this technique. A reporter was attempting to duplicate this feat and he was having trouble with it. He had to really wing it to get the glass to break. I think they must have filmed this from inside the car. I suspect that one of the edges of the ceramic has to impact the glass in order to cause it to shatter. If the rounded side hit it, I suspect it would just bounce off.

I also found a video of a guy breaking a piece of auto glass with one of those emergency escape tools. He did not have to hit it very hard, he just tapped it and it shattered. The tip on this tool might be spring mounted. I do not know why that is. Vibrates against the glass maybe, so you get a thousand tiny taps?

Tempered glass is made by cooling the outside surfaces quickly while the inside is still hot. The outside contracts more quickly than the inside. Then the outside is under tension. When the inside cools, it contracts. This allows the outside to contract as well. The outside was previously longer when it first cooled. Then as the inside cools, the whole mass contracts. The outside surface was longer when it first solidified, so it can be stretched again without breaking. Glass breaks under more easily under tension than under compression. When something impacts the glass, it will first bend. When the glass bends, the surface on the outside of the bend gets stretched. As long as it is not stretched farther than it was when it was first cooled, it should not break. Should you go beyond that, then it will shatter.

Why should a bit of ceramic be able to break tempered glass so easily? I think it must be because the surface of the glass is under tension, and the bit of ceramic is hard enough and sharp enough that it can cut the surface of the glass, which will cause a break in the tension in this one spot. This unbalances the tension in the surface of the glass, and these unbalanced tensions will just pull the glass apart.

And then we have Prince Rupert's Drops. Discovered around 1650, still quite amazing. Sorry about the lame soundtrack. I found several videos showing a Drop exploding, but this is the only one I found with a Drop being hammered.



Update December 2016 replaced missing picture.