From today’s NYTimes, on using glass as a construction material:

For flat glass, heat tempering…took advantage of one property of glass — that when it cools slowly it becomes denser. By rapidly cooling the exterior of a sheet (usually with air), the surface stays less dense.

This is about the process of “tempering” glass to make it stronger. To understand glass tempering, think of a steak curling up when you cook it because the outside of the meat puckers up (i.e. gets denser than the inside). Tempered glass is the opposite of a steak — slow cooling makes the interior denser than the outside. But because glass can’t curl like steak does, the outside surface simply stays flat under a constant compressing force. This makes the glass stronger.

Because tempering essentially creates an internal tug-of-war in the glass, it can react in interesting ways when the glass does break.

Tempered glass may take longer to crack, but it can still break. Because surface compression must be balanced by interior tension, when tempered glass does break it forms many more smaller pieces than untempered glass

An awesome example of this happens when you drop molten glass into water, to make a raindrop-shaped bead of glass called “Prince Rupert’s Drop.” The water cools the outside of the glass very quickly, making this the ultimate example of tempered glass. The drop is unbelievably strong, easily withstanding a vice grip and blows from a hammer. But it has a fatal and spectacular weakness, as seen in the two videos below: