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Shattering glass

My glass ashtray suddenly broke. Prior to this, there was a single pop sound as if a pebble hit the empty ashtray. Can you explain why this happened? Louise, College Park, Georgia

Glass breaks if it has a flaw and tension in the flaw. [Corel]

The glass was under too much tension so it broke much like a snow-laden branch will snap, startling the quiet forest.

"Most probably the ashtray was not properly annealed after it was made," says Siegfried Herliczek, safety glass consultant of Glassig, Inc., Petersburg, Michigan.

Glass forms only under extremely high temperatures — high enough to mix and fuse the basic ingredients (sand, soda, and limestone or chalk) together. Then, the trick is to cool the melt slowly enough that the glass atoms lock into a disordered, almost-liquid state — and not into a crystal’s perfect arrangement. If done right, we get glass: a substance that is not a liquid and not a crystal but something in between.

The process is downright tricky. If we cool glass too quickly, it will be highly strained and may even break while cooling. Even if it doesn’t break then, it is still strained at room temperature.

That’s what happened to your ashtray. "There was too much tension on or near the glass surface. The addition of a small scratch eventually caused the ashtray to break," says Herliczek.

To reduce that surface tension, the glass manufacturer should have cooled the thick ashtray slowly enough that its surface temperature did not differ much from the interior as it cooled.

By the way, in Mesopotamia (present-day Iraq), we found "secret instructions" written on clay tablets 3300 years ago for making glass. First, the instructions said, "Placate the gods" — sacrifice a sheep, burn juniper incense, and pour a honey and melted butter libation. Only then build a fire in the glass furnace and start the capricious business.

(Answered May 14, 2004, updated April 25, 2009)