Boat Windshields Rarely Shatter

Q: I own a boat with a curved tempered safety glass windshield. I have been using the boat for about eight months and driving it pretty hard in sometimes rough seas. The other day we had just launched the boat and were drifting getting ready to go out to sea when the windshield exploded into tiny pieces. We found this odd and would like to know if tempered glass is subject to breaking in this fashion.
— Bob, Charlotte, North Carolina

A: "It is very rare," says Siegfried Herliczek, safety glass consultant of Glassig, Inc., Petersburg, Michigan. "I have heard of it happening less than ten times in 30 years. There are tens of millions of pieces of tempered glass in use in cars and buildings in the US and it probably only happens a few times each year."

Why does it happen, ever? For glass to break, it must have (1) a flaw and (2) tension in the flaw. It's hard to break tempered glass even with a hammer since it's five times stronger than normal glass. However, as you found out, it can break spontaneously.

Here's why:

· Poor production: If the glass has a thin compression layer on the surface (it shouldn't), a small crack (that gets bigger with time and vibration) may eventually eat into the glass tension area and break the glass.

· Poor installation: If the boat windshield rubs against a metal object, then the boat vibrations can form and deepen a flaw. Finally, when the scratch penetrates into the tension area of the glass, the glass breaks.

· Nickel sulfide pellets: If the glass has a nickel sulfide pellet in the tension area of the glass (it shouldn't but nickel can creep into molten glass from contact with stainless steel, for example), the glass can break spontaneously. The nickel sulfide crystal grows, cracks the glass tension area, and that breaks the glass. Glass manufacturers take care to avoid nickel sulfide contamination.

Glass tension sounds like a culprit but tension is necessary to form strong glass that — if it breaks — breaks in a safe way. Herliczek explains the thermal tempering process used in North America to make almost all tempered glass.

Glass is heated to about 1050 degrees F (566 degrees C) and then quickly cooled. This produces glass with a different molecular arrangement in the middle than on the surfaces. It puts the outside surfaces (each about 25% of the glass thickness) into high compression and the middle 50% into tension. High surface compression toughens the glass.

Further Reading:

WonderQuest: How safety-glass is made

Glassig, Inc.: Tempered glass breakage