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Posted 9/19/2003 8:05 PM     Updated 9/19/2003 8:11 PM
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Boat windshields shatter rarely, no monthly eclipse, gray squirrels live 3 years
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, sometimes in 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.

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 U.S. 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's 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 is 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 Surfing:

WonderQuest: How safety glass is made

Glassig, Inc.: Tempered glass breakage

Q: Why isn't there a solar eclipse every month?

A: We don't have a monthly eclipse because the Moon doesn't revolve around the Earth in exactly the same plane as Earth revolves around the Sun. If it did, then we would have an eclipse at every New Moon (a solar eclipse) and at every Full Moon (a lunar eclipse).

As Earth circles the Sun, it traces an ellipse — almost a circle — in the sky. The Moon does likewise as it circles Earth.

However, the Moon's ellipse lies in a plane about 5 degrees slanted to Earth's orbit. That little difference matters. Only about one out of six new or full moons produce an eclipse. Half the time the Moon lies above Earth's orbit (called the plane of the ecliptic) and half the time it dips below.

We have an eclipse when the Moon crosses the ecliptic plane or comes close to crossing. Only then can the Moon cast a shadow on Earth (lunar eclipse) or blot out the Sun (solar eclipse).

Further Surfing:

MrEclipse.com: The ultimate resource for eclipse photography

Solar Eclipses for Beginners:

Lunar Eclipses for Beginners:

Earthview: what causes an eclipse

NASA: Amazing facts about solar eclipses

NASA: Eclipse home page

Q: What is the normal life span for a southeastern gray squirrel? For the common gray squirrel?

A: The gray squirrel (native to North America, introduced to Great Britain, Ireland, and South Africa) usually lives about three years. It can live ten years and, in captivity, up to 15 years.

By the way, the gray squirrel's hind legs are double jointed and it can leap 20 feet (6 m) from branch to branch.

Further Surfing:

Molly Gallagher: Eastern Grey Squirrel-how to care for the young

(Answered Sep. 19, 2003)

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