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Boat windshields shatter rarely,
no monthly eclipse, gray squirrels live three 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 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
Boat windshields can break spontaneously. [NOAA]
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 Surfing:
WonderQuest: How safety-glass is made
Glassig, Inc.:
Tempered glass breakage
No monthly eclipse
Q:
Why isn’t there a solar eclipse every month? — Saima, Islamabad Pakistan
A total solar eclipse, photographed at Chisamba, Zambia on Jun. 21, 2001.
At the time Espenak took the picture, the Moon had obscured 36% of the Sun’s
diameter. [©2001 by Fred Espenak, www.MrEclipse.com]
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
Gray squirrels live three years
Q:
What is the normal life span for a southeastern gray squirrel? — Gwen,
Virginia Beach, Virginia
Q: For the common gray squirrel? — Gary, Spokane, Washington
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
horizontally 20 feet (6 m).
Further Surfing:
Molly
Gallagher, Licensed Wildlife Rehabilitator: Eastern Grey Squirrel—how to care
for the young
(Answered Sep. 19, 2003)
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