I am a car wash owner.  From time to time a customer would claim that
we cracked thier windshield.  I was perplexed because there is
absolutely nothing in my car wash that would crack a windshield.  I
consulted a local windshield repair expert and interestingly he said
that small, sometimes even imperceptable imperfections in the glass
(small rock chips, etc.) can cause this.  Basically, if there's a
small chip or crack, and the car heats up inside and then you spray it
with cold water, the change in temperature can flex the glass and turn
a small crack or chip into a big crack.  Unfortuntely, when I try to
explain this to my customers, they say "yeah, right."  Hence, here I
am on Google answers.  My question is this:  Can you find any 3rd
party articles or dialogue on this topic that I might be able to point
to to at least open up the possibilities in my customer's minds that
it could have been something like that.

Dear mikeyrosenrosen-ga;

Thank you for allowing me an opportunity to answer your interesting
question. Look no more...What you are referring to is widely
understood in the scientific world and it is known as ?thermal
stress?. That?s when liquid of an opposing temperature enters into an
already weakened area of the surface (glass, in this case) and causes
it to expand or fluctuate to such a degree that an additional or more
profound defect occurs along the defect's most vulnerable edges. Here
are some excerpts from articles about thermal stress on glass
surfaces:

?If you have any small dings or cracks get them filled. As the weather
drops your windshield must deal with the "thermal stress" of a warm
interior and a cold exterior. A small ding can turn into a very
expensive, spreading crack.?
SMART LIVING NEWS
http://www.smartlivingnews.com/article.html?id=378&date=2005-01-15


?Heat also can damage windshields. Cold water on a hot windshield with
a small crack in it can make that crack much worse.?
NEWSHERALD.COM
?Some sweat when the heat is on?
http://www.newsherald.com/archive/local99/la080199.htm

According to this article in FLEET OWNER a company that produces a
washer water heater for automobiles says that its product does not
crack cold windshields because of it?s carefully ?measured and
controlled fluid flow?. Obviously the implication here is that
unmeasured and uncontrolled hot fluid on a cold windshield CAN, or
perhaps in some cases, DOES crack cold windshields:

?According to Microheat, HotShot produces hot washer fluid in about 30
sec. and sprays it on the windshield in intermittent spurts every six
to ten sec., for a total automatic washing cycle time of two and a
half minutes, or until the driver shuts it off. Thanks to the measured
and controlled fluid flow, it won't crack cold windshields, either,
the company says, even though it uses a burst of steam early in the
cycle process to unblock any frozen nozzles in the system and free
frozen wiper blades.?
FLEET OWNER
?In hot water?
http://fleetowner.com/mag/fleet_hot_water/

Here is a very revealing discussion from an extremely reliable source
? The Cluster Physics Research Laboratory at the University of
Virginia:

?One morning you wake up and there is a thick layer of frost on your
car. Rather than scrape the frost off of your windshield you decide to
pour a bucket of hot water on it to melt the frost. If your windshield
has a small crack in it this may cause the crack to run (become
longer). Why?

Answer: When you pour hot water on the cold windshield it will expand
where the hot water is being poured. This will cause stress in the
windshield, which may cause the crack to run.

Why: Again, whether or not the crack runs will depend on where you
pour the water relative to the location of the crack.?

UNIVERSITY OF VIRGINIA
THE CLUSTER PHYSICS RESEARCH LABORATORY 
PHYSICS 105 - HOW THINGS WORK - FALL, 1999
PROBLEM SET #9 - THERMODYNAMICS - SOLUTIONS
http://rabi.phys.virginia.edu/105/1999/PS9a.html

So you see, it?s not some strange phenomenon about hot and cold and
the fact that it?s glass, this is the same priciple that creates
potholes in the road. When hot and cold cause cracks or defects on the
asphalt to expand or contract the damage becomes stressed and
eventually more profound. Because asphalt is a bit more ?rubbery?
(oil, tar, etc) it can withstand quite a bit more stress that
something much more brittle and unforgiving ? like glass, which tends
to simply crack like an egg shell. As a matter of fact, if you want to
use an analogy with your customers just ask them if they ever taken a
cold egg from the refrigerator and dropped in it hot water. If there
are any weak places or barely visible cracks, they will almost
inevitably become more visible and more profound. Again, ask them if
they?ve ever poured warm tea or soda over cold ice. The cracking and
popping sound is thermal stress shattering the ice cubes. This is
thermal stress.

US DEPARTMENT OF ENERGY ? NEWTON BBS
ASK A SCIENTIST
http://www.newton.dep.anl.gov/askasci/gen01/gen01518.htm

I hope you find that my research exceeds your expectations. If you
have any questions about my research please post a clarification
request prior to rating the answer. Otherwise, I welcome your rating
and your final comments and I look forward to working with you again
in the near future. Thank you for bringing your question to us.

Best regards;
Tutuzdad ? Google Answers Researcher


OTHER INFORMATION SOURCES




SEARCH STRATEGY


SEARCH ENGINES USED:

Google ://www.google.com




SEARCH TERMS USED:

THERMAL STRESS

HOT COLD

TERMMERATURE

WATER

CRACK 

DAMAGE

WINDSHIELD

GLASS

Great answer!  Exactly what I was looking for.

What you are dealing is a form of thermal breakage.  Specifically,
glass that breaks due to a rapid change in temperature is known as
thermal shock.  Glass expands when heated and contracts when cooled. 
Expansion and contraction puts stress on the glass.  Typically, glass
with no imperfections will be able to withstand the forces placed on
it and will not break.  However, any imperfection, such as a chip,
nick, small crack can cause will create an area of weakness within the
glass that cannot withstand the thermal forces, and the glass may
crack significantly.



Thermal breakge is discussed here by glassonweb, a resource for those
in the glass industry:
http://www.glassonweb.com/articles/article/181/

My recommendation is to not wait until after a customer's windshield
cracks to let them know about thermal breakage.  A car wash I used go
to had a sign created and that warned customers that a ding in their
windshield on a hot day could turn into a large crack after being
washed.  If they are aware of the idea of thermal breakage from the
beginning, they won't feel like you are pulling a fast one after it
happens to them.  You will have also done your part by warning them of
the risks before hand.

If your customer's really don't believe you, prove it.  Keep on hand
drinking glasses that you have either hit or squeezed so they have a
crack that has already started.  The glasses should have thin walls to
ensure this will work. Pour hot water into the glass (the hotter the
better).  If your car wash has a coffee machine, the hot water from
this would be perfect.  Let the hot water sit in the glass for 15-30
seconds, enough time to completely heat the glass.  Then, dump out the
hot water and immediately pour in ice water.  You should ata minimum
see the crack in the glass grow, and may even see the glass literally
shatter.  You may even see the glass shatter just from pouring in the
hot water, as the temperature change from room temperature to hot is
enough to break glass.

If they still don't believe you, tell them to call a windshield repair
company of their choice.  Any windshield or glass company should be
able to validate the power of thermal temperature changes in glass.