MadSci Network: Engineering

Re: When a vacuum is created in a glass, why doesn't the glass collaspe?

Date: Thu Apr 19 11:52:30 2001
Posted By: Jeff Yap, Materials Engineer
Area of science: Engineering
ID: 985905217.Eg

Hi Ethan,

You've asked a very good question, and it shows that you have a pretty 
solid understanding of how vacuum and atmospheric pressure behave.

The short answer to your question is that the glass jar didn't break 
because it was strong enough to withstand the kind of pressure that Boyle put 
on it.

Why the glass was strong enough is a bit more complicated, but still 
pretty cool.  Glass typically has a tensile strength of about 2000 to 
25000 pounds per square inch (psi).  The compr
essive strength is normally much higher than that.  Atmospheric 
pressure (On average, at sea level, at room temperature) is 14.7 psi 
(101.3 kPa or 760 mmHg).  So at full vacuum, the jar would have 15 psi 
compressing in on it from all directions, which is considerably less than 
the pressure needed to crush it.  Additionally, the air pump used, (built 
by Hooke, who also did a lot of cool science stuff) 
probably wasn't able to suck out all of the air from the jar.  Even modern 
vacuum pumps like Turbopumps and Cryopumps can't suck ALL of the air out 
of a container...

In order to crush (also known as "implode" because it catastrophically 
fails inward) a glass jar using vacuum, (I highly recommend AGAINST doing 
this...) you'd have to do one or more of the following:
1) Make the walls of the jar very very thin.
2) Add scratches, chips, and dents to weaken the glass.
3) Apply extra atmospheric pressure.
4) Apply a very high vacuum in the jar.
5) Make the jar non-cylindrical.  (Cylin
drical structures are very strong because of their shape.  To see this 
better, get a two liter soda ("Pop" for most people in your region...)
bottle, and try to collapse it by sucking the air out of it.  You'll 
notice three things: 1) it collapses by having the sides cave in (also 
known as "buckling"), 2) it's easier to do if you dent or bend the sides 
with your hands before pulling the air out of it, and 3) it's hard on your 

I hope this helps!  Let us know if you have any more questions.

Jeff Yap
Mad Scientist

Cool sites:
Encyclopaedia Britannica Online
Encyclopedia - American Glass

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