Glass Fun Fact: Prince Rupert's Drops and Tempered Glass

Much of functional architectural glass applications - like shower doors, table tops, car windows, skylights, etc. - requires the use of safety glass - often glass that has through a process called tempering.  Glass is pretty wonderful stuff, but it does have some bad habits.  First, it is brittle and has a tendency to crack when struck or heated unevenly.  Second, shards of glass are really sharp and pretty dangerous.  Tempered glass solves both of these problems simultaneously.  Glass is much stronger in compression than tension.

Float Glass process

If you can cause the surface of the glass to become compressed relative to the interior of it, you can harden it by a factor of up to 10.  There are a couple of ways to do this.  One is to heat the glass and then cool it very quickly.  The surface of the glass will cool much more rapidly than the interior.  The slow cooling of the interior causes it to want to contract more than the surface, placing the surface under considerable compression.   This strengthens the glass and makes it more scratch-resistant and heat-resistant in the bargain.  Another method is chemical tempering, in which sodium atoms on the surface of the glass are replaced with potassium atoms, which are significantly larger.  This also puts the surface in compression, and can be done with glass of complicated shapes that would not survive heat tempering.

One interesting effect of the tempering process is that tempered glass doesn’t just crack.  When tempered glass encounters a big enough stress, it shatters into small granules.  If the integrity of the surface of the glass becomes compromised, the interior, which is under huge tension, will disintegrate.  This is much safer than big dangerous shards, but does make the glass suddenly an awful lot harder to see through.  This is one reason why the windshield of your car is not made with tempered glass, but laminated glass.  Laminated glass is made by bonding two or more layers of glass with an ‘interlayer’ of plastic film which will hold the pieces together if the glass should crack.

Tempered glass is an extremely useful material, but it does demand some planning.  Because of tempered glass’ all-or-nothing breakage, it must have been already cut to the size, shape, and already have any holes cut out before the tempering process.  There’s no cutting the glass down to fit afterwards. 

Tempering as an industrial process started in the 20th century, but it was a party trick far before that.  One of the first examples of tempered glass is something called Prince Rupert’s drops (or balls), supposedly named after the Bavarian prince who brought it to the attention of the Royal Society.  If you let a blob of molten glass drip into a bucket of water, it will form an extended teardrop shape with interesting properties.  The bulbous end of the drop is tempered and can withstand extreme force, such as hitting it with a hammer.  The tail, however, is very delicate, and if broken, the whole thing will shatter into tiny pieces.  

When you think about it the stuff is a bit odd, but that’s glass for you.  It’s odd stuff.

A local DC morning news television station visited Erwin Timmers' recycled glass art class, and he demonstrated the explosive properties of tempered glass by shattering a thick panel (jump to 2:47).
Click on each line below to jump to previous Glass Fun Facts postings:
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