How Much Stress Can One Take?

Artist Leana Quade has video of her in Graduate school thesis project where she uses a ratchet strap to create tension, anxiety and stress (with nerves of steel) in finding how far a piece of tempered sheet glass bends before exploding. This thesis show video won 2nd place at the International Student Exhibition at the 2016 G.A.S. conference.

Release _ by LQuade from LQ on Vimeo.

“All New” Glass Fun Facts: Part 2

The WGS Blog returns to provide More.  Glass.  Trivia!

As far as the early history of glass making in Britain goes, the Romans brought the technology with them. This led to the European-wide spread of glass manufacture. British history records glass “Manufacture” dating back to the 13th century when “Broad Sheet” glass can be located to the areas around Sussex and Surrey. In the meantime the Venetians had thrived as glassmakers, as their glass became popular due to its brilliance and creative form. By 1330 the French had also developed “Crown Glass”. This took until the 17th century to be produced in England, in London.

 In England in 1676 George Ravenscroft invented “Lead crystal” by introducing lead oxide to the glass which took on a more brilliant appearance.

 The 17th century brought a new glass process from France, “Plate glass”, a term still used today. This was a process of pouring molten glass onto a table and then rolled. Once cold, the glass was ground under large grinding disks until optically smooth, making it perfect for mirrors. The French had legislated heavy duties on imported glass products which made it impossible for the Venetians to Export, and also offered generous incentives to any Venetian willing to work for them. By the 18th century this technology was being used in England at Ravenhead, producing the first English Polished Plate.

Crystal-Palace-general-view1

The Crystal Palace was a cast-iron and plate-glass building originally erected in Hyde Park, London, England, to house the Great Exhibition of 1851

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

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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:
Glass Fun Facts: Gaffer/Composer
More Glass Fun Facts: Bullseye Glass

Float Glass Fun Facts
Glass Fun Facts – Shattered Glass Predicts Weather
Why is Glass Transparent?