Thursday 29 January 2015

Spontaneously Exploding Pint Glasses

Right, time to move away from the beer and back to the science - but I'm going to ease myself into it with a post on pub-based science.

This one has Glenmoray in it.

I have to admit to a whiff of paranoia when I
picked this glass up specifically for this post!

The classic British pint glass, the "nonic", is in my humble opinion one of the great pieces of design. It holds beer, you can stack them when empty (making a bartender's life so much easier), and the little bulge both improves grip and prevents the lips of two glasses from coming into contact, a little hygiene feature that nobody usually notices.

And they're tough, seriously tough. You can plunge a nonic into icewater straight from the glasswasher (at around 70C) and it'll cope with the thermal shock. You can stack them 30 high and they'll survive (not recommended for safety reasons, but you can).

The reason for this toughening is in the heat treatment they go through when they're manufactured. Each glass is slowly heated to around 720C and then rapidly cooled with chilled air. As the glass cools the inside (actually inside the glass, not "inside" as in where you put the beer) cools more slowly, leaving the whole structure under compression - it literally squeezes itself, resulting in a tougher glass that won't crack easily, and when toughened glass does break it does so in spectacular style, shattering into hundreds of small cubes and relatively few dangerous shards, meaning there's a much lower chance of serious injury due to accident or malice.

There is a downside to this heating process though, and it's all down to something called nickel sulphide (NiS).  This is a common contaminant in making glass and is very tricky to remove, especially when you're trying to make reasonably cheap glasses for pubs. It exists in two forms, or "phases", alpha-phase and beta-phase. The beta-phase is the everyday form, stable at normal temperatures, with the alpha-phase being stable at temperatures over 715C - crucially, this is just under the temperature required to harden the glass.

So when we rapidly cool our pint glass and it hardens we can end up with little bits of alpha-phase NiS stuck inside it, squeezed into not popping back into beta-phase because it would have to expand by about 3% to do so.

Now these "NiS inclusions" as they're technically called, will eventually pop back into the beta-phase, but it can take years. When they do though, they can cause an enormous amount of stress - up to 125,000 pounds per square inch, concentrated into a tiny area inside the glass. For comparison, the water pressure in the Mariana Trench, the deepest part of the oceans, is "only" 15,000psi.

This is easily enough to cause a crack to shoot through the glass at supersonic speeds, blowing the entire glass to pieces.

It's not a predictable effect, it can happen at any time. A slight shock to the glass (literally just flicking it with a fingertip in the right place) or even an ultraviolet ray from the Sun can set it off. Or it can just happen spontaneously, and there's plenty of reports of people's pint glasses simply exploding while sitting on a table. It's an interesting complaint to deal with as a bartender, in a decade behind bars I had to deal with incredulous customers twice and saw it happen to unattended glasses a couple of times.

The failure rate is small enough to be acceptable in pubs, especially given the other safety benefits of toughened glass, but this is a serious problem in glass being used for construction, such as modern skyscrapers and funky new bits of architecture, and is known as "glass cancer".

I believe this video is probably the result of a NiS inclusion - I'll buy a couple of pints for the first bartender to catch it on CCTV, statistics suggest it must happen weekly in the UK alone.

Further reading:
Nicely readable detail: Glass Breakage & Nickel Sulphide Inclusions  

NSFW but we're talking your-pint-exploding language: Real Life Account

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