It never works when you need it to

A couple of weeks ago my wife mistook an old glass jug of ours for a pyrex one and poured boiling water in it. The result was quite pretty, with a jigsaw of cracks across the jug rendering it incapable of holding any fluid ever again, boiling or otherwise.

That’s thermal expansion for you. Glass is a failry poor thermal conductor (in this context, anyway); the heat stays on the inside and the inside expands more quickly than the outside and the glass can’t cope with the stresses this induces. Crack.

Last week, a colleague of mine did this as a demonstration in our annual Osborne Physics and Engineering Lectures to local schools.  He chose the cheapest, lowest quality glass he could find from a well-known low-price store. He prepared it beforehand by putting it into ice to get it really cold. But would it break when the boiling water went in? No. Not in front of an audience of several hundred children.

It’s always the way – when you demonstrate things live they never quite work the way they should. But that’s what makes doing this kind of thing fun, and unanticipated failures add to the audience experience. However, during the lecture in question, we did see the successful destruction of several household objects all in the name of materials science, and that was exciting.


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