I've read that tempered glass work by heating and then rapidly cooling the glass so that the outer layer 'freezes' (stops expanding/contracting) but the core contracts further as it cools. I'm told that this causes compression in the outer layer and tension in the core but I can't see how this could be.

Surely, if the still-contracting core were 'pulling' on it, the outer layer would be under tension. For the outer layer to be compressed, surely the core must continue to expand after the outer layer 'freezes', pushing on the outer layer.

Any help would be greatly appreciated.

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    $\begingroup$ Wikipedia is pretty clear on this. If you haven't seen videos of "Prince Rupert's Drop" , you should. It's pretty cool. $\endgroup$ – Carl Witthoft May 23 '14 at 23:13
  • $\begingroup$ OK, just watched SmarterEveryDay's video on it and updated my question. Am I along the right lines? Thanks or the suggestion. $\endgroup$ – Whonut May 24 '14 at 10:40
  • $\begingroup$ Yep, I think you've got it :-) $\endgroup$ – Carl Witthoft May 24 '14 at 11:13
  • $\begingroup$ Brilliant. Thanks for your help. Should I do anything to my question since it's solved or is it OK the way it is? I'm always mindful of disobeying some sort of SE rule :P $\endgroup$ – Whonut May 24 '14 at 11:39
  • $\begingroup$ You could post your conclusions as an answer and then (after a required time period) check it off as accepted. $\endgroup$ – Carl Witthoft May 24 '14 at 12:12

Just watched Smarter Every Day's video on Prince Rupert's Drop, I think it all makes sense now.

I was under the impression that the core was the thing compressing the surface by pushing into it, which didn't make sense to me because the core contracts on cooling.

Instead, it sounds like the core is causing the surface to 'compress itself' by pulling the surface particles inwards and thus closer together. Compressive force is exerted by surface particles on other surface particles, not by the core.

The core is put under tension because it is attempting to contract but the solid surface won't let it.

Destin explains it better than me, watch the video if it still doesn't make sense :)

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