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.