How is glass tempered?

Glass is tempered by...

When the glass has been made, it is heated again until it starts so go soft and the surfaces are then cooled quickly with air blasts. Since glass is a poor conductor of heat, the surfaces cool quickly and solidify whilst the inside is still liquid. When the inside cools and solidifies, it contracts, pulling on the outer layers. As the outer layers are solid, they cannot be pulled very far, so the pulling forces stay in the glass. The outer layers of the glass are being compressed...

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