Sulfur contamination of iron and steel make them `` red short '' , so that at red heat they become `` crumbly '' instead of `` plastic '' .

This is called `` work-hardening '' . The next step is to `` draw the temper '' or `` temper '' the steel , so that it will not break like glass . After the `` heat-treat '' and `` temper '' , stones are used to grind the blade edge to make it sharp .
