The process of becoming hard or making something hard, either physically through cooling or chemically through treatment.
From Old English heardian, from heard 'hard,' ultimately from Proto-Germanic *harduz. The metallurgical sense developed alongside blacksmithing techniques, while figurative uses (hardening of attitudes) emerged in the 16th century.
In metallurgy, hardening often involves a paradox - heating steel to extreme temperatures before rapidly cooling it actually makes it harder than if it cooled slowly. This process, called quenching, traps carbon atoms in place, creating a stronger crystalline structure!
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