temperare

Latin temperare combined temper (an uncertain root meaning to regulate or cut) with the idea of due measure and proportion. Temperare meant to mingle in proper proportion, to moderate, to regulate. The word described the mixing of hot and cold, wet and dry, strong and weak in the proper ratios to achieve balance. The root also gave English temperature (the proper degree of heat), temperament (a person's natural mixture of humors), and temper (to moderate, or one's moderated character).

In metallurgy, tempering steel meant heating and then carefully controlling the cooling to achieve a balance between hardness and brittleness. Fully hardened steel is brittle; fully annealed steel is soft. Tempering — heating to a specific temperature below the hardening point and holding it there before cooling — achieves a regulated combination: hard but not brittle, resilient. The medieval smith who tempered a sword blade was managing the same physics that a materials scientist manages today.

In chocolate making, tempering controls the crystalline structure of the cocoa butter in chocolate. Cocoa butter can crystallize in six different forms; only one (Form V or β2) gives chocolate the desired snap, gloss, and smooth melt. Tempering — melting chocolate completely, cooling it to seed Form V crystals, then raising it to working temperature — ensures the correct crystalline form predominates. A chocolate bar that has 'bloomed' (turned gray and matte) has been improperly tempered or subjected to temperature changes.

The common thread in all tempering is precise thermal control in service of a desired material state. Steel tempering, chocolate tempering, glass tempering (strengthened by controlled heat and rapid cooling), egg tempering (slowly raising the temperature of eggs by adding hot liquid to prevent scrambling): all involve managing heat carefully to achieve a specific structural outcome.