By Harold McGee
As we’ve seen, heat conduction in a solid proceeds either by the movement of energetic electrons, or by vibration in crystal structures. A material whose electrons are mobile enough to conduct heat well is also likely to give up those electrons to other atoms at its surface: in other words, good conductors like metals are usually chemically reactive. By the same token, inert compounds are poor conductors. Ceramics are stable, unreactive mixtures of compounds (magnesium and aluminum oxides, silicon dioxide) whose covalent bonds hold electrons tightly. They therefore transmit heat slowly by means of inefficient vibrations. If subjected to the direct and intense heat of the stovetop, ceramics can’t distribute the energy evenly. Hot areas expand while cooler areas do not, mechanical stresses build up, and the utensil cracks or shatters. This is why ceramics are generally used only in the oven, where they encounter only moderate and diffuse heat, or are applied in thin coatings on the surface of metals, so that the metals can do the job of distributing the heat evenly.