the surface of metals, so that the metals can
do the job of distributing the heat evenly.
Spontaneous Ceramic Coatings on Metals It
turns out that most of the metals commonly
used in kitchen utensils naturally cover
themselves with a very thin layer of ceramic
material. Metallic electrons are mobile, and
oxygen is electron-hungry. When metal is
exposed to the air, the surface atoms undergo
a spontaneous reaction with atmospheric
oxygen to form a very stable metal oxide
compound. (The discoloration on silver and
copper that we call tarnish is a metal-sulfur
compound; the sulfur comes mainly from air
pollution.) These oxide films are both
unreactive and fairly tough. Aluminum oxide,
when it occurs in crystals rather than on pans,
makes up the abrasive called corundum, and is
also the principal material of rubies and
sapphires (the gem colors come from
chromium and titanium impurities). The