direct high heat can shatter them. Heat-
resistant forms of glass incorporate an oxide
of boron that has the effect of reducing
thermal expansion by a factor of about 3, and
for this reason are less affected by thermal
shock, though they’re still not immune.
Enamelware In utensils called enamelware,
powdered glass is fused into a thin layer onto
the surface of iron or steel utensils. This was
first done to cast iron early in the 19th
century, and today enameled metal is widely
used in the dairy, chemical, and brewing
industries, as well as on bathtubs. In kitchen
utensils, the metal diffuses the direct heat
evenly, the ceramic layer is thin enough that it
can expand and contract uniformly, and it
protects the food from direct contact with the
metal. Enamelware is reasonably durable,
though it still requires some care: the ceramic
layer can be chipped or damaged by
quenching a hot pan in cold water.