 The
"crystal" that's used in fine glassware is actually a glass, but
it is chemically different from the glass that's used in more
common glassware.
Both materials are formed by melting
together a mixture of silicon dioxide (also called quartz or
silica) and other chemicals and both are glasses, meaning that
their atoms are arranged haphazardly and not in the crystalline
lattices of such materials as salt or sugar. But the chemicals
that are added to silicon dioxide to make normal glassware -
sodium oxide and calcium oxide - make the glass easier
to melt and work with at the expense of strength. That's why
normal glassware is relatively soft, emitting a dull sound when
you rap it because it experiences lots of internal friction.
In
contrast, the chemicals added to silicon dioxide to make
"crystal" glassware include lead oxide, which makes
the glass easier to melt but doesn't weaken the glass nearly so
much. Lead "crystal" glassware is relatively hard and
emits a ringing tone when you rap it because it experiences very
little internal friction. |
