1102. What is the difference between crystal and glass?
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. 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 and increased damping. That's why normal glassware is relatively soft and emits a dull sound when you rap it; it experiences lots of internal friction. The chemicals added to silicon dioxide to make "crystal" glassware include lead oxide, which makes the glass easier to melt and soft enough to cut and shape easily. However, lead "crystal" glassware has less internal damping than ordinary glassware and emits a ringing tone when you rap it because it experiences very little internal friction.