May 23, 2006 > TechKnow Talk by Todd Griffin Is glass a liquid
or a solid? A very old window, like hundreds of years old, is
thicker at the bottom than the top. This proves that glass is a
liquid, right? It just flows very, very slowly. When the
TechKnow Guy was in high school (and dinosaur meat was on the
cafeteria menu) he was taught that glass is a viscous fluid,
specifically a supercooled liquid. However, most scientists now
agree that glass is in fact an amorphous solid. Nonetheless, the
myth of the liquid nature of glass has become so widespread that
some science textbooks still include it as fact, and many people
still believe it. Let's start by addressing the question of
fluidity or flow in glass. It is true that some glass windows in
very old buildings, European churches for example, are thicker
at the bottom than at the top. Some tour guides in these
buildings have propagated the "flowing glass" theory to explain
this phenomenon. After all, it does make a nice story. To
understand the true cause, we