<strong><a href="http://www.cmog.org/article/does-glass-flow" target="_blank">Does Glass Flow?</a> <img src="../scipics/_ext.png"
/></strong>
Speaking of time, just how long should it take—theoretically—for windows to thicken to any observable extent? Many years ago, Dr. Chuck Kurkjian
told me that an acquaintance of his had estimated how fast—actually, how slowly—glasses would flow. The calculation showed that if a plate of
glass a meter tall and a centimeter thick was placed in an upright position at room temperature, the time required for the glass to flow down so as to
thicken 10 angstrom units at the bottom (a change the size of only a few atoms) would theoretically be about the same as the age of the universe:
close to ten billion years. Similar calculations, made more recently, lead to similar conclusions. But such computations are perhaps only fanciful.
It is questionable that the equations used to calculate rates of flow are really applicable to the situation at hand.
. . .
It is worth noting, too, that at room temperature the viscosity of metallic lead has been estimated to be about 10 to the 11th power, (1011) poises,
that is, perhaps a billion times less viscous—or a billion times more fluid, if you prefer—than glass. Presumably, then, the lead ca[n]ing that
holds stained glass pieces in place should have flowed a billion times more readily than the glass. While lead ca[n]ing often bends and buckles under
the enormous architectural stresses imposed on it, one never hears that the lead has flowed like a liquid.
. . .
When all is said and done, the story about stained glass windows flowing—just because glasses have certain liquid-like characteristics—is an
appealing notion, but in reality it just isn't so. |
