Even at
room temperature, glass appears to be a solid
in the ordinary sense of the word. However,
it actually is a fluid with an extremely high viscosity. It has been documented that
century-old windows show signs of flow. The internal friction of fluids is called viscosity.
It is a property of
fluids by which the flow motion is gradually damped (slowed)
and dissipated by heat. Viscosity is a familiar phenomenon in daily life. An
opened bottle of wine can
be poured: the wine flows easily under the
influence of gravity. Maple syrup, on the other hand, cannot be poured so
easily; under the action of gravity, it
flows sluggishly. The syrup has a higher viscosity than the wine.
Glass
is usually composed of mixed oxides based around the silicon dioxide (Si02) unit. A very good electrical insulator, and generally inert to
chemicals, commercial glass is manufactured
by the fusing of sand (silica, Si02), limestone (CaC02),
and soda (sodium carbonate, Na2C03)
at temperatures around 2552°F to 2732°F (1400 to 1500°C).
On cooling the melt becomes very viscous and at about 932'F (500°C, known as
glass transition temperature), the melt "solidifies"
to form soda glass. Small amounts of
metal oxides are used to color glass, and its physical properties can be changed by the addition of substances like lead
oxide (to increase softness, density, and refractive ability for cutglass and lead
crystal), and borax (to lower significantly
thermal expansion for cookware and
laboratory equipment). Other materials can be used to
form glasses if cooled sufficiently rapid from the liquid or gaseous
phase to prevent an
ordered crystalline structure from forming. Glasses such as obsidian
occur naturally. Glass objects might have
been made as early as 2500 B.C.E. in Egypt
and Mesopotamia,
and glass blowing developed about 100 B.C.E.
in Phoenicia. (The Handy Science Answer Book, compiled by the Science
and Technology department of the Carnegie Library of Pittsburgh)
