Glass
Last updated: June 19, 2007.
Now you see it, now you don't. Glass is
a bit of a
riddle. It's hard enough to protect us, but it shatters with
incredible ease. It's made from opaque sand, yet it's completely
transparent. And, perhaps most surprisingly of all, it behaves like a
solid material... but it's really a sort of weird liquid in disguise!
You can find glass wherever you look: most rooms in your home will
have a glass window and, if not that, perhaps a glass mirror... or a
glass lightbulb. Glass is one of the world's oldest and most
versatile human-created materials. Let's find out some more about it.
Photo: Glass can be coloured or "stained" by
adding metallic compounds while it is molten, and
different metals give the separate segments of glass their different
colors. This modern stained-glass window, in the church of St Mere
Eglise, Normandy, France,
commemorates the arrival of the US Army 82nd Airborne paratroopers as
they were dropped over the town on D-Day, June 6, 1944.
Picture by courtesy of US Army.
What is glass?
Believe it or not, glass is liquid sand. You can
make glass by heating ordinary sand (which is mostly made of silicon
dioxide) until it melts and turns into a liquid. You won't find that
happening on your local beach: sand melts at the incredibly high
temperature of 1700°C (3090°F).
When molten sand cools, it doesn't turn back into
the gritty yellow stuff you started out with: it undergoes a complete
transformation and gains an entirely different inner structure. But it
doesn't matter how much you cool the sand, it never quite sets into a
solid. Instead, it becomes a kind of frozen liquid or what materials
scientists refer to as an amorphous solid.
It's like a cross
between a solid and a liquid with some of the crystalline order of a
solid and some of the molecular randomness of a liquid.
Glass is such a popular material in our homes
because it has all kinds of really useful properties. Apart from
being transparent, it's inexpensive to make, easy to shape when it's
molten, reasonably resistant to heat when it's set, chemically inert
(so a glass jar doesn't react with the things you put inside it), and
it can be recycled any number of times.
Photo: Glass can be used to recycle other
materials.
Uranium glass has an unusual yellow-green color and glows in
ultraviolet light.
These glass pieces were made using waste uranium from the cleanup of
the
Fernald uranium processing plant near Cincinnati, Ohio, USA.
Vitrification (turning a material into glass) is one way to dispose of
nuclear waste safely.
Picture by courtesy of US Department of Energy.
How to make glass
Photo: Lifting a pane of glass into place.
Picture by Kelly Barnes courtesy of US Navy and
Defense Visual Information Center.
When US scientists tested a prototype of the
atomic bomb in the New Mexico desert in 1945, the explosion turned
the sand in the immediate area of the impact into glass. Fortunately,
there are easier and less extreme ways of making sand—but all of
them need immense amounts of heat.
In a commercial glass plant, sand
is mixed with waste glass (from recycling collections), soda ash
(sodium carbonate), and limestone (calcium carbonate) and heated in a
furnace. The soda reduces the sand's melting point, which helps to
save energy during manufacture, but it has an unfortunate drawback:
it produces a kind of glass that would dissolve in water! The
limestone is added to stop that happening. The end-product is called soda-lime-silica glass. It's the ordinary glass we
can see all around
us.
Once the sand is melted, it is either poured into
molds to make bottles, glasses, and other containers, or "floated"
(poured on top of a big vat of molten tin metal) to make perfectly flat
sheets of
glass for windows. Unusual glass containers are still sometimes made
by "blowing" them. A "gob" (lump) of molten glass is wrapped
around an open pipe, which is slowly rotated. Air is blown through
the pipe's open end, causing the glass to blow up like a balloon.
With skilful blowing and turning, all kinds of amazing shapes can be
made.
Glass makers use a slightly different process
depending on the type of glass they want to make. Usually, other
chemicals are added to change the appear or properties of the
finished glass. For example, iron and chromium based chemicals are
added to the molten sand to make green-tinted glass. Ovenproof borosilicate glass (widely sold under the
trademark PYREX®) is
made by adding boron oxide to the molten mixture. Adding lead oxide
makes a fine crystal glass that can be cut more easily; highly prized
cut lead crystal sparkles with color as it refracts (bends) the light
passing through it. Some special types of glass are made by a
different manufacturing process. Bulletproof
glass is made from a
sandwich or "laminate" of multiple layers of glass and plastic bonded
together.
Toughened glass used in car windshields is made by cooling molten
glass very quickly to make it much harder.
Photo: Borosilicate glass, such as this PYREX®
jug, can withstand extreme
changes of temperature, unlike normal glass, which shatters.
PYREX® is a registered trademark of Corning Incorporated.
Using glass
We all know glass is used in windows, but there
are literally dozens of other uses. Here are just a few of the
applications of glass you can read about on Explain that Stuff!:
It's pretty amazing that something so simple can
have so many important uses!
