Glass
Last updated: January 22, 2009.
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: Stained glass: Glass can be colored or "stained" by
adding metallic compounds while it is molten, and different metals give the separate segments of glass their different
colors. This window is in the City Palace Udaipur, Rajasthan, India. Picture by courtesy of McKay Savage,
published on Flickr in 2007
under a Creative Commons Licence.
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 glass—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.
Is glass a solid... or a liquid?
It's a very interesting question.
The answer is both—and neither! There are widely differing opinions
on how to refer to materials such as glass that seem to be a bit like
liquids in some ways and a bit like solids in others.
In schools and in books, we tend to learn that solids all have a
fixed structure of atoms.
In fact, there are different kinds of solids that have very
different structures and not everything we describe as "solid" behaves
in exactly the same way. Think of a lump of iron
and a lump of rubber. Quite clearly they are
both solids, and yet the rubber is very different from the iron.
Inside, rubber and iron have their atoms (in
the case of iron) and molecules (in the case of rubber) arranged in
totally different ways. Iron has a regular or crystalline structure
(like a climbing frame with atoms at the corners), while rubber is a polymer (made from long chains of molecules
loosely connected together). Or think of water.
As you may have discovered, water is an almost unique solid because it
expands to begin with when it freezes. In short, not everything fits
neatly into our ideas of solid, liquid, and gas and not all solids,
liquids, and gases behave in a nice, neat, easy-to-explain way. The
exceptions are the things that make science really interesting!
Let's return to glass. Peer through a microscope
inside some glass and you'll find the molecules from which it's made
are arranged in an irregular pattern. That's why glass is sometimes
referred to as an amorphous solid (a solid without the regular
crystalline structure that something like a metal would have). You may
also see glass described as a "frozen supercooled liquid". This is
another way of saying "glass is a liquid that has never set", which is
the puzzling statement you'll sometimes find in science books. We could
say glass is a bit like a liquid and a bit like a solid. It has an
internal structure that is somewhere between the structure of a liquid
and a solid, with some of the order of a solid and some of the
randomness of a liquid.
Glass is by no means the only amorphous solid. It's possible to make
a type of water called amorphous ice that could be described as
in-between solid (water) and liquid (ice). You do this by cooling water
very quickly. The ice forms so fast that it doesn't have time to build
up its normal, crystalline structure. So what you get looks like ice
but behaves in some ways like liquid water. Other substances can be
made into amorphous solids too. Solar cells are often made from
something called amorphous silicon.
Photo: An amorphous silicon solar panel. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy
Laboratory.
