Glass 101

Glass has a sandy origin: By Marshall Brain

When you look out the window to see the outside world, you are looking through glass. It could be a single sheet of glass, two or three sheets (in double- and triple-pane windows), laminated glass in a car windshield, tempered glass or thick bulletproof glass. But they all start with the same common substance -- sand.

Glass is basically liquefied sand that has re-solidified in a way that is transparent. Glass does, occasionally, form in nature. For example, if a bolt of lightning strikes the top of a large sand dune, the intense heat of the electricity as it passes through will melt the sand and form a fulgurite (pictured below). People dig the ragged rods out of the dune and collect them as novelties.



A far more common way to create glass is in a factory that is making something like plate glass, glass bottles or light bulbs. The raw ingredients of industrial glass are quartz sand, soda ash (also known as sodium carbonate), lime (also known as calcium oxide) and additives like magnesium and aluminum. The soda ash cuts the melting temperature of the sand in half, meaning it is much less expensive to make glass.

These ingredients, along with crushed recycled glass, go into a furnace for melting. The recycled glass makes it even easier to melt the glass, and lowers the cost again. The melting temperature is still amazing, approaching 3,000 degrees Fahrenheit. Getting everything melted and mixed can take a day and is done in a brick-lined chamber that runs continuously, sometimes for years, until the bricks wear down and have to be replaced.

Once the glass comes out, it can go into moulds to be blown into bottles, or it can be floated on molten tin to create flat window glass. To make long, straight fluorescent tubes, it comes out of a bottom of the furnace in the tube shape, curves to horizontal and then cools over a long run of factory floor.

Most glass that goes into windows, bottles and bulbs is annealed glass. It is cooled slowly to keep it from shattering and then put to use. The problem with annealed glass is that it forms large, sharp shards that look and act like knife blades when it breaks. Much safer is tempered glass, which shatters into relatively harmless tiny bits. Glass is tempered either by heating and rapid cooling, or with chemicals. Tempered glass is used in everything from sliding glass doors to car windows, because it is much stronger and safer than annealed glass.

A car's windshield, however, is a special case. It wouldn't really be helpful to have a windshield shatter into tiny bits and fly into the driver's face if a rock hit it. So a windshield is actually made of two sheets of glass with a thick layer of plastic in between. Even if a bowling ball were to hit the windshield, chances are that the plastic layer is strong enough to catch it and keep it out of the car. Most of the broken glass inside the car will stick to the plastic as well, keeping it away from passengers.

Bulletproof glass is a lot like a windshield. But the inner plastic layer is much thicker and stronger. Made of a durable plastic like Lexan, the glass layers keep the plastic from scratching while the plastic layer stops the bullets from penetrating. Or multiple layers of glass-plastic sandwich can do the same kind of thing. A bulletproof window can be anywhere from two to five inches thick.

Then there is tinting. If you have ever looked edge-on at a glass tabletop, you have probably noticed that the glass is green. If you had a glass window two feet thick, that green colour would be obvious. But most glass is so thin that you don't even notice the colour. The green tint comes from a small amount of iron or iron oxide in the original sand. You can take the tint in two different directions. If you remove the iron, you get a glass tabletop that is clear when you look at the edge. The glass has no tint. Or in a car, you may want tinted glass to keep the sun out. Adding iron will increase the tint.

The very best feature of glass in this day and age is its ability to be recycled. Glass can be melted down over and over again without any degradation. Or, if it is left in the environment, glass is eventually broken, crushed and turned back into sand.