Traditional glass blowing

Most of the things made of glass that we use are made by a method called blowing. In this method, glass is first prepared by melting silica (from sand) and soda (sodium carbonate) in a glass furnace. This produces a thick, yellow-red liquid. Blobs of the liquid are picked up from the furnace with a long metal pipe. The glass blower then blows air into the blob through a pipe (just like you blow soap bubbles). The air enters the blob and makes it swell. The blower turns the glass around while blowing it till it gets the shape it wants.

Venice is the most famous place in the world for making blown glass object. Here's a video of a Venetian blowing a glass cat:

Float glass

Making really flat glass has always been a challenge though. One method was to blow the glass into a huge cylinder. Then the cylinder was cut and flattened out before it cooled. However, you could only get small pieces of flat glass this way. If you've seen the windows of an old building, you'll see that the window is actually made of small glass squares held in a wooden frame.

In the 1950s, Sir Alastair Pilkington of Britain devised a way to make really big sheets of flat glass. The idea was to pour glass onto a really smooth surface, and then to wait till the glass cooled. (This is just like how barfi is made in a sweet shop.) The challenge was to find a surface that was really smooth.

You can't use a solid surface because none is ever smooth. If you look at a seemingly smooth solid under a microscope, you'll see tiny bumps and dents. This will make the glass surface rough, and that in turn will make it vulnerable to breaking. So you've got to use a liquid surface.

The right floating material

Perfectly still water has a very smooth surface. But sadly, glass sinks in water. So you need a liquid that's denser than glass. After lot of experimenting, Sir Alastair found the perfect one. And it was something you'll never guess - molten tin!

Molten tin is denser than glass, remains molten even after the glass had cooled (thus the surface is always smooth) and is quite cheap to make. Commonly, the tin bath is about 50 metres long and 3-4 metres wide. Molten glass (at 1100^oC) is poured through a slit at one end and flows across the tin. By the time it reaches the end, it cools to about 600^oC, by which time it has cooled to a solid. At the other end, it is pulled onto rollers, and cut into big sheets.