Where does Glass come From?

Imagine living in a world without glass – or even a house without glass? No natural light and no view of the outside without being harshly exposed to the elements as well as letting in all kinds of vermin. – hard to say which one’s worse! Glass comes in many different sizes and styles, and has been around since as early as 3500 BC!

Glass provides a hard, durable, and aesthetically pleasing architectural material for structures of all kinds. I think we can all agree that appropriate amounts and placement of glass gives off a “rich” and classy vibe to a building, am I right? But one thing many of us don’t know is how glass is made.

So where DOES glass come from?

So how does perfectly smooth and flat sheets of glass of all sizes come to be? You don’t really hear of it being mined like metals or oil, and it certainly doesn’t grow off trees. So where in the world does this stuff come from? Well, perhaps some of you have been told that it’s made from sand – and you’re right.

But that’s not the only ingredient! I mean, how does grainy, sandy sand turn into perfect, smooth glass? There are many different kinds of glass for many different purposes, but we’ll stick to what is called Soda lime glass. About 90% of manufactured glass today is this Soda lime glass, and it’s the kind of glass used for window panes, glass containers and beverage bottles etc.

Ingredients used for the production of Soda lime glass include sodium carbonate, lime, dolomite, silicon dioxide, aluminum oxide, and other fining agents in smaller quantities. All these technical names might confuse you but you should know that the chief constituent of sand is silica (silicon dioxide), often in the form of quartz.

Up to 75% of the resulting Soda lime glass is silica – depending on the amount of cullet used, which is probably why sand is the first thing that many people think of when asked what glass is made of. Instead of using the pure chemicals to make glass, manufacturers simply use the cheaper minerals such as sand, which we covered above, and trona, which is the primary source of sodium carbonate.

There are 2 main categories of Soda lime glass: windows and containers. Glass used for windows are also called “float glass“, named after the process with which it’s created. Glass made for containers is made to be more durable against water due to its prolonged contact with liquids, and therefore has a slightly different and more durable chemical makeup. (Watch an interesting video clip on how glass bottles are made. This is NOT the Pilkington process – see below!)

You should know that prior to the discovery of the float glass or Pilkington process, there were processes such as crown glass and blown glass, which were the main methods used to produce glass all the way up until the 19th

Top left pane is float glass - note the distorted reflections on the rest

century. These methods had many limitations and imperfections – these include size, thickness uniformity, transparency, and flatness.

The float glass process, or Pilkington process, is the process that was invented by the British glass manufacturer Pilkington in the 1950’s. The process involves heating the glass constituents to about 1500 degrees in a massive glass furnace, after which it is fed into a bath of molten tin.

This bath of molten tin is the basis of this process and due to its high specific gravity, cohesiveness, and immiscibility, it is an ideal metal for this purpose. Molten glass flows on the tin bath via gravity and flattens itself out by its own weight. The glass continues to flow along the tin bath, gradually cooling off from about 1200 degrees to about 600 degrees.

It is then removed from the tin bath and is passed through about 100 meters of further gradual cooling. Finally, it is ready to be cut to size. Although there were several attempts at finding a workable “float glass” production method prior to the Pilkington process, the secret wasn’t discovered until it was learned that the key was in the exact amount of molten glass that was fed into the tin bath.

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