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How Glass is Made for Windows

Windows are our view on the world, but have you ever stopped to consider how the glass that is used in them is manufactured? We look out of windows every single day of our lives (many more times a day if you are a window cleaner!), whether we’re at home, at work, travelling in our cars or on public transport.

In the UK, glass only became the norm for use as windows in houses of ordinary people in the 17th century. Previous to this, most people who weren’t landed gentry had openings or wood shutters, but nothing that would truly protect them from the elements. For the nobility, glass windows were more commonplace in homes, but they were made from melted animal horns rather that by the process we use today.

Whether it’s going to be used for windows in a home, shop or in a car or bus the creation of a single pane of glass is a delicate process that needs to be made with extreme care. It’s an interesting process and starts with the most unlikely of ingredients. Here’s how it’s done.

What is glass made from?

Many people wouldn’t believe it if they were told that glass was actually made from liquidised sand. Before you all rush out and attempt this yourself, sand is used because it has an incredibly high melting point – somewhere in the region of 1700 degrees celsius, so you’re not going to be able to do this at home in your own oven.

What’s interesting though, is that when sand is heated up to this temperature and then allowed to cool down, it doesn’t return to its original structure. Instead, it transforms into a totally different inner structure – no matter how much you try and cool molten sand it will never properly solidify ever again, and it becomes what scientists call an ‘amorphous solid’. Something that is half way between a liquid and a solid structure, but not quite one or the other.

Making glass

If you were to go and visit a factory where glass was being made you’d see the following process. Sand is mixed with recycled glass, calcium carbonate and sodium carbonate. It’s then heated to melting point in a furnace. The latter has the effect of cooling the sand slightly and reducing the point at which it can melt, and the calcium carbonate is added to help the finished product solidify and toughen. If this wasn’t added and the glass was left at this stage, as soon as it hit water it would soften up and melt again.

The finished product is called Soda-Lime-Silica Glass and it’s the product that we see all around us, in our homes, cars and…basically anywhere with windows! If you can see out of it, chances are that’s the product you’re looking through.

The glass making process

Once it gets to this stage it’s then decided what will happen to the melted sand mixture. Some of it is taken away to be made into bottles, glassware or other containers. Sometimes a procedure called ‘glass blowing’ takes place to create products such as these.

Some of it is used to make windows. In this case what will happen is that the molten sand mixture will be poured onto flat metal sheets so that it sets in a perfectly smooth state to be used to fit into manufactured casings.

Depending on the type of glass needed and the purpose it will be used for, different chemicals or processes will be added to these initial steps to make it change. For instance, to make glass that has a green tint to it – for use in stained glass or glassware iron and chromium will be added to the mix. Lead oxide added into the mixture will create a fine crystal like glass that cuts easily and can be used for fine dining and glass ware. Bulletproof glass, which can be used for anything from cars through to shop windows, is made by sandwiching layers of glass and plastic together to form an extremely tough, unbreakable bond. Toughened glass, again used for windows and cars is made by cooling the initial mixture of molten glass down extremely quickly.