How is Glass Made?

How is glass made from sand? How is glass manufactured? How is tempered glass made? If you're interested in how glass is made, this is the guide for you.

How It’s Made How It’s Made
13 June 2022

Glass is everywhere. From our windows to the contents of our kitchen cupboards and even perching on our noses, this chameleon of the material world performs tasks others cannot.

The reasons for this are clear (pun intended). Transparent, durable, inert to chemical interaction, easy to shape and relatively inexpensive to make, glass can be both functional and beautiful. And evidence shows people were producing glass as far back as 3,600BC.

However, most of us know very little about this ubiquitous substance, especially as regards how glass is made. We might have a vague idea of it being made using sand, but no clue as to how something so opaque and gritty can become smooth and transparent. So, just how is sand made into glass?

What Is Glass?

Bottles in a row (Photo: Maskot via Getty Images)

Ever asked, how is glass manufactured? If so, it’s best to understand what glass actually is first.

In scientific terms, glass is an amorphous solid. Very few materials fall into this classification, with plastics and gels being two examples. This denotes that the arrangement of its atoms and molecules are not set out in a fixed lattice like other solids, but is less random than those in a liquid, making it something in between.

It is this structure that gives glass its versatility, making it ideal for such a wide variety of uses. What’s more, there are many types of glass, each of varying strength and design.

However, up to 90% of all man made glass, and what most of us think of as “glass”, is the type called silicate glass. It is named after the chemical compound silica. And this is where the sand comes in. Because sand is almost completely made up of the silica known as silicon dioxide.

Therefore, with that said, it’s time to answer the question, how is glass made from sand?

How Is Sand Made Into Glass?

Worker spraying oil in large glass press to prevent it from sticking ( Photo: Morsa Images via Getty Images)

Step 1: Mixing

Melting sand is a vital step in how glass is made. Usually, this would occur at a temperature of 1700°C. However, modern manufacturing begins by mixing the ordinary sand with sodium carbonate (soda), which lowers this melting temperature closer to 1000°C. Limestone is also added to offset the water solubility of the soda. Otherwise, much like the Wicked Witch of the West in The Wizard of Oz, the glass produced would melt in water. Other materials, such as magnesia, recycled glass and alumina are also added, although silica still makes up 70% of the mix. Glass produced by this process is known as soda-lime glass.

Step 2: Melting

Once combined, the glass batch is placed in a furnace until it becomes molten, after which it is refined. This means any bubbles or impurities are removed by means of raking. That done, the forming process can begin.

Step 3: Forming

The molten glass now has two possible routes depending on its intended purpose. If it’s to become a bottle or cup then it is set into the appropriate shape using a mould. To create flat panes, the molten glass is poured into a container of molten metal, usually tin, because the two molten materials are naturally immiscible. In other words, they cannot mix. Instead, the molten glass ribbon “floats” atop the liquid metal as it cools. This is known as the “float glass process” or “Pilkington Process” after the company that developed it in the mid-1950s. The temperature is gradually reduced until it reaches around 600°C. By then, the resulting glass is formed into a completely flat, smooth surface with an even thickness throughout and can be removed.

Step 4: The Final Cut

The glass now undergoes its final cooling stage, passing through a kiln so that its temperature is lowered gradually. This is called annealing and is meant to relieve any internal tensions slowly thus avoiding breakage. That done, the glass is sent for cutting and is then ready to be made into windows or any other use.

So there you have it. The answer to the question, how is sand made into glass? But that’s not the whole story. As there are in fact many types of glass…

Other Types of Glass

Glassblower blowing hot glass (Photo : Monty Rakusen via Getty Images)

As already mentioned, the appearance and properties of glass are incredibly versatile. There are a variety of different processes depending on the type of finish desired, sometimes as simple as adding another chemical in the mixing phase, others using existing glass.

How is Tempered Glass Made?

If heat resistance, safety and strength are required, then tempered glass – also known as toughened or safety glass – is the way to go. The idea behind tempered glass production is to compress outer surfaces while putting tension on the interior. If it breaks, it won’t create shards, but granules, much less likely to cause injury. Car windows, balcony doors, computers and even shower screens, all of them benefit from these properties. So, how is tempered glass made?

The process of making tempered glass begins with regular glass. There are two ways to go from there; thermal or chemical. Thermal involves heating the glass to around 620 °C, well beyond its transition temperature of 564 °C. Next comes a rapid cooling stage, meaning the outside is cooled before the inside.

The chemical route means immersing the glass in molten potassium nitrate. Chemically tempered glass is more malleable and tougher than its thermal counterpart.

How is Glass Made? Blown Glass

Some unusual glass containers are made by a method known as “blowing”. A gob of molten glass is wrapped around a pipe. As it slowly rotates, air is blown through an opening in the pipe, causing the hot, liquid glass to balloon. This specialist method allows for the creation of an incredible variety of shapes.

How is Glass Manufactured? Other Chemical Processes

Adding chemicals to the sand batch before melting, or to the molten glass, can give the resulting glass a variety of properties and looks. Add boron oxide and you get Borosilicate glass, the highly temperature resistant kitchenware better known as PYREX. Want to make fine crystal glass? Add lead oxide. And for stained glass, just add the right metallic compound to molten glass. Different compounds are used to create different colours. For example, iron and chromium-based compounds make green.

How is glass made naturally?

Believe it or not, while most glass is manufactured, it does occasionally appear in nature. The best known form of natural glass is volcanic glass, such as obsidian. It forms when felsic lava cools quickly.

Clear Cut

Image of glass transparent test tubes (Photo: Andriy Onufriyenko via Getty Images)

So, in this guide to how glass is made, we’ve looked at the answers to numerous questions, including how is sand made into glass, how is glass manufactured and how is tempered glass made. We hope that’s made everything completely clear cut.

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