How often do you look out of windows, without the slightest clue about how glass window panes were created? Maybe you have a vague idea that glass is made using sand, or once caught a few minutes of a glass documentary?
If you’re like most of us, that’s as in-depth as your knowledge gets. However, window glass actually has a fascinating history and is created using a unique process that’s well worth researching.
Window glass didn’t start to become common until the mid-16th century, and even then, it was reserved for only the richest households.
So, what changed? Why do we have such easy access to this luxury now? It’s down in part to improved glass production processes, and we’re here to share all the details.
Ready to shed some light on exactly how your glass window panes were made?
Let’s start learning.
There are multiple different methods for creating the glass for glass window panes, but the float process is the most common when it comes to creating flat glass window panes.
The method was invented by Sir Alastair Pilkington in 1952, and is used to create the most uniform glass possible.
Other methods include glass blowing, used to create bottles, jars and other containers, the Danner method, used to create glass tubing, and the ribbon process used to make light bulbs.
We’re going to focus on the float method here – read on to find out exactly what it involves.
Most glass for glass window panes uses the same base ingredients of sand and minerals. Green glass can include chromium, and boro silicate glass includes boron oxide.
The base materials are heated to 3090°F, at which point they become liquid. Limestone is added to the mixture to prevent it from dissolving entirely.
The melting process can last up to 50 hours, and the mixture is simultaneously refined and homogenized, to ensure that the quality of the finished product is as high as possible.
The end result should be liquid glass which is completely free from impurities and bubbles, and ready to be further refined.
Once the initial melting and refining process is complete, liquid glass moves into the float bath.
The glass mixture for glass window panes floats over a refractory spout and onto a surface of molten tin – hence the word ‘float’ in the name of the process.
The glass enters the float bath at a high temperature and gradually cools, slowly enough to prevent cracks from forming.
At this point in the process, the thickness of the glass is decided. While in the past, only thick glass could be produced, advancements mean that glass thinner than a millimeter can easily be created.
Of course, most standard glass window panes will be thicker than this to ensure that they’re durable, easy to work with, and doesn’t let out too much heat when installed in buildings.
While the ribbon of glass is still cooling, it can be altered using various coating techniques. The most common is chemical vapor deposition, which is used to lay down extremely thin coatings very quickly.
These coatings can be used to alter the optical properties of glass. For example, certain coatings can create glass which reflects infrared wavelengths. It’s possible to lay down multiple coatings one after another to achieve the exact properties needed.
When creating glass window panes, coatings are used to reduce the amount of sunlight that can travel through glass, and to increase the amount of thermal protection it offers.
While glass might have appeared in perfect condition in the float bath, stresses can start to appear as it cools. If left unattended, these stresses could form cracks and cause the glass to break.
To prevent this, a process called annealing is carried out. Annealing involves passing glass through a long furnace called a lehr and cooling it very slowly.
Glass is heated to its ‘annealing point’, which is usually between 850–900 °F. This softens the glass and helps relax internal stresses.
The glass remains at this temperature until all the stresses are resolved, at which point it can be gradually cooled until solid.
Just think of it like taking a warm, relaxing bath to de-stress after work!
The annealing treatment is essential in resolving the internal stresses that formed during the initial cooling, ensuring that the glass is as strong and durable as possible.
Glass window panes which aren’t properly annealed could easily shatter when exposed to small temperature changes, pressure, or shocks, which isn’t ideal for glass which needs to survive busy homes, offices, and buildings.
Once the glass has been annealed it can be cut to shape, polished and drilled without any risk of it shattering or cracking under the strain. Attempting any of these steps prior to annealing would be disastrous.
To ensure that every sheet of glass is as high-quality as possible, automated inspection tools are used throughout the process. These are able to spot any issues, like bubbles, stresses, unmelted sand grains, or ripples.
Many of these issues can be resolved further along the production line, so having them flagged up early on is essential for maximum efficiency.
In the case of issues that can’t be solved, the system can alter the path of cutters to remove the problematic sections of glass from production.
The ability to use automated inspection tools makes producing window glass panes many times more efficient and ensures that no faulty glass makes it into the final products.
Inspection machinery is able to spot flaws that would be impossible to see with the naked eye, saving staff time and increasing quality.
Once the glass has been cooled completely, it needs to be cut to size.
Diamond tools are used to cut glass into pieces, as dictated by a linked computer system.
To prevent wastage, the computer creates patterns which use as much of the solid sheets of glass as possible. For example, cutting identically sized sheets side by side, and cutting small sheets out of the leftover glass from larger cuttings.
In the past, this would have taken hours to calculate manually. Developments in technology have made glass production easier and more efficient.
Before being transported to its next destination, glass needs to be carefully packaged.
A thin layer of foam helps protect the surface from scratches but doesn’t do much to guard against impact. Instead, carefully cut pieces of thick packing foam and cardboard are used to protect the glass from bumps and knocks during its journey.
Packaging is clearly marked using ‘fragile’ stickers to ensure that it isn’t thrown, dropped, or handled carelessly.
Now that you know how standard glass window panes are produced, you might be wondering about the extra work that goes into creating double glazed windows.
Double glazing has been around since the 1930s, and is a popular way to keep houses warmer and decrease heating costs.
As the name suggests, double glazed windows are made from two separate panes of glass. These panes are separated by a spacer, meaning that there’s a sealed gap in the center.
The gap is filled with air or inert gas, both of which are poor conductors of heat. This means that less heat is conducted between the two panes, trapping warm air inside the building and keeping cold air out.
Traditionally, the gap between the glass window panes was filled by drilling small holes in the spacer, then filling them in. Nowadays, an online gas filler is used for greater efficiency.
The sides of the glass window panes are carefully sealed to ensure that air from outside can’t enter the unit. If they aren’t sealed properly, humid air could enter the gap and create condensation on the inside of the glass.
Double glazing has developed a lot since it was first invented. Spacers are now larger, gaps are sealed more effectively, and toughened safety glass is used for increased security.
Have you ever encountered frosted glass window panes in a bathroom?
This glass allows plenty of light in while preserving the privacy of the person inside.
It’s commonly created using a technique called sandblasting, where an abrasive material is propelled at high-pressure towards the glass, creating a rough surface that can’t be seen through.
Other techniques include acid etching, mold etching, and frost etching, although these are generally used to create specific designs on smaller sections of glass.
The frosted effect is always applied at the end of the glass production process – the starting sheet of glass would look no different to one of the float glass sheets we described above.
Learning more about the complex production processes behind everyday items like glass window panes is so interesting – even when you’re not expecting it to be!
Now that you know all the details of the float glass production process, you might be curious about other types of glass production, or different applications of float glass.