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Can You Cut Toughened Glass? Everything You Need to Know

Toughened or fully tempered glass is classed as a ‘safety glass’: it is five times stronger than regular untreated glass, and can bear heavy loads without breaking. If it becomes damaged, it is designed not to break into dangerous shards but to shatter into small harmless cubes. It is this safe and resilient quality that makes toughened glass ideal for high risk areas on all types of residential and commercial buildings.

 

Date

22 Jun 2022

By

Simon McAuliffe

However, if you are working on bespoke or custom briefs, with tight specifications around the size and shape of the glazing, the question of whether you can cut toughened glass, and how to do so, may come up.

When can glass be cut?

Any shaped glass units, glass panels with cutouts, or glass cut to non-square shapes can be cut to the size and shape you require, but it must be done before the glass is put through any toughening process. It is important to consult with a specialist early on in the project to ensure the panels can be designed and cut to the right size and shape in the first place.

 

Once the glass has been cut to the required shape, it is then placed in a furnace where the toughening process takes place. The panels are heated to 600 degrees, and then rapidly cooled with cold air. This rapid cooling contracts the surface of the glass, putting it in a state of compression whilst leaving the inside of the glass in a state of tension. The glass is much stronger under compression, and in order to break it, the compressive force must first be exceeded.

Once you temper a piece of glass, it is not possible to cut it again without shattering it or leaving it weakened and unfit for its original purpose. Various how-to guides will tell you that you can cut tempered glass by first annealing the piece. This involves heating the glass in a kiln and then letting it slowly cool over a course of eight or nine hours. After you anneal it, you might be able to cut it into the size and shape you’re looking for. However, in doing so you are essentially removing the temper and returning it to its previous state as “ordinary” glass, which means it will lose its ability to shatter into grain-like pieces, and can no longer be considered safety glass.

Glass roof over an atrium containing offices and a reception area

Cutting toughened glass - does it make sense?

Cutting toughened glass by turning it into an annealed glass is likely not worth the time and effort required. By doing so, and reversing the strengthening process, tempered glass doesn’t stay strong or safe, and becomes just regular annealed glass which would need to be re-tempered once it’s been reshaped to be able to classify again as safety glass.

As the end product will be a simple piece of annealed glazing, it makes little sense to attempt to cut toughened glass – instead, if safety glass is required on a project, liaise with your specialist to ensure the right size and shape are specified before any tempering or toughening takes place.

Having said this, in many situations working with toughened glass is generally the better option; indeed, in the UK, Building Regulations have specific requirements around when safety glass should be fitted in windows and doors. The regulations are based on the level or risk that someone (especially, but not exclusively, small children) could fall through a particular pane of glass, or that a piece of overhead glazing could cause harm if it were to break. 

What are the benefits of toughened glass?

The process of toughening creates tensions in the internal surfaces of a glass panel to increase its strength, safety and durability, by exposing regular glass to extreme heat. 

By increasing the strength of a panel fivefold in this way, it can withstand extreme weather conditions, and can be used in a variety of traditional decorative and architectural applications – including structural glass, glass floors, glass balustrades, walk on glass roof lights and glass doors.

As well as increased strength and durability, this glazing is also referred to as safety glass for good reason: the tempering process forms differential stress patterns within the glass, which means it won’t shatter into harmful and sharp glass shards on breakage. In the first place, it is not easy to break, but even if it does shatter, it will collapse into tiny crystalline, harmless pieces, which are far less dangerous than the sharper edged pieces of broken annealed glass. 

This makes it much safer to use in doors, low windows, or frames next to doors – and naturally glass floors or roof panels. In the unlikely event that someone trips in one of these locations and their hands, arms or body come into contact with the glass then if it breaks, it will break safely. The toughened glass will shatter into lots of small pieces and is far less likely to cause any serious damage.

This unique breaking mechanism of the tempered glass makes it completely safe for homes, offices or any commercial setting.

Toughened Glass VS Laminated Glass

Toughened glass is often muddled with laminated glass, as they are both types of safety glass.  Two or more panes of glass can be laminated to form a stable single sheet that keeps its integrity when broken.  Whilst most common laminates are made using standard annealed glass for cost reasons, it is still better to laminate toughened or heat-strengthened glass, for its strength. They differ in a number of ways – primarily in how easily they can get broken.

Laminated glass is most known for being used on the windscreens of the majority of the world’s cars. This global use can be put down to what happens when laminated glass gets broken as it remains in place.

When broken, laminated glass holds in place and does not break up into large harmful shards or hundreds of little pieces (like toughened glass). Instead, it keeps in place long enough for a replacement to be found. 

This happens thanks to the plastic (polyvinyl butyral) interlay that is sandwiched between two glass sheets during manufacture. This bonds the two panes together like a glue so that even when the glass breaks, its fragments keep binding on the thin film.

This highly practical safety feature greatly reduces the chances of injury occurring from falling glass. In addition to this, as the panes do not burst apart on impact, laminated glass leaves no hole behind for intruders and thieves to crawl through. 

Toughened glass exists for a reason, and it is because it offers a much greater level of strength, durability and safety. If you require a bespoke glass panel, then you’ll need to consult with a specialist who will be able to advise on the exact type of glass to use, and how to achieve the correct size and shape – this will avoid the need for any expensive repairs in the future. In this way, you will ensure you are supplying the best quality product available for your client. 

In essence, we’d never recommend cutting toughened glass – because to do so, you’d essentially need to return it to a state of annealed (or float) glass, which would make the process effectively redundant. It’s always better to get the glazing right from the earliest stages in the process, which means collaborating with your specialist early on, and taking their advice and input when it comes to the glazing specification.