I’ve recently bought a new power drill. I decided to research what I can and can’t drill through, and I came across tempered glass. Tempered glass is extremely strong glass used for car windows, phone screens and squash courts among other applications.
There are many articles on Google titled “How to Drill Tempered Glass”. Confusingly, these articles tell you that you can’t in fact drill through tempered glass.
To add to the chaos, a few articles say you can in fact drill through tempered glass and give a walkthrough on how to do this. I therefore set out to myth-bust this once and for all: can you really drill through tempered glass?
Tempered glass is around 4x stronger than normal glass and is designed to shatter on impact. No DIYers, nor even your local glass shop, would have industrial lasers capable of drilling through tempered glass. If drilling is attempted, the tempered glass would shatter before the user can complete the hole. So the short answer is no; you cannot drill through tempered glass.
What is Tempered Glass?
Tempered Glass, otherwise known as safety glass, is a heat treated glass around 4 times stronger than normal glass. Other types of glass include annealed (normal), heat strengthened glass, toughened glass and laminated glass. Check out this paper on the history of glass manufacturing.
Heating glass alters its molecular structure and causes the glass to contract. This puts energy back into the glass and causes it to shatter on impact.
In addition to being far stronger than annealed glass, tempered glass is also safer. It also has many uses. To name a few: vehicle windows, shower doors, table tops, refrigerator trays, phone screen protectors and diving masks.
The advantage of shattering is the fact smaller pieces of glass are less likely to cause injury or death. If a shower door made from tempered glass shatters, it will break into many small pieces rather than large sharp pieces.
How to tell if glass is tempered?
Tempered glass has very smooth edges due to the manufacturing process. Grinders smooth the edges of glass panels before being tempered, and so often the very smooth edges of the glass are a giveaway.
Tempered glass will break on impact and drilling. Although annealed glass isn’t easy to drill into, it is possible, whereas tempered glass will shatter immediately.
How is Tempered Glass made?
Think of Tempered Glass as an ‘addon’ to annealed glass. Many factories specialise in the tempering process and would first purchase untempered glass. The untempered glass is cut to the size and specifications for the customer.
This initial cutting requires specialist machinery and a continual flow of water. Diamond drills are used and the water keeps the glass clean and cool. Once the glass is cut to size, the factory needs to ensure the edges are nice and smooth. A grinding machine sands down these edges ensuring a smooth finish.
Once cut to size, the untempered glass is moved into a furnace. The furnace needs to be kept at 700 degrees celsius (extremely hot!). Often the furnace takes over 24 hours to heat, and so this is by far the most costly element of producing tempered glass. Each panel spends only 2 minutes in the furnace.
If there are any microfractures in the glass, it won’t survive the tempering process and will shatter. Check out this paper on why it’s so important there aren’t any imperfections in tempered glass. It can cause spontaneous fracture!
2 minutes in the furnace produces tempered glass. The end product is safety glass which will shatter into at least 52 pieces for each 5×5 square. The final part of the production process is quality control. Randomly selected panels are destroyed to analyse shattering. Depending on the customer, a 5% to 10% rejection rate for the completed product is reasonable.
How to drill tempered glass
Check out the video below. The user has an extremely sophisticated set up and puts a lot of time into trying to drill through tempered glass. He classes this as a success as he’s able to drill a little into the glass, but it ultimately smashes:
Here’s a misleading video. The user appears to drill through tempered glass with a complex set up. Having researched this topic, this sort of resource is why the answer is unclear on the internet. The user says it is tempered glass, but look at the ratings on the video. Half the people watching this don’t believe it, and there’s lots of negative comments:
Here’s an article describing how to drill through tempered glass. However, again, it’s just not factually accurate.
The only way to drill through tempered glass is to drill through glass before it’s tempered.
How to cut tempered glass
It is not possible to cut tempered glass. Tempered glass is designed to shatter on impact. The only way to cut tempered glass is to reverse the tempering process. This involves heating the glass to 700 degrees celsius and then slowly cooling it. This process is called annealing.
Once the tempered glass has been annealed, the glass would become weaker. Hopefully the glass becomes weak enough to the point at which you can cut it. The annealing process could either be completed with access to a kiln, or by taking it to a professional glass cutter.
The annealing process involves leaving the glass in the kiln for about 30 minutes. Cool the glass over a 3 hour period by gradually reducing the heat of the kiln. Cooling the glass too quickly will weaken the annealed glass. Once the glass has been removed from the kiln, it should be left overnight to cool further.
Then, cut the glass like normal. This video does a great job of taking us through that process:
Having researched the uses of tempered glass and the manufacturing process, I’ve concluded that it’s not possible to drill through tempered glass.
Tempered glass has a lot of built up energy and is manufactured to shatter on impact. The manufacturing process is bound by regulations and so tempered glass is of extremely high strength and quality. It’s therefore inherently impossible to drill through tempered glass as it is designed to shatter on impact.
Put it this way: if it were possible to drill through tempered glass, the tempering process would not be fulfilling its purpose.