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How to Cut Tempered Glass [Step By Step Process]

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Tempered glass is a type of safety glass, ​designed so that if it breaks, it will not shatter into shards. Glass shards are very dangerous, and tempered glass is made to crumble into small pieces, which is a lot safer.

Because the glass is resilient and strong, it can be hard to cut.​ And, if you don’t cut the ​glass properly, it may break into pieces. ​We’ve outlined the steps to safely cut tempered glass so that you end up with accurate cuts. 

What Is Tempered Glass

You need to understand the importance of annealing so you can become more proficient at cutting glass. During the glass-forming process, the glass may develop permanent stresses.

To ensure dimensional stability and to prevent excess tension in critical areas, the stresses must be reduced. Annealing is used to do this.

A man cuts tempered glass in a workshop

Thermal Tempering Process for Glass

The thermal tempering process happens when the glass is rapidly cooled or quenched at a specific temperature by using symmetrically placed air jets.

Because the outer layers of glass are cooled more rapidly than the inside, the glass is compressed, which strengthens the glass.

During the process, the inside of the glass is stretched. This type of process often works well for glass that is of a regular thickness. Thick glass may fracture if this type of strengthening process is done.

A large number of glass products can be strengthened substantially by the use of thermal tempering. However, again, if the glass is too thick, it may break or violently break apart into a large number of shards.

Because glass that is not tempered can break into jagged pieces and cause serous injuries, tempered safety glass is often required in certain commercial applications, such as the manufacture of shower doors.

How Tempered Glass is Strengthened​

​For a lot of uses, glass needs to be strengthened. On its own, it just isn’t strong enough. Technological advancements have led to two primary ways of strengthening glass: heat strengthened and tempered.

​Both processes involve heating the glass to a temperature above 1,000 degrees Fahrenheit. The differences between the two methods are in the cooling process:

Heat strengthening cools the glass slowly, which results in really strong glass. It doesn’t have the safety features that tempered glass has, but it is almost twice as strong as un-annelaed glass. 

Tempered glass is cooled a lot faster to create higher surface compression, resulting in glass that is 4x-5x stronger than standard, unnealed glass

​Where Tempered Glass is Used

Wherever safety is required, tempered glass is preferred. Here are the most common areas you’ll find tempered safety glass:

  • Shower and bathtub doors
  • Car windshields
  • Balcony and sliding glass doors
  • Windows and skylights

Tempered glass is best whenever there is potential for human contact with broken glass.

Tempered glass prevents sharp, jagged, dangerous glass shards

How to Cut Tempered Glass

While you can have a professional cut the tempered glass, you will find that cutting tempered glass yourself offers two major advantages:

  • You can cut the safety glass to your precise specifications.
  • You can make any modifications without incurring any extra costs.

Tools Needed

You’ll need tools for both preparing the glass and for making your cuts. For starters, here are the tools to gather to prepare the glass:

  • A kiln for annealing the glass
  • Heat-resistant vessel
  • Small amount of water (around two cups)
  • Cooling rack
  • Thick work gloves
  • Tongs

Next, gather the following tools for cutting tempered glass:

  • Safety goggles
  • Leather gloves
  • Straight edge (ruler)
  • Permanent marker
  • Glass cutter
  • Dowel (1/4 inch or .64 centimeters – nothing larger)
  • Course sandpaper – 10-grit

Preparing Your Glass

Annealing is used to cut tempered safety glass. Annealing is a heat treatment type process that changes the microstructure of the glass so it is easier to cut.

Start the process by using a kiln to ensure that the glass is easy to cut and use. Without a kiln, you cannot safely cut the glass.

1. Place the Glass to be Cut Inside a Kiln

The kiln’s intense heat will soften the coating of the tempered glass so it can be annealed. Unless you are going to use the kiln often, you should see if you can borrow one or access it. A kiln, for example, may be accessed through the art department at a community college.

NoteThe annealing process removes the stress points from the tempered glass. These stress points cause the glass to break into countless pieces whenever it is cut. Without the stress points, the heated and annealed glass can be safely cut, and will not shatter or crumble.

2. Place the Glass in a Heat-Resistant Vessel

The vessel you use can be purchased at either a glass store or art store. Place the newly annealed glass in the container and cover it with about 3/4″ of water.

A man cuts a tempered glass mirror

3. Allow the Glass to Soak inside the Kiln 30 Minutes

This part of the process is important, as the kiln needs to be set at a high enough temperature for the glass to anneal.

  • Effetre, Lauscha, and Bullseye glasses should be soaked at 940 degrees Fahrenheit.
  • Borosilicate tempered glass should be soaked at 1,050 degrees Fahrenheit.
  • Satake tempered glass should be soaked at around 890 degrees Fahrenheit.

It helps to use a temperature controller to maintain a constant temperature inside the kiln.

4. Cool the Glass Gradually

If you are not sure what type of glass you are annealing, lower the kiln’s temperature to around 800 degrees Fahrenheit. If the glass is Satake, use a cooling temperature of about 750 degrees Fahrenheit.

The tempered glass should be cooled inside the kiln for about three hours.

Remember the following points when cooling tempered glass:

  • Cooling the glass at too fast of rate will cause the development of additional stresses. This, in turn, will weaken the annealed glass.
  • The strain point, during cooling, is the internal pressure that decreases inside a sheet of glass. Once the glass cools below the strain point, it will not break.

5. Remove the Tempered Glass from the Kiln 

Once the glass has been cooled, remove the sheet from the kiln. Use tongs to remove the glass, as it will still be extremely hot.

Wear thick gloves when using the tongs and removing the glass.

Position the glass on a cooling rack overnight before you try to cut it. If you try to cut the glass when it still is hot, you can injure yourself and damage or break the glass.

One Point to Remember – As the annealed glass cools inside the kiln, the outer part of the glass will cool more quickly than the inside. Therefore, if the exterior feels cool, it does not mean the glass is ready. Use caution during handling. Cooling the glass gradually reduces any build-up of stress and will lead to a better and more uniform cut.

Cutting Tempered Glass

Now you are ready to cut the glass. You need to follow the steps below to ensure a successful cut.

  1. Clean the glass’s surface: Spray and clean the glass with a window cleaner and wipe it clean with a soft, lint-free dry cloth. You should always clean the glass first to ensure that the cut is precise and smooth.
  2. Wear leather gloves and safety goggles: After the glass has gone through the annealing process, it is no longer considered a safety glass. Therefore, any breakage could harm you. The glove and goggles will protect hands and eyes from harm.
  3. Produce a straight line with the permanent marker: Use a metal ruler and measure the exact location where you want to cut the glass. Hold the ruler along the line. Follow up by using a permanent marker to mark a straight line along the ruler’s edge.
  4. Use a glass cutter to score the surface: Keep the ruler in place, so you have a guide for cutting the glass. Place the glass cutter at the start of the line, and move the cutter over the total length of the marked line. Press down moderately for cutting. Never run the glass cutter along the line more than one time.
  5. Place a wooden dowel under the cut: After you have scored the line, place a dowel (about 1/4 inch) beneath it. This will prevent you from shattering the glass. Don’t use a dowel that is larger, or you may cause the glass to break.
  6. Use an even and sharp pressure: Use both hands to apply an equal amount of pressure on either side of the dowel. This will cause the glass to snap along the line into two smoothly cut pieces.
  7. Sand the cut glass: Use 10-grit course sandpaper to remove the roughness from the edge of the cut glass. Doing so will make the glass safer and more reliable to handle. Don’t skip this step, as doing so could cause a serious injury.

Caution: When performing any of the above steps, make sure you are wearing your safety goggles and gloves. Don’t take them off during the entire glass-cutting and sanding process.


You can cut tempered glass yourself. You just need to know what tools to use and what steps to take.

An expert at home repair, remodel, and DIY projects for nearly 40 years. His first experience came in completely restoring an antique home. Completely redone from the inside out, and restored to its original form, the home is a featured design by renowned Southern California Architect Cliff May, considered to be the father of the California Ranch Home. Now Dennis spends his time on fine woodworking projects and tool comparisons.