The short answer

Often referred to as "safety glass," tempered glass is heat-treated to alter the composition of the glass. This process results in glass that is about four times stronger than regular glass and safer since it breaks into smaller, duller pieces.

The long answer

Tempered glass is heat-treated glass. The process of heating and high-pressure cooling makes the material about four times stronger than regular, annealed glass. Whereas regular glass breaks at 6,000 pounds per square inch (psi), tempered glass generally breaks at around 24,000 psi.

In addition to being stronger, tempered glass is safer than regular glass. When glass normally breaks, it cracks into long sharp shards that can be difficult to clean up and result in injury. Because of tempered glass' composition, it fractures into small, duller pieces that are safer to sweep up.

Fun fact: The industry term for broken shards of glass is called "spall." (Keep it in mind for your next round of Scrabble or Wordle!)

For this reason, tempered glass is often used in spaces where people are at risk of injuries caused by broken glass. You'll find tempered glass used in cars' rear and side windows*, shower and tub enclosures, skylights, patio furniture, and racquetball courts.

Here's how tempered glass is produced:

Step 1: Cut to shape

Tempered glass must be cut to shape before heat treatment because it cannot be altered afterward. This is why it is much more expensive to replace than regular glass.

Step 2: Extreme heat

The cut piece of glass is then exposed to temperatures exceeding 600 degrees Celsius (or over 1,100 degrees Fahrenheit).

Step 3: High-pressure cooling

Then the glass undergoes a procedure called "quenching," or high-pressure cooling. The outer layers of the glass cool and solidify faster than the inner layers during this phase.

As the outer layers contract and solidify, the inner layers, still hotter, also want to contract. However, they are restricted by the already solidified outer layers, forcing the inner layers to compress within them. This results in the exterior of the glass being in a state of compression, while the interior is in tension, giving tempered glass its unique strength.

🧠 Bonus brain points

Why don’t car windshields use tempered glass?

* Wondering why cars' windshields don't use tempered glass? It's because there's an even safer and stronger glass available: laminated glass. Laminated glass contains layers of thin plastic, which causes the window to bend rather than break fully.

Check out some other curious questions:

Sources

Ford, M. (2001, January 22). How is tempered glass made?. Scientific American. https://www.scientificamerican.com/article/how-is-tempered-glass-mad/

TEMPERED VS LAMINATE GLASS FOR AUTOMOBILES. Smiley’s Mobile Glass Shop, Inc. (2020, October 29). https://www.smileysglass.com/blog/tempered-vs-laminate-glass-for-automobiles

What is Tempered Glass and What It’s Used For? Window Film Depot. (2021, March 15). https://www.windowfilmdepot.com/blog/what-is-tempered-glass-and-what-its-used-for/

What Is Tempered Glass?. Impact Security, LLC. (2021, November 22). https://www.defenselite.com/blog/what-is-tempered-glass/