This Is Why Porcelain Can Break Glass

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The safety glass in your car windows is made to withstand all sorts of impact, from hailstones to thrown objects. But even tempered glass can shatter if hit with the right force—and that force is found in the humble porcelain. Wondering how and why? This is why porcelain can break the glass.

Glass is innately brittle, meaning the whole pane is weakened as soon as you nick its surface. Porcelain is perfect for creating that nick because it’s dense and hard. So when you throw a shard of porcelain on glass, the impact can crack the surface and destabilize the whole pane.

This is why car burglars and vandals often target car windows with porcelain sourced from spark plugs. All it takes is a single hit in the right spot to create a spiderweb of cracks that shatters the entire window. But this principle doesn’t work with all types of glass. So let’s learn more about porcelain and other ceramics and why they can break the glass.

What is Porcelain Made Out Of?

Broken ceramic mug

Porcelain is ceramic made by firing white clay mineral kaolin at extremely high temperatures. Depending on the properties required in the final product, the clay is mixed with water and other minerals such as alabaster, feldspar, bone ash, steatite, or quartz sand.

The resulting product is a strong, translucent, non-porous material that resists high temperature and can be used to make everything from dinnerware to tiles.

Why Can Porcelain Break Glass?

Porcelain’s ability to break glass doesn’t come from its strength. Instead, it is because of how glass reacts to a sharp blow.

There are various types of glass. The everyday glassware in your home can be easily shattered by any sudden impact, including a hit from a porcelain shard, a brick, a football, or any other hard object. Here we’re talking about tempered or safety glass, which is more resistant to impact and is used in car windows and shower doors.

Tempered glass is made by quickly cooling the molten glass so that the outer surface becomes harder than the inner layers. This makes it stronger than annealed glass, but the compressive stress on the surface leaves the glass’s center in tension. So once you can surpass the surface tension, the whole thing shatters.

With porcelain, you not only get a really hard object but one with sharp edges. So even with relatively less force, you can concentrate enough pressure to create a fracture on the small point of contact.

And once there is a dint on the surface, the internal energy of the glass is released, and that dint spreads out until the entire sheet of glass shatters. That’s how porcelain can break a car window, but a hammer can’t.

Can All Ceramic Break Glass?

No, not all ceramic materials can break glass. Some are too soft, while others aren’t hard or brittle enough. However, there are a few other types of ceramics besides porcelain that have the potential to break the glass. A few examples are:

  • Alumina: Abrasion-resistant material used to make insulators, spark plugs, and cutting tools. 
  • Boron carbide: A black crystalline compound used as an abrasive and bulletproof material. 
  • Zirconia: White crystalline oxide is used to make dental implants and artificial joints.

While all these materials are harder than glass, they must be in the right form to create a fracture. So a chunk of alumina will not break safety glass, but a sharp fragment may just be able to do the job.

Other than these high-tech applications, ceramic from a toilet bowl or flower pot may also be able to break glass if thrown with enough force and at the right angle.

Why Do Spark Plugs Break Car Windows?

Broken window-pane

Spark plugs are notorious as burglars’ tools for breaking car windows. But interestingly, it’s not the spark plugs themselves that are doing the job, but rather the porcelain jackets that encase them.

Still, the car window will probably remain intact if you throw a spark plug at it. For the porcelain to do its job, you first have to break the porcelain casing into small fragments. Then take one sharp shard and fling it at the window at just the right angle to make a direct hit.

The extremely hard shard will concentrate force on a tiny area and penetrate the surface of the glass—both the strongest and weakest part of the tempered glass. Once the surface is breached, the pressure inside the glass is released, and the entire window shatters.

This method works great with the porcelain encasing spark plug because it’s designed to withstand high temperatures and mechanical stress. So it’s both hard and brittle enough to create a fracture on impact.

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