Why a spark plug ceramic breaks a car window so easily?

So many times you may want to break a car window, a hammer, a blunt object or a knife can come in very handy. It requires much force and effort but thanks to a sparkplug which makes breaking a glass easy.

What is a spark plug? According to Wikipedia, it is a device found in the car engines used to deliver electric current from the ignition system to the chamber of combustion where air and fuel are found in a spark-ignition engine. It may be so small but a small spark of electricity is what enables a car engine to start. This means without a spark plug your car will not start, rendering it motionless. Today though, we shall dwell on its other purpose of why a spark plug can break a car window easily.

A spark plug is built with ceramic, originally made from alumina a very strong material, and steel which separates the electrodes. This is the insulator part of the spark plug. The spark plug is built with these materials to withstand the high heat of the combustion chamber. The glass material during construction is converted from a liquid to solid so fast leading to high volume.

spark plug

Physics proves that the lower the density the higher the volume. As the liquid was cooling it happened so faster on the outside while on the inside it cooled slowly. This creates plenty of compressed energy on the inside from the thermal gradient effect on the material. The only important part of the spark plug needed to break a window is the porcelain chips. You can get them by breaking the spark plug against a heavy mason wall or hammering it down or throwing it on the tarmac. All that is needed in the magic of breaking a car window is a small piece of the plug that is sharp at the edge, enough to create the pressure point on the glass window.

How a spark plug works in breaking glass

Upon obtaining the chip with a sharp edge just throw it at the glass window from a distance, that’s just it. These, according to Science and Physics on hardness scale, they are heavier than glass. Just make sure to be careful when handling the porcelain chips to avoid injuring your hands with sharp edges.

Due to the compressed energy on the inside of the glass, the force that could be used to break the whole surface of the glass window is now concentrated on one point. This is a major weakness of tempered glass. Just a small tiny spot has now come into contact with the sharp edge creating a pressure point, the tiny break shatters the whole glass to pieces.

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Modern car windows are made of tempered glass, which as mentioned above, during its processing the outer part that cooled fast is in compression mode while the inside that cooled slowly is in tension mode. Crack inhibition occurs at this point, so whatever is meant to overcome this force must first overcome the compression before the tension. The glass tempering process ensures that the glass is strong by internal energy stored.

This reaction can easily be explained by cracking an egg. You cannot easily break an egg by pressing it through your hands as easily as it would be by cracking it with a spoon. A very sharp pencil pressed down on a piece of paper can cause a tear but not a crayon. In the case of the ceramic chip from the spark plug, the impact force causes a crack, leading it to penetrate the glass internally, so deep. Due to the compressed tension, it releases the crack spreads so fast in microseconds, shattering the whole glass. 

One thing worth noting is that the spark plug can only breaks the car glass window and not the windshield. Advanced technology has ensured that in case of accidents the windshield should break before the human skull due to impact. This means the material used to make windshields are very different from the windows made of tempered glass. This explained, let’s summarize the benefits of spark plugs below.

Advantages of spark plugs in breaking car windows

  • They are easily available and accessible as they are found in the engine of the car. Even though you are locked out of the car you just open the bonnet, get one, break the window, and access your car.
  • You do not need a professional to come to break the window for you. You can easily do it. Just throw a porcelain chip from your spark plug and voila, you have access to your car. Super simple yet it can saves a life at time of danger time and money.

In case of accidents, and you cannot open car doors, a spark plug comes in handy. Using the already broken ceramic chips from the spark plug, throw them at the window and you can get out through the shattered window. Firefighters and emergency evacuation teams are always known to carry them in their pockets for such scenarios.

Precautions while using spark plugs are necessary too. Always use safer gloves if possible to avoid hurting yourself. When throwing the porcelain chips at a window stay a distance from the shattering glass or take cover. Do not throw a spare plug while too close to the glass target. Chances of it failing to shatter and bounce back at you might happen, causing injury.


We are always working hard daily to make our lives easier and machines know how to do that best. Anything that is going to save you time and energy is always something worthy to consider. A spark plug has proven not just to be a part of the car ignition chamber but as an emergency tool too. 

Always keep those old, worn-out spark plugs somewhere in your car. You never know when you may need it to access the outside or the inside of a car. 

However, be careful not to get in trouble since in some countries, possessing ceramic chips is considered a minor criminal case. Also, don’t forget to take precautionary measures while breaking a window with a spark plug. You don’t want to injure yourself or anyone nearby.

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Matink is a "Jack Of All Trades" writer and admin at thegadgetsjudge.com. Matink love writing, and reading more about Technology, origin of life and understanding the universe. He is a qualified Master Electrician, Welder, Safety Professional and currently doing SEO and Google IT Support Professional Certificate at coursera.org. Follow Matink on twitter @thegadgetsjudge


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