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Urban Legend- Breaking tempered glass with Aluminum Oxide

  1. Apr 6, 2010 #1
    There are a lot of youtube videos and online forum speculations that suggest that tossing a disassembled spark plug at a car window will cause the window to spontaneously shatter. The supposed justification is that tempered glass can easily withstand from softer materials than it (such as the iron of a hammer), but even a light brush with a material of higher Mohs scale strength (such as the Aluminum Oxide within the spark plug) causes it to spontaneously shatter in a manner similar to Prince Rupert's drops.

    Is this rumor true? If so, does someone have a more detailed explanation of the physics behind this effect. Sources would be excellent- google is not turning up any credible information.

    Also, what specific properties of car door glass make it susceptible to this effect?

    Thank you in advance.
  2. jcsd
  3. Apr 6, 2010 #2
    I have no idea if it is true but if it was then I might have a guess why. Aluminum oxide (in some forms) is one of the hardest materials known to man. Glass shatters because it has very low ductility. Ductility is how much a material can undergo mechanical stress (bending, twisting, etc.) with out fracturing.
  4. Apr 7, 2010 #3
    Tempered glass is made by a process that generates internal stresses within the glass.

    The surface of the material is under compression, while the interior is under tension. In order to shatter the glass, you need to produce a crack or scratch that goes deep enough to reach the part of the material under tension.
  5. Apr 7, 2010 #4
    I think I think an experiment is in order. Who's the first volunteer to who will throw chunks of disassembled sparkplugs at his wind shield?

    Somehow I think this mythology is perpetuated by a desire not to trash ones car to dispel it.

    Neighbor: Why are you out at midnight throwing things at my car?
    Self: I didn't find it justifiable to destroy my own, but there's this this really weird thing I heard on the internet I needed to test....

    And, um, what aluminum oxide? Spark plugs are made of metals and fired clay.
    Last edited: Apr 7, 2010
  6. Apr 7, 2010 #5


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    It's not the hardness, Moh's scale and scratches only apply at small forces and low speeds.

    Breaking a window is all about force applied at a point, wood is a lot softer than glass but you would have no problem breaking a window with a baseball bat.
    The reason a sparkplug breaks a window is that it's pointed.
  7. Apr 7, 2010 #6
    You're not getting the point of tempered glass, apparently.

    It's not about supplying sufficient force to snap the window in two, it's about getting a crack down to the material that's under tension so the internal stresses can propagate the crack further.
  8. Apr 7, 2010 #7
    Agreed, that's why I mentioned Prince Rupert's drops as an example of tempered glass. Do you think that the hardness of Aluminum oxide makes it more susceptible to creating a deep scratch, even when it is tossed at a low speed? I know that a car window can withstand a decent hit from a hammer, but these guys literally just toss a tiny bit of Aluminum Oxide at the window.

    Here is a supposed video of the effect:

    Spark plugs do contain aluminum oxide. It acts as a replacement for the "clay" (porcelain) insulator near the pointed ends, as Aluminum Oxide can withstand the high temperature of the plasma arc a bit better than clay can.

    Thank you for all your replies!
  9. Apr 8, 2010 #8
    Do you really believe a spark plug could made such a deep crack? It seems excessively optimistic to me. This urban legend is what its name suggests: a legend.
  10. Apr 8, 2010 #9
    I'm not saying I believe it, but I'm not ready to dismiss it out of hand either.

    What do you mean by "such a deep crack?"
    It doesn't have to go all the way through the glass.

    Small pebbles made of softer material frequently chip windshields, and pebbles tend to be rounded rather than having sharp edges to concentrate the force of the impact.

    There are also known cases of tempered glass shattering spontaneously, without any outwardly observable cause.

    I don't think it's inconceivable that the sharp edge on a small chip of ceramic might be enough to trigger shattering in an inferior quality piece of tempered glass.

    I've seen a tool for EMTs to get through tempered glass--it's basically a spring-loaded punch that can be operated with just the strength in your thumb. On the other hand, if you try to punch through a windshield with your fist, you're more likely to break your hand than the glass.
  11. Apr 9, 2010 #10
    What has a piece of porcelain from a disassembled spark plug more than other pieces of rock? A windscreen have to resist pieces of rock coming out from other veichles, for example. That effect on youtube could be true for *some* bad constructed windows, certainly not for all, as they wanted to make us believe.
    Yes but glass windows are continuously scratched in their surface from little pieces of solid material, even from dust. Why they don't break?
    Last edited: Apr 9, 2010
  12. Apr 9, 2010 #11

    We've already established that the difference is hardness; most general rocks are not higher in terms of the Mohs scale than the galss is, but Aluminum Oxide is.
    That's what we are trying to figure out. Maybe the smaller scratches are just in hte UV protective finish, and not the actual pane?
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