# Why can one small piece of porcelain shatter auto glass?

**URL:** <https://boards.straightdope.com/t/why-can-one-small-piece-of-porcelain-shatter-auto-glass/113799>\
**Category:** Factual Questions\
**Created:** [June 13, 2002, 5:01am UTC](https://boards.straightdope.com/t/why-can-one-small-piece-of-porcelain-shatter-auto-glass/113799 "2002-06-13T05:01:52Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Chris\_Luongo](https://avatars.discourse-cdn.com/v4/letter/c/7cd45c/32.png) [@Chris\_Luongo](https://boards.straightdope.com/u/Chris_Luongo)\
**Post date:** [June 13, 2002, 5:01am UTC](https://boards.straightdope.com/t/why-can-one-small-piece-of-porcelain-shatter-auto-glass/113799/1 "2002-06-13T05:01:52Z")

</div>

[This post](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=120004) about car windows breaking with no apparent cause, reminded me of something else.

You take an old car spark plug, and smash it with a hammer until the porcelain insulator breaks up.

If you toss one of these porcelain fragments at a car window, the window instanly cracks up over its entire surface, and then, any slight disturbance causes the glass to fall out.

I remember in high school kids would mention this, but I didn’t believe it. At the local junkyard, I tried this on a car that was about to go into the crusher anyway, and it does indeed work as described. It seems to take as many as two or three porcelain-tossing attempts, but you don’t even have to throw it very hard.

Is spark plug insulator material really porcelain?

Why/how does this work?

---

<div class="post-metadata">

**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\
**Post date:** [June 13, 2002, 10:47am UTC](https://boards.straightdope.com/t/why-can-one-small-piece-of-porcelain-shatter-auto-glass/113799/2 "2002-06-13T10:47:44Z")

</div>

It sort of surprises me that this would actually work, I would think that the piece of porcelain (I think the insulator is porcelain, which has been used as an electrical insulator for a _long_ time) would be too small to actually do anything. Here’s my theory anyway:

Porcelain is a harder substance than auto glass (7 vs 6 on the Moh’s scale) and can easily scratch the surface, similar to how a glass cutter scratches a glass surface. Because auto glass is often under high stress, a significant scratch or chip can cause the entire piece to shatter.