# Spark Plugs and Glass. **URL:** \ **Category:** Factual Questions\ **Created:** [June 17, 2002, 9:42pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591 "2002-06-17T21:42:58Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![d12](https://avatars.discourse-cdn.com/v4/letter/d/71c47a/32.png) [@d12](https://boards.straightdope.com/u/d12)\ **Post date:** [June 17, 2002, 9:42pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/1 "2002-06-17T21:42:58Z")
It’s always bothered me, but if I throw a spark plug (the white part) at a pane of glass/window will the glass shatter? A friend told me he tried it on a car in auto class and he broke the window. I kinda doubt it but if it does make some sense how does it occur? ---
**Author:** ![catmandu42](https://avatars.discourse-cdn.com/v4/letter/c/3ab097/32.png) [@catmandu42](https://boards.straightdope.com/u/catmandu42)\ **Post date:** [June 17, 2002, 10:00pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/2 "2002-06-17T22:00:24Z")
Only if you throw it really, really hard. The “white part” is usually just common ceramic, and has no special glass-shattering properties that I know of. If your brother isn’t full of it, the glass he shattered was probably ready to do so anyway. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 17, 2002, 10:02pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/3 "2002-06-17T22:02:13Z")
Ummm… I don’t understand your confusion; throwing things at glass can break it, a little piece of gravel can break a windscreen if it hits hard enough. ---
**Author:** ![Malice](https://avatars.discourse-cdn.com/v4/letter/m/278dde/32.png) [@Malice](https://boards.straightdope.com/u/Malice)\ **Post date:** [June 17, 2002, 10:09pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/4 "2002-06-17T22:09:35Z")
I can’t speak to the why of it, but I can vouch for the window-shattering abilities of a small piece of hurled ceramic. My wife’s car stereo was stolen and the police officer who responded determined that the culprit had thrown a small piece of ceramic from a spark plug through the glass. The officer noted that stereo thieves use this tactic frequently because it is effective, quick and–interestingly–relatively noiseless. ---
**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\ **Post date:** [June 17, 2002, 10:20pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/5 "2002-06-17T22:20:07Z")
The ceramic of a spark plug will easily shatter automobile glass if propelled by a sling shot. A group of teens near where I live were arrested and charged with mulitple auto breakins. They might have gotten away with it till the police found a couple hundred spark plugs missing the ceramic behind one of the teens home. The teens even had a couple auto repair shops saving old spark plugs for them. ---
**Author:** ![bcullman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bcullman/32/8207_2.png) [@bcullman](https://boards.straightdope.com/u/bcullman)\ **Post date:** [June 17, 2002, 11:38pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/6 "2002-06-17T23:38:40Z")
> [@](#): > > \*Originally posted by racer72 \* > \*\*The ceramic of a spark plug will easily shatter automobile glass if propelled by a sling shot. A group of teens near where I live were arrested and charged with mulitple auto breakins. They might have gotten away with it till the police found a couple hundred spark plugs missing the ceramic behind one of the teens home. The teens even had a couple auto repair shops saving old spark plugs for them. \*\* Umm, but so will a small stone. What is the significance of using a spark plug (or a part of a spark plug) to break glass? ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [June 17, 2002, 11:53pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/7 "2002-06-17T23:53:24Z")
If there’s anything to it at all, **bcullman** , I’d guess it’s this part, mentioned by **Malice** above: > [@](#): > > relatively noiseless ---
**Author:** ![caracal](https://avatars.discourse-cdn.com/v4/letter/c/91b2a8/32.png) [@caracal](https://boards.straightdope.com/u/caracal)\ **Post date:** [June 18, 2002, 1:16am UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/8 "2002-06-18T01:16:52Z")
I’ve heard the same thing. I don’t think the force of the impact is supposed to have anything to do with it. After searching the web, most sites that mention this are vandalism and burglary tutorials. But [this site](http://www.astm.org/JOURNALS/FORENSIC/PAGES/703.htm) lists an exerpt from the Journal Of Forensic Sciences: > [@](#): > > A novel and efficient method for forced entry into vehicles using broken spark plug insulators to break the windows was reviewed. Various experiments were performed in which a number of objects, varying in their physical characteristics, were compared to the broken ceramic plus insulators in order to examine the uniqueness of the latter in the breaking of tempered glass. A plausible mechanism was presented attributing the great efficiency of the insulators to their hardness in relation to other materials (MOHS scale), and to their potential for the concentration of force in one point because of the sharp, jagged ends. ---
**Author:** ![d12](https://avatars.discourse-cdn.com/v4/letter/d/71c47a/32.png) [@d12](https://boards.straightdope.com/u/d12)\ **Post date:** [June 18, 2002, 3:40am UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/9 "2002-06-18T03:40:23Z")
Earlier someone was telling me it was because the charges between each were so opposite that it just went boom 😕 ---
**Author:** ![d12](https://avatars.discourse-cdn.com/v4/letter/d/71c47a/32.png) [@d12](https://boards.straightdope.com/u/d12)\ **Post date:** [June 18, 2002, 10:39pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/10 "2002-06-18T22:39:50Z")
Is the “white” part just ceramic? Not anything else added to the compound? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 18, 2002, 10:55pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/11 "2002-06-18T22:55:02Z")
‘ceramic’ is a generic term encompassing a broad range of materials of diverse composition. ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [June 18, 2002, 11:10pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/12 "2002-06-18T23:10:38Z")
Tempered glass can be remarkably hard to break if you strike it with a blunt object. I’ve done a similar experiment with an old side window from my VW rabbit and a ball peen hammer. I eventually gave up trying to break it with the hammer. A sharp, hardened point like on a broken spark plug or a center punch will do the trick with remarkably little force. Remember that tempered glass is under stress between the core and surface. That’s what makes it break into small pieces when shattered where ordinary glass will break into large jagged pieces. A small chip in in tempered glass forms a stress riser that allows the stress in the glass to do the rest of the work. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 18, 2002, 11:16pm UTC](https://boards.straightdope.com/t/spark-plugs-and-glass/114591/13 "2002-06-18T23:16:25Z")
Like [Prince Rupert’s drops](http://www.cmog.org/page.cfm?page=281).