# What are the black marks created by rubbing metal on ceramic?

**URL:** <https://boards.straightdope.com/t/what-are-the-black-marks-created-by-rubbing-metal-on-ceramic/677888>\
**Category:** Factual Questions\
**Created:** [January 4, 2014, 5:06pm UTC](https://boards.straightdope.com/t/what-are-the-black-marks-created-by-rubbing-metal-on-ceramic/677888 "2014-01-04T17:06:37Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\
**Post date:** [January 4, 2014, 5:06pm UTC](https://boards.straightdope.com/t/what-are-the-black-marks-created-by-rubbing-metal-on-ceramic/677888/1 "2014-01-04T17:06:37Z")

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If you rub metal on some ceramics like a plate, porcelain sink, toilet, etc, it will leave behind a black mark. The mark can be removed by scrubbing. What is that black mark? Is it metal shavings, a chemical reaction with the ceramic, or something else?

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [January 4, 2014, 5:10pm UTC](https://boards.straightdope.com/t/what-are-the-black-marks-created-by-rubbing-metal-on-ceramic/677888/2 "2014-01-04T17:10:29Z")

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It’s metal rubbing off.  
You have re-created the [touchstone](http://en.wikipedia.org/wiki/Touchstone_(assaying_tool)).

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**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\
**Post date:** [January 6, 2014, 12:01pm UTC](https://boards.straightdope.com/t/what-are-the-black-marks-created-by-rubbing-metal-on-ceramic/677888/3 "2014-01-06T12:01:31Z")

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Percelain has a very fine grain .

Ceramics used in the kitchen are covered in glass, glass has no grain, so as to avoid this effect.

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**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\
**Post date:** [January 6, 2014, 1:10pm UTC](https://boards.straightdope.com/t/what-are-the-black-marks-created-by-rubbing-metal-on-ceramic/677888/4 "2014-01-06T13:10:40Z")

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> [@Isilder](#):
>
> Percelain has a very fine grain .
> 
> Ceramics used in the kitchen are covered in glass, glass has no grain, so as to avoid this effect.

And, being a liquid also makes it a lot harder to leave marks.

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**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\
**Post date:** [January 6, 2014, 3:24pm UTC](https://boards.straightdope.com/t/what-are-the-black-marks-created-by-rubbing-metal-on-ceramic/677888/5 "2014-01-06T15:24:26Z")

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ceramic knife sharpening hones are full of those black marks. it is material removed from the metal object.