In my own experience, running or rubbing a high-carbon steel on wood leaves a pencil-like trail. Is this simply carbon rubbing off, or is it a more complex reaction involving tannins in the wood, like another user suggested? EDIT (thanks Ivan Neretin): Or something else entirely, like iron (or its oxides) rubbing off? And to be more precise: is it lamellar hematite that rubs off and looks gray, or pure iron or a combination?

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    $\begingroup$ Carbon does not react (nor does it rub off). Iron does. $\endgroup$ Commented Sep 7, 2017 at 18:14
  • $\begingroup$ @IvanNeretin: well, that's the answer then! $\endgroup$ Commented Sep 7, 2017 at 18:20
  • $\begingroup$ The reaction with tannins requires moisture and some time. $\endgroup$
    – aventurin
    Commented Sep 9, 2017 at 8:02
  • $\begingroup$ Can you describe what kind of wood you are talking about ? if it's a polished one or one with a brown colour coating it would be simply like rubbing of the color part by scratching $\endgroup$ Commented Sep 10, 2017 at 3:27
  • $\begingroup$ Not coated with anything, just regular softwood timber. $\endgroup$ Commented Sep 10, 2017 at 4:02


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