PostHeaderIcon Why Is Rust Red?

This post is at a deeper level of understanding than a lot of what I write. If you’re lost by step one or two – review spectra and come back!

The Premise

When I think of a good question, I just can’t let it rest. Here’s what I tweeted today:

Why does iron make things red? Mars – blood – iron tablets – hematite scratch tests? (I don’t know)

I’ll clarify/deepen my question: Assume your audience groks spectra and color (in depth). Now answer “Why does iron oxide make things red?”

People started sending me parts of the answer, and I just had to figure it out.

The Puzzle Pieces

Together these pieces make up an answer.

  1. All the colors that are not absorbed are the colors you see.
  2. Spectra for single atoms tend to be simple – electron energy levels are fairly well defined so one detects discrete absorption lines.
  3. Spectra for molecules (such as the various iron oxides) are much more complex, because the electron energy levels are less well defined. In fact there are significantly more energy states and vibration possibilities for molecules, so those discrete lines end up as whole bands of absorption.
  4. Iron oxide happens to be a molecule that absorbs energy in pretty much every state corresponding to the purple, blue, green, and into the yellows – leaving the reds reflected.
    • Blood is a little different, but related. Hemoglobin has hemes – those hemes carry the Iron in the middle. When that heme interacts with O2 (note I did NOT say bonds to) the whole shape of the molecule changes causing the orbitals to move, which changes the energies that can be absorbed by the electrons. These shift towards those blues again, leaving (once again) the reds reflected.
    • Oh, and hematite is grey/silver, but the scratch is reddish because in the hematite crystal structure the orbitals are stuck in one state, but once you disrupt that structure – by grinding it up – the orbital positions can change allowing different amounts of energy to be absorbed – like the blues, leaving the reds. Again.


Thanks @superacid & Doug


Doug McGregor

And I used Wikipedia to get some definitions – like “heme” and “ligand.” That doesn’t mean you should cite Wikipedia as a trustworthy source though!

I am accepting corrections and clarifications to this post – though I’m not going to go into the basics of spectra here. Maybe another time?

~ A l i c e !

2 Responses to “Why Is Rust Red?”

  • Justsomeguy says:

    Not all iron oxide is red though, if you’re a blacksmith and you’re hammering steel, the red hot steel may form “scales” that are essentially Iron Oxide (Fe3O4) these scales however are black.

    • Alice says:

      Hmm. You’re correct about the scaling … but what happens if you do a scratch test (take a scale and scrape it against a white porcelain scratch plate)? I’m going to hypothesize that the scratch itself will be reddish. This is also what happens with hematite.

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