Rust and Reflection: The Optical Properties of Oxidized Steel

SkepticJ
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A couple of days ago I burnished a very rusty old piece of steel with a fine steel wool. It's now very smooth, and there is no loose rust scale. It has a finish somewhat like a century-old doorknob in some old building at a university. Some place where hands have rubbed it many thousands of times over the years.

No oil, wax etc. was applied to the piece, yet the burnished rust is much darker than it was before the process. Why is this? The rust is much more reflective than the matte it was. I can see the blue of the sky showing in the highlights, but the shades of the rust colors are much darker--darker than wet rust.

Is this a similar phenomena to the darkening that happens to wet objects? Some rocks, for example, look much darker when wet. I've noticed this in slate, for one. Why does this happen, as well?
 
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