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Unoxidized aluminum has a shiny appearance like typical metals. However in air aluminum rapidly oxidizes, giving it a more diffuse appearance (or bidirectional reflectance distribution function). Which (if any) of the following would be a more accurate description for the mechanism driving this change:

  1. The oxidation introduces surface roughness at the scale of light's wavelength, causing what we can think of as lots of specular microfacets that scatter the light in different directions.
  2. The oxidation introduces subsurface irregularities/"particles" (at or smaller than the scale of light's wavelength). Incoming light is then refracted through the metal's surface into the metal, scattered internally by these subsurface irregularities, before refracting back out through the surface.

My uneducated impression is that:

(1) seems unlikely because of how much smaller atoms and molecules are than light's wavelength

(2) seems falsified by the absence of even faint specular highlights on the aluminum that is typical of dielectrics with subsurface scattering, such as glossy opaque plastic.

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  • $\begingroup$ Unless you're looking into a vacuum chamber at a freshly deposited or etched surface, all aluminum you see is oxidized—even shiny aluminum. So I don't really get the premise of the question. Photographs of examples would help. $\endgroup$ Commented Feb 20, 2023 at 20:57
  • $\begingroup$ @Chemomechanics I see. So aluminum is actually mirror-like even with an oxidized layer, and the typical "matte" appearance (relative to other metals) is only due to surface microfacets at- or above wavelength-scale? $\endgroup$
    – Museful
    Commented Feb 20, 2023 at 21:33
  • $\begingroup$ @Museful I think what you're describing as oxidized aluminum might be aluminum with other materials deposited onto it or that has been roughed up by repeated scrapes and scuffs. Chemomechanics is right, freshly polished aluminum oxidizes in a few minutes in air. $\endgroup$
    – g s
    Commented Feb 20, 2023 at 23:12

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Aluminum consists of a myriad of tiny crystallites (size scale ~tens of nanometers) with random lattice orientations. The lattice orientation has a slight effect on the oxidation rate, so the nanocrystals at the surface will grow "caps" of oxide at slightly different rates, which after a while will yield a somewhat bumpy or lumpy surface.

Under controlled conditions, it is possible to grow an oxide on a piece of aluminum in such a manner that the growth rate differences from grain to grain are swamped out, and you can get smooth, shiny oxide on purpose.

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