BRAND.G.CHAMELEON%MIT-OZ@MIT-MC.ARPA (06/20/84)
From: Katherine T. Schwarz <BRAND.G.CHAMELEON%MIT-OZ@MIT-MC.ARPA> I don't think anyone has mentioned this aspect (sorry if I'm duplicating anything) When something is ground up, its ratio of surface area to volume increases. This means that surface effects become relatively more important. If a substance is at all reflective, then incident light is much more likely to be reflected from the surfaces of grains of powder, rather than interacting with the material. Reflection generally does not change the spectral character of the light (because reflectivity depends on surface conductivity, which is not too frequency dependent); absorption, of course, does, depending on the material. So anything shiny will appear white when powdered, regardless of its color in bulk, just because the powder reflects much more. (Flour and cornstarch are not reflective; they are white in bulk.)