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I expect that what one can see on the outside of a just solidified piece of metal is just the "raw" surface of the inner stucture.

Solidifying metals or alloys arranges in partial christal latices in many variants.

I'm interested in how the surface ends up directly after solidification, disregarding later effects like oxidation.

How comes there is so much roughness on a metal surface?

It is possible to make a metal surface shiny smooth by mechanical polishing,
but also by moulding it into a polished mould.

What is the difference between the natural and the polished surface?


This too applies to thermoplastic polymers that are partially chrystalline,

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This photo from a proposal for stainless steel stents ( to be used in humans) is enlightening

stainlesssteel

stainless steel scanning electron microscope (SEM) photo

The crystallization is obviously random, with a lot of pores at that size ( which is what they try to increase for the purposes of the study). This means that the facets of the crystals are not on a plane so as to behave as a mirror. Polishing will cut off anomalies and allow a preponderance of crystal faces oriented in the same plane so as to get a shiny surface. The same should be true for any material with crystal structure.

Now why the metal has solidified in this manner is a function of the temperatures and pressures during its construction. Crystal growth follows random patterns starting from the seeds.

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  • $\begingroup$ Silicon, though or because(?) of not being a metal, can be produced into a crystalline / single crystal structure (boule) which looks like polished if I'm not mistaken. It is created by heating above its melting point according to Wikipedia (don't know if this means melting but sounds like it) and cooling slowly from a seed crystal. No idea if this could work for metals. $\endgroup$
    – Arc
    Commented Jul 9, 2015 at 3:46
  • $\begingroup$ @Archimedix Single crystals can be made in various ways, and they are reflective ( diamonds are the best). There exist metallic flakes that are reflective ( gold flakes for example are found in nature) $\endgroup$
    – anna v
    Commented Jul 9, 2015 at 4:02

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