HN Explorer

Ask HN: What makes some metals shine?

16 points by rhn_mk1 ↗ HN
Metal surfaces can have those geometrical patterns, like grains a few millimeters in size, appearing only when light is at the correct angle.

Here's one picture with an example, right side: https://nnnnnnnn.co/log/210514.html

8 comments

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db48x
That particular photograph is probably edited.

The grains you see in a steel item are just grains of steel. Each grain has a particular crystalline structure, and neighboring grains have different structures or different orientations.

Most other metals lack a visible grain structure because they are purer (and therefore less likely to crystallize) or the metals in the alloy do not produce as many variations in grain structure.

rhn_mk1
I've just noticed that the patterns are suspiciusly regular on this picture. Nice catch!
rnvannatta
I don't think it's edited. It's just hot dipped galvanized steel, those are zinc crystals on the surface.
db48x
I suspect the colors were manipulated though. Some of the zinc crystals appear to be reflecting a cyan light placed somewhere else in the room, but look at the one cyan reflection under the guy’s thumb.
durkie
This texture very often is the result of galvanizing the base steel to prevent/reduce corrosion: coat it with zinc so it "rusts" preferentially to the iron base metal. Similar to a sacrificial anode on a boat: "here, seawater, you need to eat something, so eat this chunk of zinc instead of my fancy bronze bolts/steel hull/etc"
mrob
Dislocations in the crystal structure. Here's a demonstration using a raft of small bubbles to simulate atoms in a metal (actual demonstration starts at 5:12 if you want to skip the introduction):

https://www.youtube.com/watch?v=sn1Y6zIS91g

You can see the same grains forming as the bubble raft is deformed.

noman-land
Very interesting demonstration. Thank you for sharing.
rnvannatta
That's hot dipped galvanized steel. The zinc crystal pattern formed on it is a called spangling.

https://www.steelmillsoftheworld.com/activities/datacenter/G...

It shines like that, with only some of the crystals being lit up at a given view and sun angle, from the surface being anisotropic. Light reflects more mirrorlike if it bounces parallel to the grooves in each of the individual crystals, and it is scattered if it bounces perpendicular to the grooves.

I think there's also an oxide layer contributing some iridescence, essentially a thin layer can cause constructive and destructive interference depending on light wavelength and thickness. If the light bounces mirrorlike, then a fraction of light bounces off the oxide surface, and a fraction transmits, then bounces off the metal. Depending on wavelength, these two light packets will interfere constructively or destructively. It depends on how many extra wavelengths the transmitted light travels (which depends on layer thickness, reflection angle, and light wavelength).