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Distinguish glass hard disk platters from aluminum without marring the reflective surface?

I have hundreds of hard disk platters from disassembled hard drives.  Some are made of glass, some are made of aluminum.  The glass ones, when flexed, shatter into thousands of razor shards which screech across the room. I would like to avoid that in the future.  I only want to use the aluminum ones, but I don't want to mar the reflective surface.

I've thought of weighing them, but the platters some from many manufacturers and eras and so the weights won't be consistent.  Magnets don't attract glass or aluminum. Does glass float? (I'm guessing no.)  Is the reflective surface transparent? (I'm guessing no again, but maybe a strong light would work?) There has to be a way....

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rickharris6 years ago
Simply waving a neodymium magnet over the disk will do - You can feel the drag from the eddy currents in the aluminium.
+1
PhilipD3211 months ago

The sound is going to be your best analysis, once you get the hang of it. The metal ones had a long resonance, and the glass not nearly so good.

PhilipD3211 months ago

Coming to this a bit late, perhaps but ... Light transmission worked for me, but only when I used a laser pointer. A very faint pin prick of light could be seen. I also checked the densities, and there was a difference, but perhaps there is variation in the glass / aluminium alloys used. I got 2.4 for glass and 2.7 for the metal ones.

arpruss5 years ago
I am not sure about the magnet method--isn't the glass coated with something metallic anyway, so it would be conductive?
Odonata (author) 6 years ago
I thought more about this and realized there is a very bright light in the sky above me. I looked a the sun through a glass hard disk platter and could see a dim red sun. I looked through an aluminum one and could only see my own reflection. So it turns out tha light is one possible answer. I will try toe magnet idea as well, so there may be more than one way which is nice to know. Thanks for the answers!
AndyGadget6 years ago
The glass ones, when flexed, shatter into thousands of razor shards which screech across the room.

Yep.  Been there - Done that.
I bent a 2.5" platter once to get a feel for what sort of 'metal' it was made of.
If I hadn't been wearing reading glasses I'd probably be typing this in braille.

I think Rick has the answer to your question.
You could use a metal detector.
https://www.instructables.com/id/Home-Made-BFO-metal-detector/

https://www.instructables.com/pages/search/search.jsp?&q=metal+detector

https://www.instructables.com/file/FHO9A5ZGH4NNAKO/

That should work without even having to touch the platters, thus reducing the risk of marring the surface of said platters.

The trick already mentioned regarding waving/moving a strong magnet near them, that will work too, but you know, your hands might get tired, especially if you've got hundreds of them to go through, and you probably wanted an excuse to build a metal detector anyway.
;-)
frollard6 years ago
spin up the drive if you can, bring a magnet near it. Through eddy currents, the aluminum platters should brake heavily when a magnet is next to them, and/or the magnet should feel considerable pull in the direction of the platter's motion.

Also, if you have a sample of each, I'd venture that the sound of clinking them should tell you

Also, don't the aluminum ones have square edges where the glass ones have a rounded edge?