I’m going to try something a little exotic for the heatsink on my next build. Only because I scored it for free and the chunk of copper that had my eye is $20 plus shipping.
I’ll be impressed if anyone gets this one (except maybe Hoop).
Picture with my S41 and a brass .38 casing for color reference.
I think it’s refined actinium.
Probably wrong, but I have some nickel bar stock that looks a lot like that.
It could be bronze, and is it for a bearing of some sort?
It’s just a chunk of bar stock squared up, not really for anything per say.
unobtainium oxide (or platinum)
That’s an unmachined brake piston cup slug for a 2007 Cadillac SRX rear caliper.
I have no idea but I can’t wait to see what you do with it.
Bob
Boss, if that was platinum there’s no way I’d be online right now talkin bout flashlights with y’all.
indeed, no one in their right mind would give it to you for free either 
Tin
That looks like the Aluminum volume control knob off of my Pioneer SA9500 Stereo amplifier circa 1976.
I dunno. 45-70? Or 45 long colt? I had to change my signature line for stuff like this.
A slug that they send through pipelines to clean them.
I’m leaning towards bronze.
Hmm… because you’re making a heat sink, I have to assume that it is a thermally conductive material like aluminum. It’s originally intended use is unknown. But there are striations in the metal, so I’m thinking that it is not a finished piece… or that it is a leftover scrap from some other product.
So basically, I have no idea.
0% alum 0% tin
high speed steel?
Monel?
Negative. HSS has terrible thermal properties.