Gold Dust on Aluminum


Suppose you have a lot of small aluminum foil pieces, each around
maybe a cm squared, with either fine gold dust deposited on it or fine
copper dust - I'm not sure which it is. The "yellowish material" on
the aluminum comes off very easily and can be usually wiped off with
your finger. I can acquire as much of this dust as i need but it
always will be on the aluminum shards. These pieces are the byproduct
of a little random-rock processing machine I use. I find it curious
these deposits are all over the aluminum, it really looks very golden!

I'd like to find a quick and easy way to first determine if it is gold
or copper or whatever. I'm pretty sure it is metallic. I also am quite
certain it is one or the other just based on the kinds of "random
rocks" i have been injecting. If it is gold, I'd like to recover it
chemically if possible (and as safely as possible too).

I have read that gold has particular reflective characteristics under
a black light but am uncertain if this is true (I have not tried).
Does copper look entirely different or have different qualities I can
identify under a black light? I would need something definte, being
that it is a fine dust of either copper or gold.

I suppose I could simple keep scraping and scraping until I had a
significant pile. Then i could burn it with a lighter or match and see
if it oxidizes to something tarish/black. if so, it has to be copper?
i know gold will oxidize but i think it would melt first? the
particles are so fine, i'm concerned they'd become quickly lost - it
is very easy to blow it all away once collected. it is also a pain to
scrap such tiny pieces of aluminum (and i have to sort out the normal
pieces from the very golden looking ones).

the other option is the use of acids. i am aware copper will dissolve
in nitric acid quickly, whereas pure gold will not. aqua regia will
dissolve gold quickly and i imagine copper as well. i'd prefer,
however, not to play with acids. one, i don't like acids and two
nitric acid is somewhat difficult to acquire these days.

well, if i find out its gold, i'd like to collect it into one solid
bit. i first tried dissolving the aluminum pieces, with dust material
on it, in a solution of sodium hydroxide. the aluminum dissolved
quickly and formed the aluminates. after sitting an hour, the grey-ish
material floated to the top and a reddish material lingered in the
middle and bottom. eventually it all began to mix again at the bottom.
I tried boiling if off and was only left with a lacquer/nail polish
looking liquid remain that i simply washed away. I am wondering if the
reddish material is an indication of copper and not gold? in either
case, why was there no more reddish (or even yellow for that matter)
material left after boiling? i did boil at high heat on a normal
electric stove - perhaps i will try at a lower heat next time?

if it is gold or copper i could simply place the pieces in some sort
of crucible and melt it down slowly at least 1100 celcius. the gold or
copper would melt second, the aluminum first. i think this would
definately alloy it however if it got that hot. hence, my search for a
chemical means to get a solid, pure piece of this mystery dust.

i have read that aluminum-copper alloys look golden but the dust is
clearly not a alloyed with aluminum in my case. i have also read that
aluminum-gold alloys can look golden or even white but i have yet to
see this either. again, the dust easily rubs off most of the time.
some pieces require a little more force, like that of a razor scrape
against a kitchen counter for example. nonethless, very easy to get
off.

i also suppose i could take it to a local jewelry dealer and see if
they'll use some of their carat testing kits/devices on it. if it is
pure gold, it should definately be detectable - it is definately
golden looking stuff.

any comments or ideas would be greatly appreciated. thanks.

.

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