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Testing to determine if a metal is gold


#1

Greetings all! I have searched the archives and can’t seem to find
what I need, so if the answer is in the archives please point me to
it.

My father recently passed away and when we cleaned out his workship I
found half a dozen or so large sprues. One of his hobbies was
casting. I think these are probably 10K gold as his favorite method
of obtaining metals was to buy old high school rings. These things
have the heft and color of gol d and I would not imagine him wasting
his time on base metals… but I als o don’t want look like an idiot
sending Nu Gold or some brass alloy to a re finery. So, my question
is - is there a quick test for gold? I don’t care much about the
karat content, just want to confirm it is gold. I do some etching of
silver from time to time, so have access to varying concentratio ns
of nitric acid ranging from fuming on down. Yes I know how to work
with nitric acid. I’ve read one tests for gold using nitric acid -
but how. Wh at concentration, what do you look for?

Again, if this is in the archives please just point me to it.
Thanks!


#2
So, my question is - is there a quick test for gold? I don't care
much about the karat content, just want to confirm it is gold. 

Given that they look roughly like 10K gold (yellow color) the
choices are limited to what you suspect, versus something like brass
or bronze or nugold, etc. So the basic test is simple. Run a file
over the metal to expose a bit of clean metal, then put a drop of
nitric on it. Concentration isn’t critical, but reasonaly strong. If
it’s higher than 14K, not much of anything will happen. 14k itself,
with concentrated nitric, will slowly turn dark. 10K will quickly
turn dark. Lower than that, you’ll start to see a greenish tinge to
the acid drop and it may start to be more actively etching. But if
it’s brass, bronze, nugold, the drop of acid will, after a moment,
start bubbling fairly vigorously, and turning quite distinctively
green. if you’re not sure, use a bit of known brass or bronze as a
comparison.

You can also, if you’ve got something to use as a touchstone, rub
the metal onto the stone (an unglazed tile works fine too) to leave a
fairly substantial streak (takes rubbing back and forth until you’ve
got a good mark, heavy enough so it’s the color of the metal you’re
testing. Put the drop of acid on that. You’ll see the same reactions,
but there’s a difference. If the metal has no gold, by the time the
acid stops reacting (add more if you’re not sure), the streak will
have completely dissolved, leaving at most, a faint smudge, and
possibly not that, after you rinse the acid off (don’t scrub. Just
gently rinse). If there was gold in the metal, it won’t have
dissolved, and you’ll see some of the streak remaining, usually as a
dull brownish color, but still something there that did not dissolve.

If the drop of acid causes no reaction, and you suspect it may be
higher karat gold, add a few grains of table salt to the acid drop.
That is chemically the same as adding a bit of hydrochloric acid, so
then what you’ve got is fainly resembly dilute aqua regia. That will
allow it to react with even up to 18K gold. Higher than that, and you
might have to add actual Hydrochloric acid to get that reaction, and
by then, you’d be perhaps using HCL first, then adding a bit of
nitric…

If your beginning tests suggest gold, take the trouble to carry it a
bit further, with the test stone, by finding some samples of gold of
the karats you think it might be. Take a rubbing/streak of these
samples next to your unknown. It’s much easier to decide whether a
reaction is one thing or another if you’ve got a known sample’s
reaction to compare it to. Be sure the samples are essentially the
same color range as the unknown. Using white gold 14K samples to see
if a yellow unknown is also 14K won’t work so well, for example.

Hope that helps
Peter Rowe


#3

If you put it in ammonia and the ammonia turns blue, it is not gold.
But it does not confirm that it is gold. Start with that. Then go to
the acid testing.

If anyone knows a simple test to confirm silver without the acids, I
would like to know that.


#4

Brent, Just place a drop of nitric on the piece and watch the result.
If it bubbles and turns green it’s not gold. If no appreciable
reaction it is. In the case of some low karat golds it may turn
black.

Jerry in Kodiak


#5
If anyone knows a simple test to confirm silver without the acids,
I would like to know that. 

very off the cuff and imprecise, but…

Look at it first. The color is unique. whiter than any other metal.

Second, heat it up on one end, and see how fast the other end starts
to burn your fingers. Silver’s thermal conductivity exceeds that of
other metals significantly. aluminum, I think, is next, but it looks
different, and still conducts heat more slowly.

And, with mild heating, you also get an idea of how easily it
oxidizes. That will give you a pretty good clue telling between fine
silver and sterling.

If you have a laser welder handy, just hitting it with a pulse who’s
power settings you already know have a certain effect on silver, will
give youa pretty good idea too. With that, you can distinguish fine
from sterling, and even argentium from standard sterling.

Peter