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