Hi there Poetic,
Gold will dissolve only in a mixture of Hydrochloric Acid, (HCl) and
Nitric Acid, mixed in a 3:1 ratio. The resulting mixture of acids was
named Aqua Regia by alchemists because of its unique ability to
dissolve gold.
So, when the acid mixture is applied to a piece of gold jewelry, if it
bubbles, the gold is pure. The actual karat of the gold cannot be
determined by this method, but if the acid mixture happens to turn
blue, Copper has been detected and the piece is not pure gold.
The test for simple purity is performed this way, with the following
results:
Scratch the object with a file and apply a drop of Nitric Acid. If the
object turns a bright green it is gold plated or gold filled on base
metal. If the object turns a pinkish cream colour it is plated or gold
filled on silver. 10 karat gold will turn dark brown. 12 karat gold
will turn light brown. 14 karat or higher will have little or no
reaction.
To find the karat of a particular piece of jewelry, the following
technique is used: File a clean spot on the article to be tested and
rub it on the test stone, (usually a small slab of flat slate). Doing
this will leave a small “streak” of the gold that will be used to
perform the karat tests, rather than testing on the actual piece of
jewelry.
The next steps involve testing the streak left behind on the test
stone. Test kits come with “test needles” made of differing karat
gold. Rub a test needle nearest the karat you think the article is
next to the mark. Apply acid to both marks simultaneously. (Pure
Nitric Acid for 12 karat or under; Aqua-Regia for 14 karat or higher).
If the colour does not match try another needle until it does. White
and green gold react slowly, but will still leave a brown mark after
applying acid.
The following site has more information on the acid test, and
information on how to test for the number of karats in a gold ore
sample; you wouldn’t want to use this technique on Grandma’s old ring.
It calls first for weighing the sample, but then crushing it and
mixing it with 7 times its weight in silver:
http://about-the-web.com/shtml/reports/09-049.shtml
Here is a page that lists the reactivity of gold with different
substances:
http://www.webelements.com/webelements/elements/text/Au/chem.html
The phrase “Acid Test” came from the Gold Rush days in California
because of the Nitric Acid test for gold:
http://www.webelements.com/webelements/elements/text/Au/chem.html
Hope this answered your question!
Kutsavi
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