Specific gravity is a good guide to gold purity, but not that
accurate for modern alloys. The specific gravity will vary with
composition (ie the nature of the alloying metals which today can be
v. diverse,) and mode of manufacture ('stamped' objects are more
compact and thus have higher SG than some types of casting). So SG is
only accurate if you know the composition, in which case you don't
need to do the SG ...
Archimedes had simpler alloys to contend with - Iron Age gold
objects are characteristically gold plus silver, rarely more than 2%
copper. Prospectors and mineral hounds can take comfort in the
thought that much the same is true with gold nuggest, 'cos those are
usually just gold plus silver, rarely much more than 1 - 2 % copper.
Other testing techniques include that good old standby the
touchstone and acid (still available in nice little boxed sets from
suppliers). But again modern alloy types make this problematic - some
commercial red karat gold alloys, for example, contain enough copper
for them to froth up green and be mistaken for brass. AND, pet
bandwagon, any jeweller who files a flat on a decent piece of
jewellery and dabs a acid onto it should have the same acid inserted
into each ear. I've even seen folk do that to antique jewlelery
That other modern jeweller's stanbdby, the electronic gold tester,
can produce varied results and the general view seems to be that they
are not that reliable. Any feed back on this area would be great.