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Verifying Gold Alloys


#1

I have to verify the gold content of medals and orders to determine
whether they are real or fake. I want to determine the karat value of
the gold alloy.

I use a precision scale, hang the part on the scale, zero the scale
and then immerse the part in water. The weight reduction tells me the
volume of the part. I then weigh the part itself in air. Dividing the
weight by the volume gives me the density. A comparison with known
densities of metals and alloys tells me the composition: 8.9 is brass
or copper, 10.4 is silver, 11.5 to 19.3 is gold between 10 and 24
karat.

Is this approach commonly used or are there other non-damaging
methods? In my opinion, the test must not just check the surface
(like the stone-rub and acid test, which in addition damages the
part) because plating can falsify the results.

A discussion of gold-alloy testing or a reference to any earlier
exchange in this Forum would be appreciated.

Thanks,
Dietrich


#2

There are thermodilution testers available Very NON destructive Look
in Rio, or Stuller Best Regards, BobnCyn Wallace


#3

Dietrich, I’m sure you are proceeding in the right direction,
especially with being concerned that touchstone methods may cause
damage (although only important I would think in the case of
absolutely mint examples) and, more importantly, that they only test
the surface.

Your proposed method gives you the overall density of the piece,
subject to experimental factors that I’m sure you’re aware of
(accuracy and precision of the balance, temperature, method of
suspension, no adhering bubbles …). The question then has to be
how happy are you to take density as an indication of metal
fineness?

For gold items there shouldn’t be too much problem, as it’s not easy
to find a base material sufficiently dense to serve as a fake. The
only one that springs to mind is tungsten. This wouldn’t be alloyed,
but I suppose a disk of tungsten, which is becoming increasingly
available today (OK, in the form of a tungsten composite together
with some other metal, sometimes cobalt) could be given a coating of
gold so as to have the right density for a fairly high carat item.
Unlikely maybe (maybe not)… But bear in mind that for a given
high density, although it indicates that the item is (probably)
composed of noble metals, there will be a huge range of possible
alloys that give that density, comprising gold, platinum, palladium,
silver, copper, so you wouldn’t be able to assign a gold carat value
on the basis of density alone. Touchstone testing would help
considerably.

Silver is much more problematic, on at least two counts. First,
lead has a density of about 11.34, and has a low melting point,
making it pretty well ideal for fakes. Secondly, a wide range of
silver alloys have been used for medals (indeed for all silver
items). So, a perfectly genuine silver medal may be only say 70
percent silver, and so have a lower density that Sterling.
Presumably reference books on medals would (might?) list the actual
fineness used. Remember also that some genuine medals are actually
made of base metal with a silver or gold plating. Again, you’d need
to know this from reference works.

X-ray fluorescence wouldn’t help either, unfortunately. The metal
plating industry (especially printed circuit manufacturers) has
bench top instruments that will give the gold content of a small spot
on an item, but unfortunately it’s only looking at the top most layer
of the surface, just a micron or two. The only certain answer, as
you know, would be to melt the item down and analyse a representative
sample…

I imagine that density determination would be just a part of the
procedure needed to determine whether a medal is genuine or not. A
necessary part, but not sufficient. I can imagine that it wuld also
require a careful optical examination by a medals expert, who would
consider things such as exact dimensions, crispness of detail and
similarity (or not, hopefully) with known fakes. For example, for a
medal of sufficient value it could be worthwhile for a forger to use
18ct gold or whatever the original had.

Thanks for bringing such an interesting and problematic topic to the
forum, and I wish you good luck in your work.

Kevin (NW England, UK)