Specific Gravity

From: Dr Gyula Petranyi 
Date: 1998 Apr 23 00:36:09 +0300

Specific gravity can be used as a simple measurement to find out the ratio
between the components of a bimetallic alloy (remember Archimedes and
heureka). The method has been used for coins, too, but I wonder if striking
changes specific gravity. Hammering can already make the flans more dense
and then striking the coin may add to this. Is this effect big enough to
make specific gravity measurement unreliable in numismatic research? Does
anyone know more about this?



From: Jxrgen Sxmod 
Date: 1998 Apr 22 23:33:20 -0700

I am often using specific gravity as a non-destructive testing of an
alloy. Gold, which has 19.3 can by striking be highened to 19.6. I
don't know the changings for other metals. But this is not the main
problem.  Impurities and gas bobbles can really change the specific
gravity. 

So, I remember many years ago a beginner had bought a genuine
minor 17th century silver coin and now tested the specific gravity,
which in this case was only 7.x or 8.x and he thought it was a
forgery. It gave a lot of troubles for some people, who thought they
were experts! - And guess how my flaming was against them.



From: David Garstang 
Date: 1998 Apr 22 20:10:56 -0700

Specific gravity won't change due to striking solid metal with no
bubbles.  Metal is not compressable.  It is malleable -- it will
"squish" into a new shape -- but the volume won't be altered unless it
contains gas bubbles.  It seems to me that the striking would tend to
squeeze out the bubbles, making it closer to the correct specific
gravity reading.

My question is, how do you get accurate enough volume and weight
readings on an object that small?  What type of equipment do you use,
where do you buy it, and is it horrendously expensive?  If not, I'd
like to create my own setup.



From: Ibpanda 
Date: 1998, Apr 1998 18:52:17 EDT

Well, the method of Archimedes will still work. You need a graduated
cylinder of adequate diameter, callibrated in milliliters or
finer. For tetradrachms that might be hard to find -- standard lab
glassware would fit a denarius or an obol, but not a big old honking
sestertius.  If I pour 20 ml. of distilled water into the cylinder and
then put in the coin and read the resulting volume as 21.6 ml I know
the coin has displaced so many ml of water. Now the book says the
density of pure silver is 10.49 grams per cc.  (1 cc ~= 1 ml at 20
deg. C) So a nearly pure silver tetradrachm that weighs 17 grams
should displace about 1.6 ml, unless I did the math wrong.

A good laboratory supply catalog should have balances accurate to a
hundredth of a gram -- not sure what they cost -- probably a few
hundred US$.  Most jewelers only use balances accurate to a tenth of a
gram.  I have no idea how analytical laboratories measure the volume
of very small objects, but I imagine it's just some refinement of the
displacement method.



From: Martins, Ken/SCO 
Date: 1998 Apr 23 17:21:50 -0600

Expanding on Ibpanda's suggestion to use liquid displacement to
measure the volume of the coin:

I would suggest adding a small amount of surfactant (soap) to help
remove air bubbles attached to the coin.

There is still a big issue of accounting for thick patina.  This
method may not be suitable for heavy patinated coins.



From: Warren Esty 
Date: 1998 Apr 23 18:58:26 -0600

Contrary to a recent suggestion, I do not think "the displacement
method" commonly used for finding the specific gravity of coins
directly measures the volume of the liquid displaced. Weigh the coin
with a balance (to .001 gram if possible). Then weigh it again
immersed in a dense liquid while it is suspended by a very fine
thread. The displacement will cause it to weigh less by the weight of
the liquid displaced (I think carbon tetrachloride used to be used
because it is much denser than water and so gives a greater weight
difference and thus more accuracy, but I don't know what is used now,
given the hazzards of chemicals). Then the density of the liquid can
be used to determine the volume of the coin from the difference in
weights in air and liquid, and, with its weight in air, the density of
the coin follows.



From: J. Mike DeWeever 
Date: 1998 Apr 23 22:28:34 EDT

A "grain" scale used by all firearms cartridge reloaders, will
accurately measure to +/- 1/10th (.1) of a grain, which is
1/ 70,000th of a pound (1 lb.= 7,000 grains )
1 / 154th  a gram (1 gram = 15.432 grains)
these start at about $45.00 with a top end of 450 grains, there are
also balance and electronic models which may go higher ... 
weight and price, call a local sporting goods store that sells
firearms (you will be discussing "reloading scales") if they dont have
one on the shelf they will gladly order one for you,

Most are balance type so you could remove the "pan" and adapt for a
"string and immersion" type situation.  one mfr would be R.C.B.S.

Just a quick thought as I am familer with this type scale and these
are somewhat inexpensive for the degree of accuracy.



From: Peter Gaspar 
Date: 1998 Apr 24 09:00:55 -0600 (CST)

Warren Esty is completely correct in his description of the method and
his statement that it is the weight of liquid displaced that is
measured directly.  Carbon tetrachloride is a dangerous carcinogen and
should not be used.  Other perhalocarbons are not as dangerous but
still toxic.  The trouble is that to use them safely requires a
well-ventilated room, but air currents disturb the weighing.  I use
water despite its lower density and higher surface tension, the latter
a source of systematic error in the weighing.  The advantage of water
is that its density is known to many significant figures as a function
of temperature.  Since the density of a liquid changes enough over a
few degree temperature range to affect the final specific gravity of a
coin by >0.01 s.g. units, the temperature of the liquid must be
measured and the density of the bath liquid at that temperature known.

       With a semimicro balance that weighs to 0.00001 gram, and using
water, my specific gravity determinations of silver coins has an
estimated error of 0.02 grams/cc.  Each weighing must be repeated at
least five times and the results averaged to achieve that level of
accuracy.



From: Tony Clayton 
Date: 1998 Apr 27 22:43:30 GMT

As a metallurgist by training, I would doubt very much whether
striking a coin would significantly change the specific gravity (or
relative density, to use a more modern term) unless the struck
material was porous in the first place.

A refence book I have regarding cold worked nickel indicates a density
change of only 0.05%, which is recovered on annealing.

Someone mentioned implosions.  In these cases the pressure is
exceedingly high for a short period of time, and this will compress
the crystal structure significantly, as used in nuclear devices.
However, when the pressure is removed the interatomic spacing would
revert to normal.

The biggest problem in determining composition by density is measuring
the volume of the coin. The upthrust on a typical denarius of volume
approx 0.2 cu cm is 0.2 grams when immersed in water.  The presence of
even a small bubble attached to the coin in the crevice of the
inscription would make a significant error in the calculation,
especially compared with the density differences as the silver content
changes.

The bulk modulus of gold is about 1.6E11 N/sq m if my calculations are
correct, which means that the volume drops by 1% if the force exerted
on a 1 sq cm coin is 16 tonnes, but this is elastic and thus reverts
on the removal of the force.

(My calculations are based on a Young modulus of 11.4E6 lb/sq in and a
Poisson's ratio of 0.42 for gold, and were carried out well past
bedtime, so I cannot be certain of their accuracy right now!)



From: James A. Schell 
Date: 1998 Oct 31 14:15:45 -0600

I am attempting to calculate the composition of certain coins by
determining the density/specific gravity using a Mettler H70 analytic
balance to measure the mass/weight in air and the mass/weight in
water.  I have encountered some problems.  

The Mettler H70 is readable to 0.1 mg.  By calibration with precision
masses, I am confident that the measurements are accurate to within
0.4 mg.  My test coin is 1/4 troy ounce American Eagle bullion coin
with the following composition: Au 91.67%; Ag 3.00%; Cu 5.33%.  Using
SG Au = 19.42 g/ml, SG Ag = 10.50 g/ml, SG Cu = 8.92 g/ml, and SG
water(distilled and degassed) = 1.00 g/ml, adjusting to 22 Celsius,
and assuming the ratio of Ag:Cu is 300:533, my measurements suggest a
gold content of 90.72% (range:90.55% - 90.86%).  If the only source of
error is the balance, I should get a value in the range of 91.63% -
91.71%.  Assuming the composition is as stated and reversing the
calculation, the measured weight in water would seem to be 4.4 mg low.

I added a drop of purified detergent to the water to reduce surface
tension with no change in results.  A second test using a sterling
silver replica, yielded a composition of Ag 91.89% and Cu 8.11%
(versus Ag 92.5% and Cu 7.5%).  Again, the weight in water would seem
to be 1.8 mg low.  Does anyone have an explanation for the
discrepancies, assuming, of course, that the compositions are correct?



From: Paul 
Date: 1998 Nov 1 13:43:19 +0000

Do check for other sources of error :

You could always repeat the experiment using pure ethyl alcohol in
place of water, which should reduce the surface tension problem.  If
you still get the same result, you can begin to assume that the alloy
is wrong, not the method or the weighing.

The reason that I say this is that the mix of metals within an ingot
of an alloy is anything but homogenous.  Although the mix within the
alloy may be good whilst the temperature is high, there will be
crystallisation as the ingot cools, and depending on the solubility of
one metal within the other - and in the solution of the alloy,
selective crystalisation will occur, the amount depending on the
temperature, so pockets of high and low content, of one, or more of
the constituents can and will occur.  It will vary most towards the
top centre of the ingot which is one of the last places to cool and is
further complicated because of the presence of dissolved gases and
other light impurities.

The British Royal Mint did some measurements on silver ingots, back in
the 1870-90 period.  They are published in a Royal Mint Annual Report.
It is more than a few years since I read the article, but as far as I
can remember, the variation could be anything up to 7.5% off from the
average, depending on the position from which the sample was taken
within the ingot.  Providing the your lab work is clean, which it is
if you have checked your weighings to within 0.4 mg, I would assume
that the error lies in the alloy rather than your technique.

I have just checked our library and cannot find the article in
question - so we must have it somewhere as an offprint - or it may
have been reprinted elsewhere; it is certainly not with our Mint
Annual Reports.  I suggest that you try writing to one of the US
mints, who will certainly know about the subject of homogeneity within
of ingots of alloys.  If that fails the try the Librarian and Museum
Keeper of the British Royal Mint, G Dyer, who will almost certainly
know the paper.



From: James A. Schell 
Date: 1998 Nov 1 15:15:39 -0600

Thanks for the informative reply.  Several respondents indicated that
my assumption that the SG of an alloy would be the proportionate
algebraic combination of the SGs of the components was incorrect.  I
am now in search of empiric SG v. composition data.