|
Specific Gravity—making determinations of the relative
density of a material in order to distinguish alloys and group like materials.
Taking specific gravity measurements is an important
(though not the only) means of establishing whether a particular medal is
genuine or a fake. If you have specific questions about how to perform the
following, please feel free to email me
for help.
 |
I have an Ohaus Cent-O-Gram balance
scale. It reads to the hundredth of a gram and seems pretty accurate. It's
sensitive to room drafts, so if there is even the slightest breeze you need
to close the window or otherwise isolate it from the air currents. It has
the advantage of being able to take specific gravity (SG) determinations as
shown in the following photographs. |
 |
To determine the SG of an item first
weigh it normally. The Ohaus has a pan into which you place the specimen (I
didn't do it for this photo, but you get the idea). On the Cent-O-Gram you
manipulate the poises (the little weights on the bars at the top) to
calculate the weight. You can buy electronic balances now that also have the
capability to do SG, but not all of them do. Check before you buy a scale to
be sure
that it can perform this useful function. I'd also restrict my choices to a
scale that can read to the hundredth of a gram (X.XXg). |
 |
Take a very thin piece of wire and bend
it so it can hold the specimen. I use a piece of copper wire from my wife's
beading hobby, but you could probably use fishing line or whatever. The nice
thing about the copper wire is it's flexible, but retains the shape you bend
it in to so you can reuse it over and over. Make a big loop to hold the specimen and a smaller loop on top to
attach it to your scale for weighing.. |
 |
Hook your wire up to the specimen. Make
sure it's fairly well centered and tight enough that the specimen won't slip
out. |
 |
Following the directions for your scale
suspend the specimen from the appropriate place on your device and make sure
it's completely immersed into a glass of water. Before you do so it helps to
wet your fingers and liberally rub the specimen so to reduce the number of
air bubbles once you immerse it (air bubbles introduce error). Weigh your
specimen while it's suspended into the water. Don't let any part of it rest
on the bottom of the glass or stick up above the water. I wouldn't even
allow it to touch the side of the glass (though it can come close)—all of
these would introduce error. |
Once you finish with the weighings you'll have two
numbers:
A = weight of
the specimen in air (the way you would normally weigh the item)
B = weight of
the specimen while suspended in water.
Now you can perform the following calculation:
Specific
Gravity = (A/(A-B))*0.9983
For example, if the weight in air (A) is equal to 60.89
grams and the weight in water (B) is equal to 54.13 grams, then:
(60.89/(60.89-54.13))*0.9983 = 8.9921
This value is representative of a good quality bronze such
as Goetz used. Note that SG is a "unitless" value in that it simply expresses a
ratio of the mass of an item to the mass of water of an equal volume. It's an arbitrary number, but
useful for determining the type of alloy.
| 