Specific Gravity Test

Veep

After watching my ANA Counterfeit Detection video for the umpteenth time and a few other similar videos borrowed from the ANA, I'm finally motivated to get a scale and stereo microscope. Between the videos, other printed sources, diagnostic photos of key dates and my experience, I think that I'll be able to root out all but the best counterfeits and alterations.

My question is: Do any of you do specific gravity testing? With scale and microscope, under what circumstances do you feel that an SG test is still necessary?

mgoodm3

An accurate SG test is not that easy to do. Weight's easy. Volume is hard to do accurately.

lathmach

Good luck finding anyone able to do a specific gravity test.
Even Jewelers don't use it anymore.
If you're wanting to learn how to detect counterfeits, just buy some and study them.
That's what I did. I have a "collection" of counterfeits.
The most common counterfeits are cast, and they're usually the most easily recognized as not genuine.
The ones I like the most, are those struck from hand cut dies.


Ray

Geoman

I do specific gravity tests all the time. It isn't too difficult, after a little practice. But you need the right equipment, and most people don't have that available to them. I am a grad student in geology, and needed to do specific gravity on 100's of samples. Basically, how I do it, is weigh the object on a scale in air, record that weight in grams. Then, you need a scale with a long spring, and a platform to place the object on, which you lower it into a beaker of distalled water. You record the water displaced, and also the weight of the object in water. Plug all 3 measurements into a simple forumla, and you have your specific gravity. I also did this for a research position on several 100 samples. It gets very boring.

Edited to add: Actually, I have the ANA video, but haven't ever watched it. I'll watch it and see how they do it.

Conder101

Yes, I do specific gravities and I echo what Geoman said, it really isn't that hard. i have a good analytical balance accurate to four decimal places. I weigh the coin in air and I weigh it suspended in distilled water (Make sure the coin doesn't touch the sides of the beaker and that you haven't trapped any bubbles on the coin). The Specific Gravity = weight in air / (weight in air - weight in water) For accuracy sake you should run the test two or three times. This should give me figures that are accurate to two places +/- .02 If I wanted to be even more accurate then I would have to take the water temperatue into account and a couple other items, but for what I need my set up does just fine.

mgoodm3

The problem is the weighing while suspended in water part. Not easily available for most.

Conder101

Just takes a little thought about how to build a little something you can suspend the coin from, it's not that hard. It can even be done for those flat plate electronic jewlers type scales that weigh items directly.

Veep

The ANA video has a pretty simple example of a set-up and proceeedure.

construct

Is there a set-up you can take to a coinshow to test a coin you are thinking about buying? Do you think a dealer would be offended if you tested the specific gravity of his coin on the bourse floor?



Is this test really practically useful, and under what circumstances?

Geoman

construct...

There isn't a portable set-up to take to coin shows. If you did, the dealers would probably chase you put of the building. I don't see this as a practical test for coins. However, I have done 1000's of specific gravity tests for school and work. Besides weight, you need to get a volume. And the most common way to do that is in water, by submersing your item you want to test in water, on a spring-scale, and then using a ruler to measure the amount of water displaced.

Just imagine taking an expensive coin, and dropping it in beaker of water at a coin show. The best way to test for counterfiets are by what others have suggested. Study, get the counterfeit guide books, learn for dealers that are good at counterfiets, etc.

construct

Thank you, Geoman.

cladking

The difference between the weight of an object in air and its weigh in water is its volume
expressed in weight of water. These will cancel out if you use Conder101's formula.

SFDukie

<< <i>Is there a set-up you can take to a coinshow to test a coin you are thinking about buying? Do you think a dealer would be offended if you tested the specific gravity of his coin on the bourse floor?



Is this test really practically useful, and under what circumstances? >>



Whenever you might suspect that the coin is of a metal different than represented. See the story of archimedes and his eureka moment
http://www.engineering.usu.edu/jrestate/workshops/buoyancy/buoyancy.php
I could be wrong, but don't imagine it is very practical on the bourse floor.
Don

mgoodm3

No problem. Just put your handy scientific balance and a big jug of water in to your backpack before you go to the show and you're in business.

KurtHorn

No not very practical for the bourse floor. But maybe that is why ANACS graders are so wishy washy about giving opinions on Trade dollarrs at shows. They run SG tests on them when they get them back to Ohio.

cladking

It shouldn't be overly difficult to invent a device to do this. You'd just need a barometric chamber with
a scale inside and a leveling device. Geologists would be very interested in it also.

sumnom

What about slabs?