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 Replies: 12 / Views: 6,797 |
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Pillar of the Community
United States
548 Posts |
Being a scrap gold / silver buyer, I routinely get the opportunity to buy coins. While I'm not a coin expert like so many of you here, I AM a hobbyist and love coins. But how to avoid making a mistake when buying gold and silver coins for their metal value... (there's a bunch of fake Pandas out there). Besides the usual "weigh the coin," and "measure its dimensions," I created my own density scale. Sure, one can buy one (upwards of $2k), but being a chemical engineer, I felt I could make one and save a bit of $. I'm sure ya'll have heard of Archimedes, the Greek scientist who developed the mathematical concept of buoyancy? I've got the story captured in my website, see siglines. Here's some pics of the scale at work. 1. Englehard 999 "Prospector" silver round  Weight in air:  Weight under water:  This corresponds to a specific gravity (or, density, since we're using water as the fluid) of 10.58 g/cc. Pure silver has a density of 10.56 g/cc. So it's a pretty safe bet this round is pure silver.  2. 50 Peso Mexico gold coin  weight in air  weight in water  This calcs to a specific gravity of 17.07 g/cc, which is 12% copper (8.93g/cc) and 88% gold (19.32 g/cc). Not too far off the 90% gold that the coin is supposed to be.  3. 50 Singold, 1/2 ounce Singapore coin  weight in air  weight in water  Specific gravity = 18.43 g/cc, or 5% copper and 95% gold. For my purposes, 95% gold is good enough to convince me the coin is real gold.  My sigline has links to the density pages of my site. Also to the Wiki page where it shows you how to calculate the specific gravities from the weight differences. Please let me know if you have questions. |
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Pillar of the Community
United Kingdom
2624 Posts |
Cool, as is Archimedes..its a shame what happened to him...
After his town was conquered the invaders knew that he had invented a crane to hoist their ships out of the water, rather than wanting revenge for this they acknowledged he must have a wonderful mind and so they ordered that he be spared.
One of the solders saw an old man drawing lines in the sand and told him to move, he refused and was killed by the point of the soldiers sword. That particular soldier was unaware who he was.
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Pillar of the Community
United States
2077 Posts |
So if I understand your rig correctly, the tank does not sit on the balance platform, just the sling that holds the coin either in the air or water, correct?
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Pillar of the Community
 United States
548 Posts |
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Pillar of the Community
United States
2077 Posts |
Thanks. I'm going to give it a shot. Tank could also rest on the table too from what I gather, just as long as it straddles the platform and leaves room for the legs of the sling to reach the platform..
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Pillar of the Community
United States
548 Posts |
My initial design had the tank legs resting on the table. The problem I had with this was that the system didn't stay "connected." No biggie, if you're only doing a few measurements every now and then. But we do a bunch daily.
For the "cable" holding up the submerged weighing tray, I used fishing line. That new braided stuff. After tying the knots, I super glued them.
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Pillar of the Community
United States
1195 Posts |
Can we get some side views of this rig?
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Pillar of the Community
United States
548 Posts |
I think what you want is a view of the basket (tray) and its support stuff.  There's more pics in my website. |
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Pillar of the Community
Canada
1411 Posts |
@david
and the Soldier was either killed for disobeying orders or sacked and starved to death
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Bedrock of the Community
United States
20753 Posts |
Quote:
Cool, as is Archimedes..its a shame what happened to him... Which no one knows for sure since accurate records from 287 to 212 BC are rather vague. This does look like almost a professional set up. And as long as it works, great. I just wonder how many coins a person would have of enough value to make that practicle. I would think most would just purchase slabbed coins. |
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Pillar of the Community
United States
548 Posts |
Quote:
This does look like almost a professional set up. And as long as it works, great. I just wonder how many coins a person would have of enough value to make that practicle. I would think most would just purchase slabbed coins. In my case, I use this scale more for confirming gold jewelry isn't plated than for coins. There's so much hanky panky on gold jewelry, that a gold buyer needs as many techniques as he can put together to stay ahead of them. The silver coins I buy are for their silver value; folks won't generally bother counterfeiting them, so I hardly ever put a silver coin in the scale. I'll put every Panda that comes across my desk in the scale. Just to be sure. The odd gold round and bar, also. |
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Pillar of the Community
United States
548 Posts |
A customer came in my shop last week with some unmarked silver rounds. He was referred to me by a local coin shop who couldn't confirm that his rounds were actually silver. They scratched the heck out of one. Listened to the sound it made when they dropped it on the floor, but they couldn't confirm anything. Using the density scale, we confirmed that they WERE silver. Good for him, too, as he had 200 of them. There's a bunch of silver rounds out there that aren't marked at all, much less 999. Here's an example of a density test on one of them. This is a Christian / Jewish piece celebrating David and Bathsheeba. Supposedly 999 silver  Weight in air  Weight in water  Calculates out to a density of 10.32. Pure silver is 10.53. I'm comfortable saying this piece is silver. (Of course, I have other means of confirming this). ps, the guy's silver rounds had a density of 10.44. Pretty close to pure silver. |
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Bedrock of the Community
United States
20753 Posts |
For jewlery and small amounts of coins might be a little fun playing with that. With many people like me with vertually many thousands of coins, checking each one would take the rest of my life. Yet if good at it and for someone that deals with Jewlery of bulk metals, should be profitable.
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Replies: 12 / Views: 6,797 |
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Posted 08/14/2012 10:50 pm
