I’ve always seen gold bars being classified as 99 point something (99.9). Some Japanese gold bars are classified as 999.9
What is the point of this?
If you don’t understand what I’m talking about just google “japan 999.9 gold” and do an image search. You’ll find some examples.
999.9 parts gold out of 1000. What else could it mean? The point is higher purity.
Googling “gold bar purity” produces lots of hits. Here’s one:
I always wondered how you can tell if a gold object is actually pure-ish gold, without melting it down. Sure, you could check density, but couldn’t someone mix lead and a lighter metal to create a core with the same combined density as gold? Then electroplate or melt on a thin outer gold layer, depending on the melting point of the eutectic metal core?
Does NMR work on bricks of gold? I only ever got to use it in school on tiny test tubes.
Gold purity on that level is generally inspected through X-ray fluorescence spectrometry, because we live in the 21st century. If you just need to check that it is “pure-ish” gold, it can be tested for electrical conductivity.
Or you could just put it in a full tub of water, measure how much water it displaces, and calculate the density from there. Eureka!
Lead is a ‘lighter metal’ when it comes to gold. Densities: Lead, 11.34 g/cc; Gold, 19.3 g/cc. You’d need to use something like osmium or uranium to hit your target density.
No idea on NMR. I too, just handed my O.Chem samples to the tech.
No, because gold is significantly denser than lead. There are some elements denser than gold, but most of them are more expensive than gold. Tungsten is pretty cheap, but has a melting point of over 6000 F.
Tungsten would be better (Uranium is slightly less dense).
Either way, and to your point, the metals that are currently known to have a greater density than gold were discovered in the Industrial Age or more recently. That’s why the tale of Archimedes’ break through with the development of the concept of density was such a “Eureaka!” moment, worthy of running naked through the streets. At the time, gold was the densest known substance, so it was impossible to fake “solid gold” with something hollowed out or an alloyed substance. Knowing the density of gold and then coming up with anything less than for a supposed/claimed solid object would be proof of a fake. Coming up with anything “more than” was, to the ancient mind, impossible given the limits of their knowledge. Or at the very least it would have been groundbreaking new science and pointed to a new element which, purely due to novelty and rarity, might have been taken as even MORE valuable than gold.
For reference, here’s a chart of elements and their densities. Of note, everything beyond gold was unknown in antiquity:
Lead just ain’t gonna do it.
Thanks. I just assumed without checking because, well, lead is pretty known for being really heavy and dense.
It would take a lot of work and expense to get the density just right.
Most people don’t have any up-close-and-personal experience with gold sufficient to know that.
I once read of a gold mine in South Africa that had a gold bar on display in their main office. They had a standing offer to the effect that they would give it to anybody who could pick it up with one hand. It was never claimed.
Well that is a pivotal point in the field of science, though perhaps more legend than fact, but there are higher and lower density materials which can be combined to have the same density as gold and then merely coated with gold. Now in Archimedes time, it was a breakthrough to see the relationship between density and volume so no one was trying to fake gold that way, they were simply mixing it with other metals. This whole subject could take me off on a tangent that I’d rather not get into now.
Just to mention, not all that much gold is kept in pure form. Jewelry and the like are often 14K gold, an alloy with other metals. Even the gold in Ft. Knox is not all high purity gold, much of it is melted down coins containing other metals.
It always annoys me when you see actors handling gold bars in films.
A classic example: Kelly's Heroes (1970) Official Trailer - Clint Eastwood, Donald Sutherland War Movie HD - YouTube
Collectors want high-purity gold because of the value. Industry wants high-purity gold for industrial purposes. As mentioned, most commercial gold is lower purity because pure gold is too soft and heavy to really want for applications like jewelry.
I’ve heard stories of thieves who stole industrial gold, went to sell it, and were caught because the purity was so high it was obvious it wasn’t a personal collection.
I don’t see why you’d use anything other than XRF to check purity of gold or gold solutions. Could you do NMR on gold, either in solution as a salt or solid-state? Sure, gold has a spin number of 3/2 but it’s nearly 10,000 times less sensitive than proton NMR. You’d be there forever doing a bad analysis.
Because of the weight? That ingot might weight 15-16 pounds, not ridiculously heavy, but almost everyone that picks up a piece of gold that size would be surprised at the weight. I expect them to do that sort of hefting up and down thing people do when something is heavier than expected.
Regarding the OP’s claim that you’ve seen gold classified on a 100-point scale, like 99.9 – where have you seen that? As far as I can recall, I’ve only ever seen gold classified on a 1000-point scale, like “999 fine”. Wikipedia calls the 1000-point system “millesimal fineness” and says “Various ways of expressing fineness have been used and two remain in common use: millesimal fineness expressed in units of parts per 1,000 and karats used only for gold.” It doesn’t say anything about a 100-point system being in use.
As was recently mentioned here at SDMB, that story is mythical. Archimedes did determine the density of the crown, but he used a method far more clever — and more precise — than measuring water displacement: He weighed the crown with a balance scale while it was under water.
Video illustrating the density of gold:
A museum in Japan offers a challenge to pull a 12.5 kilogram bar of gold through an elbow-sized hole in a box.
And there are a lot of tungsten gold bars out there.
There is a huge Chinese industry making fake gold bars of various qualities, and they certainly have the technology to make tungsten gold bars (which, although too expensive for cheap fakes, are much cheaper than real gold bars). And the Chinese have famously accused the USA of being the source of tungsten gold bars.
Real gold bars come with serial numbers now. But serial numbers are easy to fake, so there is now also an industry creating fake gold bars made out of real gold — to increase the value and disguise the source of black-market gold.