How reliable is neodymium magnet test?

[IanS]

Just been testing a silver bar with neodymium magnet.

Holding the bar at quite a steep angle and allowing the magnet to slide down the bar the magnet slides down but spookily slower than expected. So, I guess the bar is real.

But then I found this page

https://terpconnect.umd.edu/~wbreslyn/magnets/is-silver-magnetic.html

which suggest that aluminum and copper have the same effect. Even though they're not magnetic a magnet would slide slowly down copper or aluminum.

So. I don't have any copper to test this with but tested that theory on an aluminum box. and, sure enough, the magnet slides down slowly exactly the same as it did on the silver bar.

So. How reliable is neodynium magnet test. Magnet would show if the bar was tungsten. But what if it was copper? Anyone tested a copper bar with magnet?

[Seasider]

I just tried it on a copper round and you are right I could not tell the difference.  No doubt someone will be on to say the speed is not the same but I could not tell.

Even so the magnet is a good way to eliminate a lot of fakes. 

[IanS]

So if copper also causes the magnet to slide slowly we'd need to be using a number of tests. Can't rely on just the magnet. Weight?

Silver is a little heavier than copper. So a silver plated copper bar would need to be around 14% thicker to get the same weight as the genuine silver bar. In a 100g silver bar a copper fake would need to be a whole millimeter thicker. We can measure that. Unless the bar was all round the same size but with slightly shallower stamping.

So. Maybe all the tests - including specific gravity is the only way?

In any case, silver bar in sealed mint case is pretty much impossible to verify unless you buy from trusted source.

[sovereignsteve]

Which is why it has been stated on this forum many times that stacking silver using bars is a very risky business unless you have a fancy tester of some sort.

[47Ag79Au78Pt]

The slowing down that you can see is a phenomenon called, "The Lenz Effect."  It creates a kind of 'damped' movement and it's used in things like car speedometers to stop the needle bouncing around every time you go over a bump. I've attached a link to one of many videos on the subject from Youtube here.  It is no indicator at all of what the metal is made of and will work with any metal like copper, aluminium, zinc, (and worryingly for gold stackers) tungsten too.  

3 hours ago, sovereignsteve said:

Which is why it has been stated on this forum many times that stacking silver using bars is a very risky business

I'm with @sovereignsteve with this one.  You either do some fairly detailed electrical conductivity and density testing, which IS doable if your scales measure to the milligram, and you can calculate the volume of the bar to less than a cubic millimetre, or, alternatively, you rely on a complicated, beautiful, difficult-to-forge bit of minted coinage from a reputable dealer as a sort of guarantee of provenance... which is, I suppose, why coins were invented in the first place!!

If you are concerned, I have read that there are people on TSF who have machines that can test for the purity of your bar.  Alternatively there are assay offices which may be able to help.

[MrUpwardlyMobile]

Sigma Metalytics machines are a pretty good way to test. They’re just expensive

[IanS]

It sounds like you're all saying that a magnet - any cheap ol fridge magnet is fine for checking that a bar isn't iron or steel. But the slide test with neodymium can't tell the difference between silver, copper and a host of other metals.

Meaning the sliding magnet test is pretty much pointless?

There's dozens of videos on u-tube showing people doing slide test on bars and rigging up drop testers for dropping coins through - they're all pointless?

 

[HawkHybrid]

come to think about it, I've never used the paramagnetic test to test the silver content of an item.

for me it's always been does it look right compared to photos of known genuine examples. then

it's a specific gravity test on borderline cases.

the problem I have with the paramagnetic test is that it's not easily conclusive. internal cracked

coins might fail and the difference between copper and silver isn't that big, imo it's a judgement

call similar to that of the ping test.

the thing I like about the sg test is that it gives you a number which you can then compare with

others who have done the same test.

 

HH

[IanS]

What about those Test Stones and the testing fluid/acid.

Maybe many people wouldn't want to be rubbing their good coin on an abrasive surface but, ignoring that, Is it an accurate test?

 

[swanky]

On 29/10/2020 at 13:29, IanS said:

But the slide test with neodymium can't tell the difference between silver, copper and a host of other metals.

Meaning the sliding magnet test is pretty much pointless?

Of course it isn't pointless, it narrows down the metal content to just a few possibilities. Then you can weigh and measure to check density (just cross-reference weight and dimensions with numista). Only lead passes both tests, but that fails the coin ping test. Much cheaper than a Sigma Metalytics machine.

[SilverSlanger]

8 hours ago, swanky said:

Of course it isn't pointless, it narrows down the metal content to just a few possibilities. Then you can weigh and measure to check density (just cross-reference weight and dimensions with numista). Only lead passes both tests, but that fails the coin ping test. Much cheaper than a Sigma Metalytics machine.

Just posted on this exact topic. Got two 1oz OPM bars. They pass the weight and magnet test. But don’t pass the ping test. Gonna take them to my LCS and have them take a look. If they are fake I am going to pull the trigger on a Sigma. I know it’s expensive, but I’d rather have peace of mind.

[IanS]

15 hours ago, SilverSlanger said:

Just posted on this exact topic. Got two 1oz OPM bars. They pass the weight and magnet test. But don’t pass the ping test. Gonna take them to my LCS and have them take a look. If they are fake I am going to pull the trigger on a Sigma. I know it’s expensive, but I’d rather have peace of mind.

@SilverSlanger. Please, please do come back and tell us what you discover.