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Using an Ultrasonic Thickness Gauge
to Detect Fake Silver


Introduction

Around April, 2010, we starting thinking about non-destructive ways to detect lead-filled 100 ounce Engelhard silver bars. A lot of people discussed XRF, but that is very expensive, and can be dangerous. We heard about ultrasonic thickness gauges, and became intrigued.

An ultrasonic thickness gauge (UTG) is designed to tell you the thickness of a known substance. For example, you can use it to determine the thickness of a steel oil tank, even though you cannot access the inside of it. You just tell it the metal you are testing, or tell it the velocity of sound in the material you are testing. It sends a sound wave through, which bounces back on the other side of the metal. It knows how long it took to get the echo, and can easily calculate the distance traveled (the thickness).

However, we realized that you could do the opposite -- without knowing the substance, but knowing the thickness, you can determine the material. You measure the thickness (with a caliper or even a ruler), tell the UTG that you are measuring silver, and see what the results are. If the results from the UTG match the actual thickness, you can be extremely confident that you have real silver.

Theory Proved

We posted that UTGs should be able to detect lead in silver bars. We haven't (as of this writing) been able to obtain one. However, we were fortunate enough to obtain a sample of a silver-plated lead Engelhard 100 ounce silver bar.

We then tested the fake bar. Sure enough, while the bar measured 21.0mm thick, the UTG displayed that it was 33.9mm. The UTG has an accuracy of about .2mm at this bars' thickness, so clearly the bar is not made of real silver. A quick calculation shows that the velocity of sound in the bar was around 2,260m/s (vU=vS*(tA/tG), where vU is the unknown velocity of sound, vS is the velocity of sound in silver, tA is the actual thickness, and tG is the thickness per the gauge).

Velocity of Sound

Sound travels in different materials at different speeds. This is the key to how ultrasonic thickness gauges work.

It is like determining the speed limit of a road by driving down the road and back at the posted speed limit, and seeing how long it took. If the road was 30 miles (60 miles round-trip), and it took an hour, then you know the speed limit is 60 miles per hour. Except with UTGs, the sound must travel at a specific rate; unlike the car, it isn't allowed to start and stop.



Alphabetical By Velocity of Sound
MetalVelocity of Sound
Aluminum6,400 M/s
Brass4,300-4,700 M/s
Copper4,760 M/s
Gold3,240 M/s
Iron5,950 M/s
Cast Iron4,600 M/s
Lead2,160 M/s
Nickel5,600 M/s
Platinum3,300 M/s
Silver3,650 M/s
Stainless Steel5,790 M/s
Tin3,300 M/s
Tungsten5,200 M/s
Zinc4,200 M/s
   
MetalVelocity of Sound
Lead2,160 M/s
Gold3,240 M/s
Tin3,300 M/s
Platinum3,300 M/s
Silver3,650 M/s
Zinc4,200 M/s
Brass4,300-4,700 M/s
Cast Iron4,600 M/s
Copper4,760 M/s
Tungsten5,200 M/s
Nickel5,600 M/s
Stainless Steel5,790 M/s
Iron5,950 M/s
Aluminum6,400 M/s

Doing it Yourself

Most people won't need to go to the expense of buying an ultrasonic thickness gauge to test their silver. However, if you are in the bullion business, or often deal with 100oz bars (especially from less reliable sources), you might want to consider investing in one.

The first step is to measure the thickness of the bar or coin you are testing. Do not use a guide such as a website, unless you have compared the thickness to an identical bar or coin that you know is real. It is best to measure the thickness with a caliper, but a ruler may be accurate enough.

The next step is to either tell your UTG that you are testing silver, or enter the velocity of silver (3,650 M/s). Then, you should calibrate the UTG (on ours, we press the 'Calibrate' button, put glycerin on the calibration button, put the probe on there and wait for a good reading, and press the 'Calibrate' button again).

Next, put a drop of glycerin on the item you are testing. It needs to be done on a flat surface. Place the probe on the glycerin, and press down to get a good reading. If there is a coupling light, it will let you know when you have a good reading.

If you are testing a silver bar that you are concerned may contain lead, you should test several areas of the bar (otherwise, you may get a reading in an area that is pure silver from the probe to the bottom of the bar). With both types of lead filled 100 ounce Engelhard silver bars that we have encountered, testing at the exact middle of the bar should always work properly.

If the results on the UTG match the thickness measurement you took at the beginning, the bar or coin is real silver. Otherwise, you can use the table above to help determine what the metal actually is.

Applications

You can use this to test for drilled-and-filled or hollowed-out 100 ounce Engelhard silver bars.

You can also use this test to see if many larger silver items are really silver. For example, we used the test against what appeared to be a 100 ounce Engelhard silver bar, but turned out to be silver-plated lead.

This test could also be used to detect tungsten bars with a layer of gold on them. These are very rare, but the Heraeus Refinery discovered one.

Note that it will not work well on smaller coins, as you need to have a flat space (typically around 5-10mm diameter) that the probe will fit in. Many coins do not have that much flat space for testing.

Also note that you should not test any numismatic coins (any coin worth more than the precious metal content). The glycerin used on the coin could potentially cause damage (although likely not any more so than a drop of water would). And, the probe could potentially cause scratch marks.

Lead Filled Silver Bars and Gold Plated Tungsten

These bars are typically hard to detect items, as normal silver and gold testers will 'see' the pure silver or pure gold coating (which is much, much thicker than silver or gold plating; probably 1,000s of times thicker).

Although we have not yet found a lead filled silver bar or gold plated tungsten bar to test with, an ultrasonic thickness gauge should easily detect these.

One of several things would happen when the sound sent from the UTG hit the lead or tungsten. First, it could simply bounce off properly -- in this case, you would get a reading that was much less than the actual thickness of the bar. Second, it could 'scatter' off (for example, if there was a bit of air in the bar, or a rough surface connecting the two metals), in which case you get a bad reading (e.g. no reading or a constantly changing reading). Finally, it could (in theory) go into the other metal before bouncing off, but that would cause a thickness reading that would not match the actual thickness.

UPDATE: We were contacted by a certified Non-Destructive Testing technician, who clarified that the first case would be true (the wave would bounce off the lead, so the reading would show up that the bar was much less thick than it actually is).

So whatever actually happens, there should be no way that the reading of one of these bars would match the actual thickness of the bar.

Type of Ultrasonic Thickness Gauge to Use

Fortunately, our needs for a UTG are very simple here. It doesn't need to be very accurate; the cheapest UTGs available are plenty accurate.

The UTG we bought was a model TM-8811, however, just about any UTG should work just fine. Here are some from Amazon:





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