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Steve Herschbach

Nokta Impact - Some Gold Nugget VDI Numbers

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I had somebody ask me to test a couple gold nuggets on my Nokta Impact - preferably a 1 gram nugget and a 2 gram nugget. The person wanted to know the VDI number for each in the DI2 mode and Deep mode.

This by accident may be very informative possibly for people who do not know exactly how VDI numbers are generated. The short story is they are a combination of the target conductivity and the size of the target. In other words, silver is more conductive than lead, so two identical items, one made of lead and the other silver, the silver item will have a much higher VDI number.

For two items made of silver (or lead, gold, whatever) where one item is larger than the other, the larger item will generally have higher VDI numbers.

The Nokta Impact is unique as far as I know in that it has two different target id (VDI) number scales that are used in the various modes. The Impact VDI scale runs from 0 - 99 but as I said there are two different ways the scale is divided. One set of modes uses 0 - 15 as ferrous, all else non-ferrous. The other modes use 0 - 40 as ferrous, all else non-ferrous, expanding the ferrous range.

I did not want to sort nuggets all night so came up with a 0.90 gram nugget and a 2.03 gram nugget, both from the same Nevada location, and both of similar shape although one is obviously larger than the other. Here are the results...

nokta-impact-gold-nugget-vdi-numbers.jpg

The 0.90 gram nugget using the DI2 0 - 15 equals ferrous scale has a VDI number of 37. The same nugget in the Deep 0 - 40 equals ferrous scale reads 63.

The 2.03 gram nugget using the DI2 0 - 15 equals ferrous scale has a VDI number of 32. The same nugget in the Deep 0 - 40 equals ferrous scale reads 56.

Here we have a smaller gold item giving a higher VDI number than a larger gold item. How do we explain these results?

Gold nuggets are not pure gold. These nuggets are probably around 85% - 90% gold. The rest is mostly silver. Here is a strange fact. Silver is more conductive than gold. You would think adding silver to gold would raise the VDI number. However, reality is pure metals conduct electricity better than alloys, and so adding silver to gold actually lowers the conductivity and therefore the VDI numbers. In this case the size, shape, and most importantly, alloy composition of the larger nugget is causing it to read lower than the smaller nugget.

This is a great example of why if you are looking for gold nuggets you at a minimum dig all non-ferrous targets. And because weak nugget signals (very small nuggets or large nuggets deep) in mineralized ground can read as ferrous, the only truly safe approach is to dig everything.

Here is a photo of a large variety of gold nuggets and some coins I tested with the White's DFX, which uses the same -95 to 95 scale as many White's detectors. Again, you can see that size and VDI numbers do not correlate due to shape and purity differences. In general the paler gold has higher silver content. Click on photo for larger version.

gold-nugget-vdi-numbers-herschbach-dfx.jpg

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8 hours ago, Steve Herschbach said:

However, reality is pure metals conduct electricity better than alloys, and so adding silver to gold actually lowers the conductivity and therefore the VDI numbers.

Also true is that you can't determine, by simple arithmetic, the gold content(fraction) from the density and weight of an alloy, even if you know it is comprised of exactly two metals (such as gold and silver).

Electrons decide how far apart atoms are, whether in an alloy or a pure metal.  Electrons also determine how conductive a metal is.  Electrons follow rules (quantum mechanics), but those rules aren't as simple or intuitive as we would like.

However, if you have a non-homogeneous combination of two materials (e.g. quartz and gold) and you know the densities of the two components, then you CAN figure out the fraction of each.  Unfortunately, in nature, you have neither pure gold, but rather an unknown alloy, nor pure quartz.  So even in that case you will only have an approximation of the content, assuming you do the measurements (weights and volumes) and arithmetic correctly.

 

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