Lessons in Buying Precious Metals - Ping Test

facsimile spanish doubloon
Fake Spanish Doubloons
Photo Credit: Northwest Territorial Mint

In response to my article Five Dinar Gold Coin Shows ISIS Has No Interest in Polar Ice Caps reader Gary Rumain had a question regarding the counterfeiting of coins in the 15th and 16th century:

Some years ago, I read a story of salvagers finding some old Spanish gold coins in a shipwreck from the New World and noticed that they were partly corroded.

I don't recall the details now but the author reckoned that the Spanish were losing so much gold to pirates and storms that they must have started diluting their gold coins to keep up appearances and they did it so well that nobody noticed.

I don't know if this was really true and don't recall any other details but I figure if anybody would know, you probably would. So is it possible to have a gold alloy coin where you couldn't tell it wasn't pure by just judging size and weight without testing it?

I responded that the subject requires more room than a comment section could accommodate, to which Gary shot back with "Well, I'm not trying to encourage you to write a book on this. I was only a little curious."

So there's a short answer and a long answer. For the long answer I am starting a series called Testing Gold & Silver which I believe should take at least 13 articles exploring the subject of counterfeiting of gold and silver products. As for fake kilo bars of gold - I will cover these in a future article.

But to answer Gary with the short answer, yes it is possible to have a gold alloy coin where you couldn't tell it wasn't pure by just judging size and weight without testing it. There are two time periods we need to consider:

  • Present day -

    The Chinese are now making gold coins and bars that have the correct weight and dimensions using tungsten plated with gold. Gold is hundreds of times more expensive than tungsten which has almost the exact same density as gold and so it is nearly perfect for counterfeiting gold. What the Chinese can't fake for coins is the sound or ping of nearly pure gold.

    In this YouTube video, skip to the 6 minute 40 second mark to hear the ping when two 22kt gold coins are struck together:

  • Starting in 1537 A.D. - Spanish gold coins during this time had a fineness of slightly better than 22kt gold, or more specifically .920 fine gold, with a specific gravity of 17.67 grams per cubic centimeter. The standard widely accepted gold coin was the eight escudo piece, called a 4-doubloon or a Spanish doubloon by English colonists, In 1537 its weight was set at 27.4680 grams.

    One of the tests used to determine the authenticity of such coins was first to weigh them, and if they had the correct weight, to submerge them in water and see if the coin displaced less than 1.555 cubic centimeters of water. If it displaced more than that, it was considered adulterated.

    Tungsten wasn't discovered until 1783, so it wasn't available for counterfeiting, and regardless it won't alloy with gold, that is, gold and tungsten are not mutually soluble, however there was a worthless metal at that time called platinum (and denser than gold) that could melt with gold and copper to help make coins that had the exact same density, weight, thickness and diameter of doubloons - in this percentage, for example:

    74.6% Gold
    9.4% Copper
    16% Platinum

    The above alloy is basically 18kt gold but with the exact density of .920 fine gold. Of course, today no one would adulterate gold with platinum, both metals often trade within a few hundred dollars of each other and at certain times platinum has been even much more dear than gold.

    The fewer different metals in an alloy the less likely two of the metals will be further apart on the galvanic series and the less potential for corrosion. Pure gold will corrode least. Perhaps this is why some of the coins corroded.

    The reason platinum was worthless in 1537 is that there was no use for it:

    Johnson Matthey , History of Platinum Group Metals

    For the Spanish Conquistadors of the 16th century, platinum was a nuisance. While panning for gold in New Granada they were puzzled by some white metal nuggets which were mixed with the nuggets of gold and which were difficult to separate. The Spanish called this metal Platina, a diminutive of Plata, the Spanish word for silver. Some thought that the platinum was a sort of unripe gold, so that for many years it had no value except as a means of counterfeiting.

The ping test in modern times serves well if you have a genuine coin to clink against a test coin and does not require expensive test equipment. It may be helpful though to have a collection of mp3 samples for various karats of gold. For example, a 22kt gold coin ping vibrates at 4498 Hertz - almost like a fine crystal goblet. A fake gold-plated tungsten coin simply gives a dead thwack sound like your fingernail tapping a piece of glass.

In my business, when I buy precious metals, it may sometimes be necessary to use more than a dozen other tests for gold or silver, especially if the objects are thick bars or nuggets that are impossible to get an accurate sound by clinking or are of low karat. These tests I will cover in future articles.

I ignore in this article cob coinage which requires an article all of its own.

### End of my article ###

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