Metallurgist/Jewelers/Metal Smiths: I have a question about gold.

[TalkingDog]

I was over at zero hedge looking at this article on the austerity measures in Europe.

http://www.zerohedge.com/article/visualizing-austere-europe

And I came across this story: (images and video at link)

Rust Discovered On Bank Of Russia Issued 999 Gold Coins
http://www.zerohedge.com/article/rust-discovered-bank-russia-issued-999-gold-coins
Text:
Here's a head scratcher: as everyone knows from elementary chemistry courses, gold is the most inert metal in the world - it does not rust, nor corrode. Yet this is precisely what Russian commercial precious metal trading company, International Reserve Payment System, discovered on thousands of (allegedly) 999 gold coins "St George" (pictured insert) issued by the Central Russian Bank. The serendipitous discovery occurred after various clients of the company had requested that their gold be stored not in a safe, but in a far more secure place: "buried under an oak tree." As the website of IRPS president German Sterligoff notes: once buried, "the coins began to oxidize under the influence of moisture." And hence the headscratcher: nowhere in history (that we know of) does 999, and even 925 gold, oxidize, rust, stain, spot or form patinas, under any conditions. Furthermore, as IRPS discovered, Sberbank of Russia released an internal memorandum ordering the purchase of the defective coins with the spotted appearance. Sterligoff concludes: "It should be noted that the weight and density of the rusty coins coincide with the characteristics of gold that would be expected after after conventional testing methods would reveal. We think that the experts will be interesting to determine the nature of this phenomenon." So just how "real" is 999 gold after all, either in Russia or anywhere else?

/snip

I know enough about gold and silver to know that the 999 is highly pure and refined metal. Sterling silver has a number like 925 and "fine" silver a number like 999. I'm assuming gold is the same.

My question: How common is rusting in a 999 metal like gold? Thanks in advance. Sorry for my ignorance, but that's why I love DU. All the smartest people hang around here.

[The Magistrate]

1. If This Is True, Sir, the Metal Involved Is Not Gold Of That Fine-Ness

Fraud is always a possibility. Simulating gold is a very old human endeavor; old chemical texts dating to Egypt in the classical period contain numerous recipes for contriving to fool the assay methods of the time; there is no reason to believe persons in the modern era are less ingenious. Nor, put bluntly, can honesty on the part of assaying firms engaged be assumed.

[hobbit709]

2. Sounds like someone gold plated some slugs.

Archimedes devised a simple way of checking to see if it is real gold.

[Electric Monk]

3. With the price of gold what it is, there are bound to be more scams

Anyone buying gold as a long term investment now is a fool. Buy low, sell high, not the other way around.

[Warpy]

4. With the huge bubble in gold that has been created

all the thieves have come out of the woodwork and I doubt any gold out there being sold now unless it is in jewelry and already mixed with other metals for strength is reliably pure.

Tungsten is the favorite metal for salting gold coins and bars because it doesn't oxidize as obviously as other choices do.

Russians involved in fraud. Gee what a shocker.

In any case, the only gold any ordinary consumer should be buying now should be in jewelry one intends to wear frequently. Anything else is a waste and likely to be a setup for being cheated.

[MineralMan]

5. Gold can react with acids containing halides, such as chlorine, bromine,

etc. Add to that that almost all gold contains some impurities (it's almost inevitable), and it could easily be that the tannic acid in the soil where those coins were buried could produce some oxidation of the alloying mineral, probably silver.

Even a tiny, tiny percentage of silver could allow the tannic acid to produce an oxide film on the coins.

Gold is not completely non-reactive. Sea water contains minute quantities of dissolved gold., probably associated with the salt content of seawater (NaCl).

On further research, I discovered that humic acid, also found in soil, can also etch gold over time, and may be a factor in the dissolving of gold into solution. This could easily explain the presence of gold in seawater. Tannic and humic acids are also used in the preparation of colloidal gold.

This information brought to you by MineralMan...

[PCIntern]

6. All I know as a dentist is that

I have personally placed hundreds of 16 and 18 carat restorations into 100% humidity 100% of the time and have never seen rust or corrosion. Ever. Slight tarnish is possible over MANY YEARS. Never pitting. I have placed and witnessed hundreds of 24 carat gold foil restorations which do not tarnish or corrode - some have been in my patient's mouths since the 1940's. Gold foil restorations are not routinely performed in dental schools to my knowledge, except perhaps as an exercise or for some particular reason peculiar to a given patient.

[Turbineguy]

7. So......

somebody in Russia discovered some rusty gold.

These ex KGB types, they do have some crazy ideas.

[bluerum]

8. Gold does react under certain conditions. Certain chemicals, pressure, temps etc.

could cause some type of reaction - not necessarily oxidation.

Also, I think that platinum is less reactive than gold.

[Cassandra]

9. Jeweler, here.

Rust is the oxidation specific to iron. There may be some conditions under which gold will oxidize slightly but it will never rust. Also, pure gold coins should be stamped 24KT. I suppose they could be stamped in decimals but that is more common in silver and platinum than in gold.
Gold content can be tested with the correct acid and a touchstone, but if plating is suspected, filing a cut into the metal and testing there is recommended.

[JDPriestly]

10. Cassandra, very interesting. Thanks. Not that I have much gold,

but still, your post is very interesting.