Red spotting on gold coins explained
derryb
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When gold and silver move together, it signals the coming end of fiat money.
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derryb
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Anode - the metal with the lower electrical potential on the galvanic series chart
Cathode - metal with higher electrical potential
Electron Path - a connection between anode and cathode (alloying is definitely a connection)
Electrolyte - a means to carry the charge, usually moisture but can certainly be a contaminant such as sulfur.
Remove any of the four elements listed above and you'll get no corrosion. Obviously the only thing you can control with an alloyed coin is the electrolyte, however relative humidity is certainly a factor almost anywhere.
Good informative article. I personally think corrosion is a fascinating science and I worked for DOD in a Materials Science program for many years teaching active duty folks corrosion control. Kind of wish I was still doing it sometimes but along came a much better job with a much higher paygrade and much more responsibility.
John
Never view my other linked pages. They aren't coin related.
John
Never view my other linked pages. They aren't coin related.
<< <i>and of course you won't have that on a .999% gold coin. >>
Then explain why the red plague appears more on 24k gold buffalos than on 22k gold eagles.
Link mentions nothing of copper being involved with the scientists' discovery, only silver remaining in the gold reacting to sulfur. Based on their research copper has nothing to do with the red plague.
When gold and silver move together, it signals the coming end of fiat money.
I personally have to call in question .999% purity of gold if they corrode with red plague. Copper is added to gold to make it hard.
Look up the definition of 24 karat. It is only 99% gold in some accepted markets. That leaves a whole percent for other alloys.
I won't claim to know more than a trio of lab coat wearing scientists but I did my time in US Navy labs here and there and I still think there is some copper present in these corrosion cells.
Silver is anodic to gold. Mix it with sulfur and it does what? It turns black. Tarnish.
Copper is anodic to gold and silver. Mix it with one or the other and it turns bluish green.
Those are pretty well accepted facts. If it were only silver and gold you'd get a black tarnish corrosion product.
I wish I still had access to my old job as I know a couple guys who could easily put this to rest one way or the other. Here's a link to one guy I used to know who is basically the subject matter expert for the US Navy on corrosion. Dr Agarwala. I found some old paper he wrote at a Tri Service Corrosion Conference I went to once. Just a quick search and nothing definitive but the Dr. said it was red cupperous forming over silver or something like that.
We can break it down to elemental science but the real bottom line is this. No matter what it is, its darned annoying on our collectable coins and we don't like it at all.
John
Corrosion Paper
Never view my other linked pages. They aren't coin related.
"What Causes Red Spots
Red spot can occur on almost any gold coin, it certainly happens on 22 carat gold coins. We have never seen it on .999(9) fine gold coins, and presume it is virtually impossible for it to occur on fine gold.
From our knowledge of metallurgy, we can tell you that when gold is alloyed, usually with copper or copper and silver, the alloying is obviously done in the molten state, and then has to cool. During cooling, crystalisation occurs, the crystal forming around "seeds" which are molecules of the elemental metals. There is a slight tendency for the elements with the highest melting point to start to crystalise first, and this can lead to small localised areas with slightly higher or lower concentrations of the constituent elements. In ternary alloys (three elements) three pairs of binary (two elements) alloys can also form. These areas of variable alloy are usually of microscopic proportion, but can sometimes be large enough to be visually discernable.
Copper oxidises and forms other salts fairly readily whereas gold is almost completely inert, and silver lies in between, although it is fairly unreactive. This means that if some parts of the alloy are copper rich, and are exposed on the surface of the coin, then it is possible for these parts to exhibit toning or tarnishing. The red spots are areas with a higher copper content, and as copper is a red coloured metal, this shows itself in an area which is less yellow and more red than the rest of the coin. If this area subsequently tarnishes, it would almost certainly go towards a deeper red or brown colour. Whenever we have seen red spot, it has always been an area about the size of a pinhead, sometimes with more than one spot on a coin. "
Worry is the interest you pay on a debt you may not owe.
"Paper money eventually returns to its intrinsic value---zero."----Voltaire
"Everything you say should be true, but not everything true should be said."----Voltaire
That is pretty much totally correct. The individual grains or crystals of metal are laying next to one another in the alloyed state and the areas between these grains is called the grain boundaries and all kinds of interesting things chemically can happen here. So basically in this state you possess an anode and a cathode in electrical contact and all you need is a contaminant or little relative humidity (moisture) to kick off the corrosion process.
I think no matter how much the gold content there is there is going to be some copper, some silver, and probably several other metals and certainly carbon present. The more of this red plague you see the more the copper.
Some people here have touched on some good points, one of which is that why don't the 22K Eagles, which have a lot more copper and silver exhibit this red plague when .999% gold does. And that's a darn great question in my opinion.
The answer basically is in what you posted. It has to do with the molten state, the cooling, annealing, heat treating and whatever else they do (or maybe more importantly DON'T do) to the metal.
I stand fast in my belief that red plague comes from copper/silver/gold.
Some years ago the Dr. I mentioned (and another scientist) invented an aluminum treatment (untreated aluminum is tough to paint) which uses something called trivalent chromium or Chrome 3 they called it. Normal treatment is done by something called Hexavalent Chromium or Chrome Six.
Ever see the movie Erin Brockovich? Hexavalent Chromium was the evil chemical in that movie. Anyway these guys came up with this stuff and they were basically big brained guys untouchable to mere Fleet personnel. When they spoke people listened and didn't talk.
They asked for some field evaluations of their material and I jumped on it. One of their mixtures was a complete failure as a pre-paint coating however I noted that the corrosion resistance went WAY up and told them their stuff failed but technically was also a raging success.
Guys like me weren't supposed to notice stuff like that and that was my "in" to the lab coat wearing world. I co authored some scientific paper on trivalent chromium with the other guy which was a high point in my career. Imagine some dumb airplane mechanic from Indiana published in a scientific journal for his findings.
Anyway these guys taught me a lot about grain boundary actions and we specifically spent a lot of time talking about purple and red plague which to me was a pretty unknown phenomenon at that time that seemingly defies (the colors of the corrosion product I mean) the laws of nature.
Yes. I am a geek.
John
Never view my other linked pages. They aren't coin related.
FWIW...
When gold and silver move together, it signals the coming end of fiat money.
One more bit of info for you. The electromotive force (emf) shows the potential of the metals. The highest metals have a more positive voltage. Gold exhibits about 1.62 volts. Silver is close to .8 volts and copper is about .3 to .5 depending on what kind of copper.
Put any two metals together in the presence of an electrolyte and the metal with the lower electrical potential will shed or give away electrons. This is pretty much the definition of corrosion. It's also pretty much the definition of a battery.
As you can see there is a lot of difference in electrical potential between copper and gold. The more the difference in electrical potential the greater the corrosion.
In my world of aluminum airplanes, aluminum has a potential of a -1.6 volts. Putting aluminum with noble metals like gold or silver is problematic. Mostly you see stuff like this on electrical connectors.
Anyway you can get the mixture right in coins to make a nice ding resistant coin but then it may be susceptible to red plague. Fix the red plague problem and maybe the coin isn't so resistant to dings or damage. If I was seeing all my new gold coins from the mint spotting red I think I'd tell the mint they have a quality control problem and let the egghead metallurgists take a look. They can solve problems like that by turning the heat treating oven up or down a few degrees or whatever it may take to correct. If the mint isn't told, they likely don't know.
John
Never view my other linked pages. They aren't coin related.
Buddy of mine came by and shook my hand and told me he was leaving. Asked him to where and he told me "The US Mint in Philadelphia". That really got my attention and I asked "doing what?" and he responded that he would be working the heat treating ovens or annealing processes or both. We were a couple of aircraft metal workers and certainly were familiar with heat treating things but I had never thought about it much but the US Mint clearly has all manner of metal shops and performs all manner of metal forming processes. So maybe the fact that old, nearly out of work airplane mechanics are in charge of metallurgy at the mint may have something to do with the problems.
Just kidding.
John
Never view my other linked pages. They aren't coin related.
So, just to be clear, what you and some other people refer to as the "red plague" IS the same thing as what some of us call "copper spots"?
<< <i>Thank you, John.
So, just to be clear, what you and some other people refer to as the "red plague" IS the same thing as what some of us call "copper spots"? >>
Correct.
Never view my other linked pages. They aren't coin related.
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