Author Topic: Corrosion at sea  (Read 2712 times)

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Offline Figleaf

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Corrosion at sea
« on: May 23, 2007, 11:50:53 AM »
The article below was published on the back of Odyssey's large coin find.

source: Slate

Peter

Sunken Treasure, Mint Condition
When do coins from a shipwreck get corroded?
By Torie Bosch
Posted Tuesday, May 22, 2007, at 6:34 PM ET


On Friday, a team of deep-sea treasure hunters announced the discovery of a shipwreck containing 17 tons of Colonial-era coins worth as much as $500 million. The team's sponsors haven't revealed where in the Atlantic they recovered the cache or what ship carried all of that loot. They did, however, note that the gold and silver coins were in pretty good condition. What factors affect the quality of shipwreck coins?
Where the ship goes down and what kind of metal the coins are made of. Coins that spend hundreds of years submerged can end up getting scratched, worn down, corroded, covered by sea life or lime deposits, or damaged by acid conditions. The warm waters of the Caribbean and the tropics are likely to cause the most damage, as warmer temperatures speed up oxidation and corrosion. These waters also contain coral and micro-organisms that can encrust the coins, depleting their value, usually permanently. Cooler northern seas?like those off the coast of England, where some speculate this treasure was uncovered?are more likely to help keep all kinds of coins looking good.
Conditions at the sea floor can also make a difference. A muddy bottom might help preserve coins by encasing and protecting them, but an environment of swirling sand can cause scratches and wear down markings and designs. (The depth of the wreck also comes into play: Deep waters tend to have weaker currents, so the sand at the bottom doesn't move around as much.) In some cases, though, sand can be a good thing. The S.S. Central America sank in 1857 amid calcium carbonate sands that helped make the surrounding water slightly alkaline, keeping potentially damaging acidity at bay. As a result, the ship's coins were close to pristine when they were uncovered in 1987.

Salt water can seriously damage silver and copper coins pretty quickly, but it has almost no effect on gold. But even a gold coin can suffer damage if the ship's wood breaks down and makes the local environment more acidic.
Bonus Explainer: Why did these ships carry so much gold? For security, in many cases. Many of the wealth-laden ships that sank in the Caribbean and the Florida Straits were part of "treasure fleets" operated by the Spanish to protect loads of gold en route from the New World to Europe. The Spanish placed large quantities of treasure under the protection of these packs of ships, which were more likely to scare off pirates.

Explainer thanks John Albanese of the Numismatic Consumer Alliance, Bob Evans and Douglas Mudd of the American Numismatic Association, and Scott A. Travers, author of The Coin Collector's Survival Manual.
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline bruce61813

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Re: Corrosion at sea
« Reply #1 on: May 25, 2007, 11:03:23 PM »
Acids from wood breaking down would not effect gold. In general the ocean bottons are pretty calm, only very shallow, less than 30 meters gets stirred up, and then by very bad storms like hurricanes.

Bruce

Offline Figleaf

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Re: Corrosion at sea
« Reply #2 on: May 25, 2007, 11:22:28 PM »
I was impressed by the recovered wreck of the "Mary Rose", a Tudor ship of the line preserved in Portsmouth. The part that had been covered by mud was intact. What stuck out was gone. I have been told that the mud was closing off oxygen.

Another story that made an impression on me was told by my grandfather. He was involved in the lifting of a sunken ship in the outer port of IJmuiden after the end of the second world war. The ship had been there for five years. When it was lifted, it was washed with fresh water as it surfaced. According to my grandfather, if this had not been done, it would have rusted away in days. With the salt washed off, the ship could be recovered. Again, the scarcity of oxygen had preserved the ship.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline bruce61813

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Re: Corrosion at sea
« Reply #3 on: May 26, 2007, 12:28:44 AM »
You must separate things.
Anoxic environment [no oxygen] will preserve metals, they cannot rust in the case of iron articles.

Wood is preserved by saltwater, fresh water rots it, they is why wooden sailing ships were 'scrubbed' with salt water every day, the salt was embedded into the wood and that preservered it.

Gold is impervious to almost every naturally occurring chemical, even few things will discolor it.

There was a case where a small reseach sub was  'lost' and sunk, but the crew of two had not boarded it yet, but their lunches and other gear was already aboard. It sunk on 300 feet of water and stayed there for 11 months before it could be retrieved. The lunches were in perfect condition, even the lettuce was still crisp, so was an apple. But once things were on the surface, they rotten within a week. I was an article I read while studying oceanography at the University of Washington.