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Shipwreck Coin With Low Specific Gravity (1736 8-real "Rooswijk")

 
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Pillar of the Community
Australia
618 Posts
 Posted 04/06/2019  04:51 am Show Profile   Bookmark this topic Add Squire Wilson to your friends list Get a Link to this Message
Recently I purchased the below 1736 8-Real "Rooswijk" shipwreck coin from a reputable dealer. This included a Certificate of Authenticity with embossed "Undersea Search and Location" and signed by RE Bryson the Deputy Project Administrator and GJ Ralph the Numistmatic Recorder. However, the Certificate did not include an image of the coin.

As you can see below the coin is somewhat "worse for wear", and this appeared to add to its "authenticity".

So I started some research.
Firstly I obtained an XRF analysis.
For the "Shield" side of the coin this came out at:
93.3% Ag, 5.87%Cu, 0.3% Ag, 0.2% Pb.
For the more corroded "Pillar" side this came out at:
Ag 95.7%, Cu 3.7%, Au 0.3%, Pb 0.2% and Ge 0.1%.

Next I bought myself a set of scales to accuracy 0.01 gram.
After many measurements the coin weighs an average of 24.62gr and suspended in water as 2.46gr. This indicates a Specific Gravity of 10.02
Also the coin does not "ring" when I suspend it on my fingertip and hit it with another coin.

At first I became highly suspicious.
I compared it to a 1783 2 Real Shipwreck coin from the El Cazador shipwreck and this has a Specific Gravity of 9.85
It also does not "ring" when I hit it with another coin in the above manner.

Then I read the post at https://forums.collectors.com/discu...-silver-coin , specifically stating that "In the past decade, I have taken the specific gravity of at least sixty 1883 (1783) 8 Reales from the El Cazador in all conditions from decent to a corroded lump of metal. Not one coin has had a specific gravity over 9.8. They range from 8.9 to 9.8. These coins are all over the place both raw and certified. Can anyone explain a reason for their LOW specific gravity? Does saltwater "leach" the alloy?"

I would really appreaciate the feedback from CCF members on this interesting variation to a fascinating topic.

Squire


Edited by Squire Wilson
04/06/2019 04:53 am
Pillar of the Community
United States
1994 Posts
 Posted 04/06/2019  12:04 pm  Show Profile   Bookmark this reply Add thq to your friends list Get a Link to this Reply
The deterioration of silver metal with time interests me too. I've observed that early US coinage "weeps" black crud in contact with other coins in my pocket. Here's an 1807 quarter, which developed these black spots over a period of a week:



I assume that the spotting is black silver or copper oxide weeping to the coin's surface via microporosity/cracking, but I have not tested it. This would explain both a loss in density and an inability to ring like solid silver.

I'm not sure what 250 years in seawater would do, but I have seen the heavy corrosion dry table salt causes on a sterling salt spoon over a year.

"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Pillar of the Community
United States
2585 Posts
 Posted 04/06/2019  5:35 pm  Show Profile   Bookmark this reply Add tdziemia to your friends list Get a Link to this Reply
Wouldn't a more likely explanation be a reaction on the surface rather than something migrating from the interior?
Pillar of the Community
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946 Posts
 Posted 04/06/2019  5:50 pm  Show Profile   Bookmark this reply Add llewellin to your friends list Get a Link to this Reply
I think the black spotting is a result of wear of the metal, not a chemical reaction. You see the black stuff on a lot of junk silver. What happens is when the silver rubs against other coins, it wears the surface away, creating very fine (micro-nano scale) particles of metal that have to go somewhere. If there's any kind of oil on the surface the metal particles could form a sort of black putty that I've removed from junk silver with acetone. Alternatively the particles also stick together and to the surface. So not a chemical reaction but a physical one.

But in principle I agree it is not likely a solid silver coin undergoes any significant metal loss via leaching from the coin's interior.
Pillar of the Community
United States
1994 Posts
 Posted 04/06/2019  6:00 pm  Show Profile   Bookmark this reply Add thq to your friends list Get a Link to this Reply
It happened rapidly llewellin. The coin was clean and after a couple days in my pocket with other coins it was spotted. I haven't seen it happen with 20th century silver. As a result I quit carrying 200 year old silver and coppers (which form similar spotting). The deposits scrape off with a fingernail.

The subject has been studied for a long time. Silver objects change over time and become hardened and brittle. The crystallinity goes through the metal. The hardening effect can be induced in typical sterling and coin silver/copper alloys in a period of days at high temperature (600 hours at 200C).

https://core.ac.uk/download/pdf/53034503.pdf
"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
04/06/2019 6:22 pm
Pillar of the Community
United States
2585 Posts
 Posted 04/06/2019  9:27 pm  Show Profile   Bookmark this reply Add tdziemia to your friends list Get a Link to this Reply

Quote:
Does saltwater "leach" the alloy?


There are probably others out here who have read much more literature on this topic than I have, but I will go out on a limb, and say the one article I found that appeared to be well researched and referenced says that the effect of seawater on a silver coin is often more or less the same as atmospheric corrosion: a layer of silver sulfide builds up on the surface of the coin.
In shallower, more oxygenated water, there can be other compounds like silver bromide and silver chloride present.

In cases of extreme corrosion, much of the silver might get converted into a ixture of these compounds.
http://nautarch.tamu.edu/CRL/conser...l/File13.htm

There is no mention of leaching, so maybe we rule that out.
As for the effect of a surface layer of Ag2S, AgBr or AgCl on the specific gravity, you would need over 10% of these compounds to reduce the specific gravity of coin silver to a figure equal to, or below 9.8 (Ag2S has a density of 7.23 and AgBr is 6.47).

In my opinion, a layer of corrosion products this thick would likely render the coin quite disfigured (I did notice that the linked reference earlier said that some of the shipwreck coins measured at very low specific gravity were a "corroded lump of metal". At an SG of 8.9, we are talking about nearly 50% corrosion products).

Just my 2 cents' worth, again, with the recognition that I am likely far less experienced than other folks who might comment.
Pillar of the Community
United States
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 Posted 04/06/2019  9:45 pm  Show Profile   Bookmark this reply Add tdziemia to your friends list Get a Link to this Reply
Thanks for posting that article thq.

One thing we need to remember is that reactions that take place at time scales like weeks at 200C, are likely to take place thousands to millions of times slower at ambient temperatures (for chemical reactions, the rule of thumb is a factor of 2 for each 10 degrees. However, these phenomena are not chemical reactions, but physical effects like diffusion of impurities to phase boundaries, but I suspect a similar rule of thumb applies).

I did run across a summary of that 1954 paper on embrittlement of ancient and medieval coins due to phase separaration of a lead rich phase. Very interesting.
Edited by tdziemia
04/06/2019 9:47 pm
Pillar of the Community
United States
1994 Posts
 Posted 04/06/2019  9:55 pm  Show Profile   Bookmark this reply Add thq to your friends list Get a Link to this Reply
The following study shows some good pictures of silver enrichment at the surface of old corroded low silver content (15-40% Ag) coins.

https://www.jcms-journal.com/articl...cms.1021204/

Copper has been leached, leaving high purity silver at the surface.

Another presentation describes the final deterioration generated by cold striking of silver. It starts 75 pages in, but is worth looking at to see crumbling objects and fracture microphotography.

https://web.wpi.edu/Images/CMS/MCSI/2006reti.pdf

"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
04/06/2019 10:41 pm
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United States
946 Posts
 Posted 04/06/2019  10:01 pm  Show Profile   Bookmark this reply Add llewellin to your friends list Get a Link to this Reply

Quote:
It happened rapidly llewellin. The coin was clean and after a couple days in my pocket with other coins it was spotted. I haven't seen it happen with 20th century silver. As a result I quit carrying 200 year old silver and coppers (which form similar spotting). The deposits scrape off with a fingernail.


I think we're talking about the same thing. Silver coinage forms a black gunk/paste/mud like grime on the surface when it's thrown in with other coins. I have seen this on 20th century silver coins and do not think it is unique to older coinage with different impurities or embrittlement. The fact that you can scrape it off with a fingernail makes me think it's the same thing I commonly see on junk silver.

Intuitively, it is much more likely that given their formation in just a few days these deposits are formed by mechanical rubbing of the coins and the sweated silver rather than a chemical reaction or solid diffusion, which are incredibly slow processes at room temperature as tdziemia notes, especially for a metal as inert as silver.
Pillar of the Community
United States
1994 Posts
 Posted 04/06/2019  10:07 pm  Show Profile   Bookmark this reply Add thq to your friends list Get a Link to this Reply
It's not from sweat llewellin. The coin was in a clean coin pouch, in contact with modern silver. I have never seen anything like this on mine-run Kennedy and Franklin halves. And while coins get grimy, they do not form spots on smooth surfaces like this.

tdziema, some of the microphotos of silver show voids in the structure. But what is more interesting is the embrittlement failure of the metal at points where it was stressed in cold compression, such as hammering and chasing.
"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
04/06/2019 10:33 pm
Pillar of the Community
United States
946 Posts
 Posted 04/06/2019  10:13 pm  Show Profile   Bookmark this reply Add llewellin to your friends list Get a Link to this Reply

Quote:
It's not from sweat llewellin. The coin was in a clean coin pouch, in contact with modern silver.


There is no reason why it is not from silver sweat. The conditions you describe, this is by far the best and only reasonable explanation. Just because you haven't seen something or don't understand it doesn't mean that it doesn't happen. I have seen this plenty of times on worn barbers with exceedingly smooth surfaces.
Pillar of the Community
United States
1994 Posts
 Posted 04/06/2019  10:29 pm  Show Profile   Bookmark this reply Add thq to your friends list Get a Link to this Reply
I have no idea what you mean by "silver sweating" llewellin, other than in the sense of soldering.

Do you mean the release of weakly bound silver corrosion products? I'd buy that argument, except that in the years I've carried silver in my pockets the coins have always gotten shiny, not spotted. Maybe it takes 100 years to generate the crudding potential of silver coins? I cleaned and repeated this several times with that same coin and it continues to generate abundant spots.
"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
04/06/2019 10:36 pm
Pillar of the Community
United States
4869 Posts
 Posted 04/06/2019  10:42 pm  Show Profile   Bookmark this reply Add swamperbob to your friends list Get a Link to this Reply
As I understand the problem of low density applied to silver coins salvaged from sea water wrecks, the problem should always be anticipated to some degree. Electrolysis of reactive metals in sea water is commonplace and a silver copper alloy can itself act like both the anode and cathode in the reaction. The silver being less reactive is not removed in this way but the copper is. This means that microscopic copper grains that form in the alloy when it is cooled will leach out of the alloy starting at the surface first. The longer the item remains in salt water the deeper the effect can go. Hundreds of years in salt water can result in leaching that penetrates deeply into the coin. This does depend on a number of factors in the environment - temperature, salinity and the presence of other elements.

The result of such corrosion often is a coin which yields a higher apparent percentage of silver and a lower percentage of copper when tested using XRF. XRF is only a surface reading but it is indicative of the loss of copper.

The total weight of the coin 24.62 grams shows considerable loss of metal. We can account for some of that as leaching but some must be due to mechanical causes (abrasion etc.).

Intuitively given only the two percentages of silver and copper, one would expect a higher SG as a result. Yet the reverse is true as attested to here.

It should be noted for accuracy that there are more than two elements present. The two XRF results add up to 99.6% and 100.0% so there is some small uncertainty. The good news is that the gold to silver ratio is correct for Mexican silver of the era.

The question is why the density drops while the silver to copper ration increases significantly? It seems not to make sense.

The reason is that such intuitive thinking fails to take into account the voids that run throughout the alloy as the result of the leaching away of the copper in the alloy. The problem is that sea water salvaged silver coins are actually NOT SOLID. They are more like sponges in their structure. The loss of copper internally results in the vacated space being replaced with the only thing available. The replacement is salt water with a density near but not exactly equal to 1.

I did some very rough calculations to determine how much copper would have to leach out of the coin to reduce the density to 10.03 instead of 10.31. To do so I made the assumption that the volume of the coin measured using pure water would not change due to the presence of the microscopic voids at room temperature. Surface voids would fill easily but microscopic voids would not.

My results were about 0.74 gram of copper would need to be replaced with water to yield such a result.

I also presumed that the reported figures 24.62g represented the weight of the coin in air and 2.46 g represented the apparent loss of weight when the coin was immersed in pure water. SG = 24.62/2.46 = 10.03.

One question remains in my mind. If you weighed a salvaged coin right after it comes up; Does the weight of the coin change over time?
My book on Counterfeit Portrait 8Rs is available from Amazon http://ccfgo.com/TheUnrealReales or from me directly if you want it signed.
Pillar of the Community
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4869 Posts
 Posted 04/06/2019  10:45 pm  Show Profile   Bookmark this reply Add swamperbob to your friends list Get a Link to this Reply
There are two different topics intertwined in this thread.

The black spots raised by thq are caused by a totally different process than the initial question of copper loss in sea water salvaged coins.
My book on Counterfeit Portrait 8Rs is available from Amazon http://ccfgo.com/TheUnrealReales or from me directly if you want it signed.
Pillar of the Community
United States
1994 Posts
 Posted 04/06/2019  11:12 pm  Show Profile   Bookmark this reply Add thq to your friends list Get a Link to this Reply
The water would not stay in the coin bob. The voids are full of 0 specific gravity air once it dries out. Like a dried out sponge. Though the coin has lost 10% of its original weight, it appears to have lost very little of its volume. squire's piece looks much better than most shipwreck coins.

The deterioration of silver over time is different in seawater that in pockets and purses, and seawater is arguably the worst of all environments.
"Two minutes ago I would have sold my chances for a tired dime." Fred Astaire
Edited by thq
04/06/2019 11:26 pm
Pillar of the Community
United States
4869 Posts
 Posted 04/06/2019  11:32 pm  Show Profile   Bookmark this reply Add swamperbob to your friends list Get a Link to this Reply
thq I am not as certain as thq that ordinary air drying results in all of the microscopic voids drying out until some length of time passes.
My book on Counterfeit Portrait 8Rs is available from Amazon http://ccfgo.com/TheUnrealReales or from me directly if you want it signed.
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