Need coin expert! Pulled from the Gulf, old silver.

[Crispin]

First here is the coin...then the story.



So I dug this up over a week ago at my antique spot. I thought is was a fried zincoln so I threw a bunch of them in vinegar and put it outside. Forgot about it until today. I brought the cup in and this is what I found (obviously the dime was not in there, I put that in for size reference:




So I'm thinking this is remarkably intact and rather odd. I bust out the oxidation test and put the all the coins from the test and the dime is a control, lots of bubbles on the unknown coin:




Okay, I'm convinced enough that it is silver to move on the acid test. I bring the zincolns as controls, control first:




Unknown coin second:




I snap a couple pictures to document and then rapidly pull it out as silver is starting to accumulate on the top, this is what it looks like back in the house (minus the control which completely disintegrated):




I cleaned it up as best I could back in the oxidation-reduction pan but I was afraid to keep cleaning because I was losing the coin. Got the wife to take the hi-res pics that are first posted. Anybody know what it could be? I realize there are no markings on it. Thanks for looking and thanks for any helpful comments. Click on the hi-res pictures to enlarge and then magnify for a better look.

PS: If anybody calls this an Aluminum fire nugget they have lost their mind.
PPS: I went through other finds from that area that I thought were zincolns and have them soaking in vinegar now. Will leave them for a week.

 

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[Crispin]

This the coin in the raw without any cleaning. I know it is not a great picture but I thought it was zincoln:

 

[Scrappy]

I know very little about silver/copper blends used for planchets, but maybe look for for old coins known for a heavy copper percentage silver. That may explain the pitting. Also check the weight and diameter and compare. Good luck

 

[Crispin]

I know very little about silver/copper blends used for planchets, but maybe look for for old coins known for a heavy copper percentage silver. That may explain the pitting. Also check the weight and diameter and compare. Good luck

Thanks for the suggestions. It looks like both mercury and pre-65 rosevelt dimes were both 90% silver and 10% copper. The diameter of all dimes then and current is 17.91mm. I will check it when I get home. If it is more then that then it must be something else. I'm not sure how much weight will help because I've lost so much of the coin.

What is the relationship between the copper and the pitting? Did the copper cause the pitting or is it that the copper distengrated and the silver was left behind?

Thanks for looking.

 

[releventchair]

Sorry bud I don't know what it is. 'Toasted"by galvanic action my best guess.
You had me laughing so hard at "fire nugget" = lost mind the dogs are barking like I lost mine. Oooh wait, careful where you step now....it's here somewhere.

 

[Bigcypresshunter]

OK crispen. I will agree that the coin shaped item is silver. The salt will corrode the coin but the round shape will remain. They are commonly called cookies or cookie coins. I found some like that here on Hollywood beach. Nice find. http://www.treasurenet.com/forums/today-s-finds/30507-trench-beach-produces-silver.html

 

[Bigcypresshunter]

PS: If anybody calls this an Aluminum fire nugget they have lost their mind.

This time I wont argue with you. Nice find. I dont know what causes the pitting on .925 silver coins. Maybe its the copper like you suggested. Spanish Reales caught in wave action will wear very thin.