Why the different colors?

k2gleaner

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I've dug up my first few coins this past week. at one site, behind my kids school, the coins came up all copper colored no matter what type of coin it was, except for a 1975 penny. It looked like it had just been in my pocket after being washed off. Across the road, at my church, the same type of coins came out looking much more like I'd expect. Both of the soils are quite organic, black, and very wet.

The attached photo shows a nickel, quarter, and a penny.
a55bb20031ee89f3c92a9b9264aff603.jpg


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I can’t give you a super scientific explanation of the exact chemical reactions that are going on, but what you are seeing is a natural tarnish and corrosion resulting when any “clad” and copper coins are buried in the ground for awhile and begin to react with the minerals, chemicals, and rain water. Modern U.S. nickels, dimes, and quarters (1965 and later) have enough nickel content that they always come out of the ground with some varying degree of grey, brown, or even reddish or orange staining or tarnish even after the dirt is washed off. The process happens pretty quickly. I once buried a shiny new modern nickel in my backyard as part of a “test garden” - after a few weeks, I decided to remove the nickel, and it had already started to permanently tarnish!

Pennies from 1982 and earlier are mostly copper, and can often clean up pretty nicely with just water, although the older ones gain a nice green patina or other kinds of natural copper corrosion depending on the soil. Pennies after 1982 are mostly zinc and literally get eaten away by corrosion in the ground in very short order. I’ve dug 2015 “Zincolns” that are already destroyed by corrosion.

Silver coins (mostly dimes and quarters prior to 1965) often come out of the ground with little to no real corrosion, depending on soil - a sight I never tire of seeing!

Anyway, I would guess that different lawn treatments or other slight chemical variations in the soil might be the reason for the different colors on the coins despite the soil looking similar.
 
I find red coins in places that are constantly high in moisture...as in soil..not beaches...oxides make red...iron oxide in the wet soil
 
Chemical grass applications will also impact what they look like

Yeah.. fertilizer is bad..burns up a zincoln fast.

Moisture in the soil can cause soluble salts in the soil to dissolve and form an electrolyte that conducts electricity resulting in electrolytic corrosion..

Cuprous oxide compacts into purplish-red cuprite

Basic green ( due to malachite) or blue carbonates (from azurite, but only in high concentrations of the hydrogen carbonate ion HCO3)
Sulphites make green also

Sulphides make a brown or black patina

Oxide turns it red

Expanded blue zincoln from salt water beach..
 

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