Info on coin degradation in soil?

[OcdChaos]

I'm new to the hobby, and was wondering if anyone could point me to a website link or article that discusses the impact of soil degradation over time on different types of metals in coins. I see where people can dig up hundreds of year old coins that look nice, as opposed to one my first digs at a park yesterday where I find a newer penny that the edges were corroded away, like eaten by acid. Just looking for an article or something that talks about the metals in coins and what may hold up over time and what doesn't, and what factors may affect coin condition in soil. Attached is a photos of a few coins I found yesterday as an example. Thanks.

 

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[Jason in Enid]

I can't point you to an article, but I can provide some insight on what you are seeing....

1- US pennies changed from a copper alloy to a zinc planchet with a copper cladding in 1982. So any 1983 (and a few 1982s as they were mixed that year) and newer like to self destruct because the addition of conductive moisture (aka ground water) causes them to become a tiny battery which eats its self. 1982 and older still look like solid coins.

2- copper corrodes from exposure to the combination of moisture, minerals and oxygen. As these can vary from one location to the next, the amount of corrosion will always vary.

3- modern clad coins (copper core with cupro-nickle front and back) are typically very stable in most ground conditions. Highly corrosive ground and salt water will eat away the copper core must fast so you can find ones that look like oreo cookies.
- The cupro-nickle cladding reacts with moisture and minerals to create a dark brown/black staining

4- older silver coins very frequently come out of the DIRT as shiny as the day they were dropped, but it the ground is constantly wet, the can turn black from sulfur contamination. Silver LOVES sulfur.

Every site is different and soil conditions can change very quickly within a single site. I have dug IH cents in almost perfect condition as well as old wheat cents that still have some original shine to them. Then 50 foot away you can dig another of the same that is so corroded its almost impossible to determine what it is.

 

[Loco-Digger]

Welcome to the world of crusty zincolns

 

[hvacker]

Moisture, soil type, compaction, chemical analysis, coin metal type, you just gave me a great idea for a Masters Thesis.
I live in a very dry sand box so not much happens except for steel.

 

[Dave Rishar]

Attached is a photos of a few coins I found yesterday as an example. Thanks.

Climate, soil, time, and coin metallurgy.

Those coins look awfully similar to what we dig around here - verdigris on copper, badly rotted zincolns, and red dimes and quarters.

 

[Geochem]

The majority of metallic objects (except for pure metals like pure silver and gold) are not stable to air (i.e. to oxygen) and water (i.e. hydrolysis). And because of this the transition metals and metal alloys will revert back to a mineral (compound) form.

This means the "coin composition" influences what will happen when it is exposed to the atmosphere and water. In this case to soil moisture and aeration over time.

Within the variations of soils and their geographic location, there will be a multitude of factors that will influence the history of the oxidation and hydrolysis to the specific coin's transition metals.

For example, if the coin is in arid soil the reaction to air will typically form an oxide film on the coin. Arid soils usually are alkaline and no acid degradation (elemental leeching) will occur.

Soils that receive regular rainfall and lateral water movement drainage conditions, and compounded with plant root secretion activity induces acids (i.e. oxidation agents) will contribute to more hydrolysis of the transition elements composing the coins. The more plant debris that winds up in soil over time (roots, leaves, etc.) the more microorganism activity will occur (i.e. increase secretions of biocompounds) that will further induce by multi-parameter conditions coin surface elements degradation. The water chemistry solutes (type and amount of cations and anions) of the soil moisture there will then be more reduction/oxidation reactions that will occur to oxidize the metal elements. Periods and cycles of soil aeration will influence soil moisture pH.

 

[BigWaveDave]

Everything just posted is true....
Very good experience and insight...
So, now that you understand the environmental effects on common clad and zinc coins, now comes the real fun.
Roll them up, and deposit them to the bank. With any luck, a coin roll hunter will be all wtf!

 

[Charlie P. (NY)]

Nitrates in the soil form acids that eat away the zinc in modern cents.

One park I hunt uses grass fertilizer that destroys a post-'82 cent in a few years.

 

[sprailroad]

This is what's great about this forum, you can get some really good answers from some really solid guys, they summed it all up.