9/4/26 Report - Various Patinas and Crusts on Dug Coins. Copper, Silver, U.S. and Shipwreck Examples. Ancient Bronze Coins.

 

Written by the TreasureGuide for the exclusive use of the Treasure Beaches Report.

Gold Patina on Dug Silver Quarter.

Over the years I've talked about the patinas that are found on "duggies" - my name for coins I dug.  Duggies can have a lot of different kinds of patina, depending upon the metallic composition of the coin and the soil or environment the coin was buried in before being uncovered.  

Time is another variable that affects the patina.  It will take a while for a patina to form or deepen.

A patina tells a part of the coins story - not only something about where it was buried, but it also gives a clue as to how long it was there.

I found a great article about patinas on ancient bronze coins (Ancient Coin Patina Explained: Types, Chemistry, and Collector Value).  The quotes below (in italicized blue) are from that article.

Ancient coins are something like shipwreck coins.  Shipwreck coins are made of a variety of metals, but bronze coins exhibit some of the characteristics of copper coins because copper makes up somewhere between 88 to 95 percent of bronze.

In technical terms, true patina refers to surface oxidation that begins forming on a metal coin almost immediately after striking. On modern copper coins, this same process produces the familiar Red, Red-Brown, and Brown designations.

However, among ancient coin collectors, the term “patina” has a broader meaning. In common usage, it describes any layer of oxidation or mineral deposition that forms over centuries while a coin remains buried...

Green patina is the most frequently encountered surface on ancient bronze coins. It forms primarily from copper acetate (Cu(CH₃CO₂)₂), which develops when copper reacts with acetic acid present in air or soil.

Importantly, green patina can vary dramatically in tone. Shades range from nearly black to bright or pale green, depending on environmental exposure and alloy composition...

Here are some pennies that show a variety of patinas.  

Dug Copper Cents with Various Patinas.

Among my copper cent finds red patinas are less common than green.  The 1970 S above is more green than it looks in the photo.  Green patinas are pretty common.  I call them greenies.

Red patina appears far less frequently than green patina. It forms when cuprous oxide (Cu₂O), also called copper(I) oxide, develops from reactions involving cupric oxide (CuO) in the coin or surrounding soil.

This patina can appear bright red or dark crimson. However, coins displaying only red patina are extremely rare...

Some copper oxides form stable blue or blue-green surfaces, similar to those seen on untreated bronze statues...

Most of the coins with a blue patina that I've found came from fresh water northern lakes, such as some in Minnesota, where the silver coins and silver artifacts all a nice blue patina. The patina left the silver uncorroded, so they coins looked very nice.  Those coins, unlike those found on saltwater beaches, didn't turn black.

Silver Dime with Blue Patina

I really like the blue patina on the silver I dug in these Minnesota lake.  The coins and religious medallions all had the same nice gunmetal blue patina.

The wildest patina I have is the black and gold silver Standing Liberty quarter shown at the top of this post.

Unfortunately, I don't remember when or where I found this one, and I didn't follow the progress of the patina over the years, so I don't know how much of it was there when I dug it.  It has been stored in a coin holder for years, but it doesn't show the characteristic signs of PVC contamination.

Most older “flip” holders and vinyl pages were made from PVC with added plasticizers that can migrate onto the coin's surface. Over time, these plasticizers migrate out of the plastic and onto the coin and can form hydrochloric acid compounds that react with the metal.

Copper coins are most vulnerable because copper reacts readily with the acidic byproducts, forming copper chloride that pits the surface.

Silver coins are more resistant to PVC contamination but can still develop dark spots or hazy films.

Gold and platinum are the least affected.

PVC contamination usually causes a green or greenish blue, and maybe a greasy texture, hazy patches, or tiny green blobs. So, I don't think that is what happened to the gold and black quarter.

Duggies can have something heavier and thicker than what I would consider a patina. It is more like a crust.  My best example is the Potosi half-reale from Jupiter shown below.

Shell Encrusted Potosi Half Reale
Metal Detected at Jupiter Beach.

That was a really hard crust. Only one side had the crust.  The other side looked more like the typical beach metal detected reale.  It was black.  So the two sides looked very different.  I discussed my theory about that before.  The cob was just washed out of the dunes, where I would guess it had spent many years.

Once the hard shell crust was removed, the surface was black but the design was sharp and clear like it was newly minted.  The shell crust protected the surface from the deterioration.  The other (exposed side) showed wear of corrosion or wear from tumbling in the surf.  

Another type of crust was found on a bunch of silver U.S. coins found on Ambersands beach in beach renourishment sand.  The sand removed from the Sevastian River inlet before being trucked in and dumped on Ambersands.

Below is the negative image of a mercury dime on the inside of the crust after the crust was cracked and removed from the coin.

Image of Mercury Dime on Removed Black Crust.

The crust fully encased the coin and was impenetrable.  Acid didn't affect it at all. The crust was like a ceramic.

Here is the other side (outer side) of the same crust.  No detail showed on the outside of the crust.

It seemed this crust was made from a type of fine clay or something like that, but I'm not sure.

The patina, just like mint marks or contextual clues, can provide a part of the coin's story and add charm and character to the piece.

So, there are a variety of types of patina and other coin coatings found on metal detected coins.  Maybe I'll present additional examples in the future.

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Once again, here is the link to the article about patinas on ancient bronze coins.  

Ancient Coin Patina Explained: Types, Chemistry, and Collector Value

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There is nothing on the National Hurricane Center Atlantic map right now.  No storms or any developing systems.

The Treasure Coast surf will remain small for a few days.

Good hunting,

TreasureGuide@comcast.net