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I’m always amazed at how old beach finds can be in better condition than the same things found on land. It makes you wonder if it’s the lack of oxygen under water or the action of chemicals in the fields. Probably both.
Full XP Deus 11" coil / hf elliptical coil
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So here is a reply at sffchronicle to someone's question about unburried bronze in the sea
"Bronze, as you most likely have realised already, is an alloy of tin and copper. Tin can react with oxygen over time, creating a white powdery type stuff. Copper reacts very slowly indeed with oxygen, barely at all. Instead, it reacts more quickly with carbon dioxide, to create copper carbonate, which is the green colour, or it can react with salt to create copper chlorite (green again) or with sulphur to create copper sulphate (black tarnish. very weird looking). Now, the chlorine and sulphur will react much quicker than the carbon dioxide with this.
In a shallow marine environment, you'll find very little sulphur, so this is effectively null. On the other hand, there's plenty of salt, and the shallow depth means the water will be rich in oxygen and carbon dioxide compared to a deeper marine environment. Assuming temperate temperatures (western europe, for example) and a normal level of salt in the sea (atlantic ocean) we're probably looking at pretty quick reactions here. Unless the mass of bronze is very large it won't last more than a few hundred years, and within a few decades it will be practically unrecognisable. A sword, for example, would probably become unusable within ten years. "
Copper does form a resistant layer of copper oxide, that is how water pipes of copper survived over four thousand years in the pyramids apparently. However acids dissolve that, carbonic acid, sulphur dioxide I think from pollution, ammonium chloride and other fertilizers, some pesticides and herbicides etc. Some studies say the last hundred years are responsible for a disproportionately large amount of corrosion of relics.
It gets much more complicated and you start to need a phd in chemistry to understand what anyone is saying, but studies on copper corrosion are online. Instead here is a slightly more basic look at some of the environments that affect copper
https://www.copper.org/resources/proper ... round.html
So the variables are many, but I think well conserved coins on the beach are likely freshly uncovered from depth in sendiment, or washed down by rivers maybe? One roman coin I found on a beach near a river was not far off yours in terms of being preserved. Another I found in pebbly shore had at least three mm of rock hard patina to it, and looked like a pebble. Underneath was not pitted though, the outlines were emphasised but any small detail erased
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XP Deus I use the HOT programme on 17khz, reactivity at 2.5 on v5.21 with the X35 11” coil
Hammered silver 2019 - 23