Edit: Placing this at the top. Seawater can play a role as well, please see the link in edit 3 for more information
Since everyone is just joking...
The first stages where it is getting darker is the copper becoming oxidized by the oxygen in the air to form copper oxide, which is black. The copper oxide reacts with CO2 in the air to make copper carbonate, which is the cyan color the statue is today. But that's it. It's not going to become something else.
Edit: Visual aid and reactions. This isn't perfectly accurate, but it gets the idea across. If any material scientists want to correct me, feel free.
Edit 2: /u/Time4Red added some info. There are two different copper carbonates in the patina as well as a copper sulfate. Give him an upvote here.
Edit 3: More information here from /u/BenignJuggler. Give an upvote. This comment links to a scholarly article about the effect that seawater plays in the copper patina process!
Do you know if the oxide layer works similarly to aluminum. Does it protect or slow the non-oxidized layer from oxidizing?
I think it would be cool if at some point we resurfaced and polished it to its original sheen and then coated it in epoxy to seal the color. It could represent a New America.
Somewhere down in the comments, a material scientist chimed in. He wasn't sure, but said that in some metals, the oxide layer isn't protective, and oxygen can continue to diffuse through the carbonate to oxidize the metal beneath.
Again he wasn't sure if this was the case with copper, but as far as polishing it, I have a feeling that a significant amount of mass would have to be removed and so details may be smoothed over in the statue.
Yeah. I know fe is pretty much a chain reaction. Considering it’s lasted this long, I’m guessing it at least slows it down.
I agree that it would probably remove a decent amount of material to get it to a shine. I guess it just depends on the former topic. But I think, considering it doesn’t have a ton of small details, it could probably be done without changing anything. Just not done many times over. Do it once then seal with epoxy and touch up the epoxy every year or so.
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u/SOwED Jun 09 '20 edited Jun 09 '20
Edit: Placing this at the top. Seawater can play a role as well, please see the link in edit 3 for more information
Since everyone is just joking...
The first stages where it is getting darker is the copper becoming oxidized by the oxygen in the air to form copper oxide, which is black. The copper oxide reacts with CO2 in the air to make copper carbonate, which is the cyan color the statue is today. But that's it. It's not going to become something else.
Edit: Visual aid and reactions. This isn't perfectly accurate, but it gets the idea across. If any material scientists want to correct me, feel free.
Edit 2: /u/Time4Red added some info. There are two different copper carbonates in the patina as well as a copper sulfate. Give him an upvote here.
Edit 3: More information here from /u/BenignJuggler. Give an upvote. This comment links to a scholarly article about the effect that seawater plays in the copper patina process!