When searching for ways to stop this process, the most common suggestion I encountered suggests simply drying the coin and to halt the cycle of corrosion. This approach targets a specific step of the reaction where cuprous chloride reacts with atmospheric moisture and oxygen forms a green cupric chloride/cupric hydroxide compound and hydrochloric acid. The idea here is if you remove moisture and oxygen then the process can't continue. Since it is really impossible to prevent all atmospheric moisture and oxygen, this approach mostly slows down the process but doesn't really stop it.
The other treatment I've found involves using sodium sesquicarbonate to remove the the bronze disease and benzotriazole to chelate the copper to prevent future reactions. Unfortunately, I found very little documentation detailing the entire process. This series will document the steps I'm taking as I attempt this approach.
I am sacrificing the coin above for science. I will start with a 5% sodium sesquicarbonate solution that I plan on changing every few days until all signs of green encrustations are removed.
5% Sodium Sesquicarbonate Solution
Ingredients
- 10.6 g sodium carbonate
- 8.4 g sodium bicarbonate
- 100 ml distilled or deionized water
Steps
- Measure out the dry ingredients and set aside. I just put it on a small piece of paper.
- Measure 100ml of distilled water.
Optional: heat the water for improved results. I used a small mason jar and heated the water in a microwave for 2 minutes. - Pour in the dry ingredients and mix until dissolved
- Place the coin in the water and cap it
You should see some of the green material dissolving from the surface of the coin pretty quickly.
Within a few hours the water will turn a bright blue-green color.
After 1 day
I poured out the water from the first day and prepared a second batch following the same steps above.
After 2 days
Here's the coin after soaking for two days. The coating of corrosion on the obverse is a lot thicker than I would have guessed when I started with this coin. The reverse is almost clear now and the dragon design is clearly visible.
The water was bright blue again so I poured it out and made a new batch of sodium sesquicarbonate.
After 4 days
Most of the green/white deposits have now been removed only a few of the deepest or thickest patches remaining. The coin has darkened a lot. I'm guessing 1-2 more days and the last of the deposits will be be removed.
After 6 days
This is the coin after soaking for six days. There is still a single bright spot of the green/white deposits just below the lower-right corner of 大 (dà) on the obverse. I poured out the solution since it's a pretty bright blue now and made a fresh batch to get the last bit of this.
Overall the coin has a kind of greenish haze covering it. There are also some redder patches of likely cuprous oxides. Once what looks like the last of the copper-chloride deposits are gone I'll explore some ways to even out the color and maybe clear up the haze.
After 7 days
This is the coin after soaking for seven days. The last of the chloride deposits appear to be gone now. As can be observed, there are a lot of black cupric oxides and red cuprous oxides covering the surface. The next step I might explore are a pickling solution to even out the surface oxidation and stabilizing the copper to prevent future bronze disease progression.
Full Progression
You say "Sacrificing" this coin at the beginning. Do you feel that there was damage done by the Sodium Sesquicarbonate Solution?
ReplyDeleteAccording to what I've found, the darkening observed is from the sodium sesquicarbonate reacting with copper to form various sodium copper carbonates. Over the long term, this can start replacing components of the natural patina or reacting with the underlying copper.
DeleteWhat I might try with a different coin is sodium carbonate instead of sodium sesquicarbonate. According to what I've found it also removes cuprous chlorides and neutralizes the hydrochloric acid but is less reactive with copper.
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