Numismatic and History Discussions > Coin Photography, Conservation and Storage


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I'm not sure this is the proper subcategory but seems the closest to me.

I've had a half dozen coin soaking in distilled water for months. They are all really poor shape so more of something to do than an active project.

After these few months, the coins started looking dry and ugly so I placed them in a bowl of olive oil (can't call it virgin cuz already been used soaking other coins in the past - sorry bad attempt at humor).

Unlike the water, changing it every few days these sat in the olive oil for about 2 months forgotten.

Today I took a look at them and the oil turned blue.

I pulled one coin out and wiped it - looks nicer than the "dry" look from soaking in water, this disturbed the blue. Originally it was just close to the coins - pulling the coin spread the blue around.

Anyone know why? What would cause the olive oil to change to blue?

Thanks for the chemistry lesson in advance.


Olive oil is slightly acidic, so the blue (green) is the copper in solution from the breakdown of the corrosion.  This can also happen when you put silver coins in ammonia if they have copper in the alloy, because ammonia dissolves copper.


Nikos P2:
try to put a little hydrochloric acid in the water. in 10 minutes it is ready ..! This is how I do all my coins.

Even if your intention is to, for some reason, strip the coin to bare metal, Hydrochloric acid is absolutely the worst thing you could use.  This is a recipe for bronze disease (copper chloride), and the chlorine is almost impossible to remove completely from the pores of the coin.  I have tried HCL on modern corroded coins found on the beach and it does work fast, but the green corrosion always comes back within a few days.  It is also toxic, especially when combined with the trace amounts of lead found in the alloys.


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