Green Copper Corrosion - Chemistry

It's pretty cool to see a mintmark revealed by the process...I did not realize it was at S at first!
I kind of like that "ancient" patina to it now--it doesn't look "too light", but probably still a goner, lol.

Thad: As to the useage of Mineral Spirits. Note that when I did use it, it was a paint thinner type sold at a place called Menards. Since Mineral Spirits can contain numerous other substances, and the coins I tried it on had a greenish discoloration, I am not sure exactly what that substance was. I've noted that around where I live the normal Green substances on anything containing Copper is usually due to the excessive SO and SO2 in the air. We do have a really horrible acid rain problem in the Illinois, Indiana areas. Without a spectroscopic analysis of anything containing a greenish substance in this area and the numerous possibilities of the variations in MS solutions, tabulating exactly reactions would be difficult to say the least. Keep up with your experiments. Eventually I'll print them all out and use them in a chem class.

I'm done with this coin. It's hopeless! I let it sit for 12 days and have concluded the current formula is not strong enough for this. I even changed the fluid out everyday, it didn't matter. It seems my suspicion was correct, the verdigris has eaten into the surface. As more and more verdigris was removed, some of the coins detail was "lost". the verdigris was actually filling in the craters. I noticed each time I changed the fluid out it had the distict bluish color characteristic of copper solutions. Along with the verdigirs removal came copper!

Some coins just cannot be saved without resorting to a much harsher cleaning process. I could certainly do that, but it's not really the intent of my fluid. I will no longer be testing on coins this bad, I'm going back to the lighter verdigris specimens.

I've just been too busy lately and I need to get back on track. I have some other ideas for the removal of light verdigris. Perhaps someday I'll work on a much harsher cleaner for these extreme cases.

Initial



After 1 day



After 12 days



Initial Close




Close up after 12 hours



For fun, I just put this coin into 0.02M nitric acid. Bubbles were observed over the surface which confirms the remaining verdigris to be cupric carbonate. I'm going to let it stand in the acid for a few hours (maybe longer) just to see what the surface looks like.

Vinegar is acidic and will damage a copper surface. The acidity is due to acetic acid which will indeed form verdigris in the form of copper acetate. Vinegar can be neutralized with any base, such as sodium hydroxide (aka lye).

I'm soaking the coin in nitric acid, a MUCH better choice than vinegar.

Back to work again, finally some time! I soaked the 1968S in some dilute nitric acid for a couple of days. It's confirmed this coin is a "goner". The corrosion was so bad even the acid couldn't remove all of it and it's clear to see the coins surface has been eaten away! This is a good example of what a harsh cleaning will do to a copper coin:

The second forumla 500409 also is looking pretty good. However, it's back to the white residue reaction product I was getting earlier.

Initial



After 24 hours



SUMMARY

I had such luck with the last formula, I'm not overly excited by this one. It did a decent job, but the white verdigris is fairly persistant. I need to get this onto my SEM-EDXRF to confirm my suspeciions that this residue is cupric carbonate. It's deinately more difficult to remove than the sulfate/acetate verdigris.

Wow Thad! The 1973 looks great! Thanks for keeping us posted!

That using a pick tool or acid will permanently damage the coin! My goal is to remove verdigris in a gentle, but somewhat rapid fashion WITHOUT damaging the coin at all. Also, I want to leave the brown, oxidized surface intact. My method is conservation not cleaning...... in similar fashion to what NCS might do. With my knowledge of chemistry and metalworking fluids, I think I can do it.