Some Questions About Verdigris

Except for the 1919 One Cent, none of the coins pictured are likely to have corrosion pits in them.

The nickel alloy in the other coins has somewhat protected them.

Nevertheless, the green stuff is indeed copper hydroxycarbonate, otherwise known as verdigris.

On Roman bronze and orichalcum (brass) coins, a thin, but rich green patina (which is a thin film of verdigris) can enhance eye appeal, if the coin is in otherwise VF or better condition, and as a result, can actually command a higher price at auction. The very obvious proviso is that there are no corrosion pits.

Red flags arise when the green is in spots, and is deep and powdery, because it is almost certain that there is corrosion pits beneath them.

Museums are obliged to try to save and conserve valuable copper, bronze and brass exhibits for display, and have developed a number of processes to passivate the verdigris, and conserve them to prevent further damage.
Some of those processes are also applicable to coin conservation. Chemistry is used in the passivation process, but coverings such as Renwax is somes used for conservation. Renwax is also used for furniture conservation

Verdigris is the pale green, powdery form of corrosion that is also known as "bronze disease". It typically only occurs on copper, bronze and brass coins; cupronickel coins don't have enough copper in then to qualify.

My observations on what's happening with these coins:

1. possibly PVC damage, possibly a corrosion spot.

2. Verdigris. Yep, this is the one. Make it wear a mask, put it in quarantine and send contact tracers to find its close contacts, because this one's got The Disease.

3. Looks like a blob of sticky goo has got stuck there; can't tell if it's solid corrosion byproducts or just goo. Acetone would tell.

4. It's an ancient, so it would have come up out of the ground looking like a little green rock. I can't tell if the green patch is simply a small untreated part of the surface or if it's subsequent corrosion. The colour is a match for verdigris, so I'd carefully monitor it.

5. Environment damage, by the looks of it. These things are copper-plated, and copper-plated things don't get verdigris and survive to tell about it. Conditions harsh enough to give a solid copper coin verdigris will likely make a plated coin corrode into rubble.

6. This Greek brass one looks like classic PVC damage, from sitting in a cheap'n'nasty coin album.

7. Aiee. Cupronickel ain't supposed to do that, captain. Some serious environmental damage here. It's the "right colour" for verdigris, but the nickel prevents it from spreading or turning more serious like actual verdigris would be doing at that stage.

8. This here's a typical "metal detecting find" environmental damage. It might devolve into verdigris, but it hasn't yet.

9. Not sure what's caused this, probably severe environmental damage, maybe even a bad case of "1960s RCM-packaged coins being sent to a humid environment". Judging by the exposed pits of raw metal (like above the "II"), it looks like someone's already had a go at trying to clean it; without before-and-after pics, it's hard to say if it's an improvement.

Thanks everyone for the help and for the welcome.



I'll look into Renwax and see where I can get it. Does it have any side-effects on coins?



Covid jokes aside, can verdigris spread to other coins if I don't "quarantine" it?



It's definitely something solid and hard, although I agree that it doesn't look like it. It was hard enough to tear the cotton off of my q-tip when I tried to get it off with it (I know, I know, don't clean coins, but this is only worth 5 cents anyway), and I was able to chip off flakes of it when I took a screwdriver to it. I'll try using acetone anyway just to see what, if anything, will happen.

Okay, thanks again!

Interesting. Thanks for the info!