Copper content threshold for BD?

Showmetheway

Greetings folks,

Went digging thru a world lot sale of coins and stumbled upon some alarming, and confusing, cases of bronze disease. As a disclaimer, I understand that BD has that powdery light substance that regrows back as opposed to the hardened and darker verdigris. And man, the fuzziness of the Jefferson nickels and some Mexican/Italian bronzital coins in the bin scared me away to not buy anything.

…How much copper does a coin need to have in order to develop the ugly disease? From ancients/medievals, I’ve seen billon types that were asserted as less than 50% silver (>50% copper/bronze) yet never developed this issue. Modern coins, w/ better metallurgy and heavier usage of tin, nickel, aluminum, etc. should be more resistant. 🤔

John Conduitt

Any copper at all can corrode. If there's less, there's less chance of the chain reaction continuing, and whatever the copper is alloyed with can make it less likely to react, but it can. Obviously, most copper coins won't develop bronze disease, let alone billon, but they will in particular conditions (high humidity with chlorides), especially if they don't have a patina, or they have a copper core. A nickel is 75% copper, so no different to a billon coin.

On the other hand, it might not be bronze disease. It might be that the coins were in a bag with copper coins that corroded, especially if they all got wet.

Showmetheway

@JohnConduitt said:
Any copper at all can corrode. If there's less, there's less chance of the chain reaction continuing, and whatever the copper is alloyed with can make it less likely to react, but it can.

Well, that’s worrisome to know! 😬 Even if the chain reaction occurs in more non-reactive alloys, I’d assume it wouldn’t result in total disintegration of the coin, or even easier to resolve? Had a war nickel that was turning pale and fuzzy, dipped in acetone and washed away w/ distilled water. No more BD since, though hopefully the BD particles aren’t floating around to land on another coin…

Sapyx

I've seen a sterling silver shilling turn green, when in contact with bronze-diseased coins. That's .925 fine silver, or only 7.5% copper.

I've seen a nickel-plated-steel New Zealand 20 cents dated 2006 do the same thing; it was growing some funky green crystals out of it's side and rim. No copper at all, just pure nickel, and it still caught it.

So you don't really need much copper or nickel present, for contagious corrosion to spread. What's critical for development are the conditions - warm and damp. The two coins I mentioned above were sitting in a bag of coins for several years in a hot, damp basement carpark here in Brisbane, Australia. Yeah, I call that bag the "bag of tears". There were lots of formerly nice coins in that bag; about a third of the coins had caught it, including most of the coppers and bronzes. Coin dealers around here must get used to such pitiful sights.

Showmetheway

Oh dear, my condolences to those poor “bag o’ tears” coins 😔. Thankfully it’s not too salty or moist in the mid-Atlantic states here in the U.S., so not too much worry w/ silver or nickel being affected. Airtites and little silica packs have been lifesavers for what few bronzes I still keep.