Green surface - cooper

Is the green surface on cooper coins considered as something bad? For example on modern (19th century) Austrian kreuzers.

When asking such questions photos are needed. It could be bad, it could also be patina.

Corrosion of copper (pipes or coins) is generally green in color. Corrosion is primarily copper oxides and can be a very thin layer (patina) or a thick mass on top.  Which are you referring to?

rsirian1

Corrosion of copper (pipes or coins) is generally green in color. Corrosion is primarily copper oxides and can be a very thin layer (patina) or a thick mass on top.  Which are you referring to?

Something like this.

Copper corrosion can be oxides red brown to black or hydroxy species with many other compounds also organics mixed in ranging from deep blue to light green.

On ancient coins stable and hard green copper patina safed the coins from being dissolved entirely. On modern coins you usually have bronze desease that perpetually eats away on the surface because of the brittle nature.

CzechLord

rsirian1

Corrosion of copper (pipes or coins) is generally green in color. Corrosion is primarily copper oxides and can be a very thin layer (patina) or a thick mass on top.  Which are you referring to?

Something like this.

Overall, “the market” prefers coins without the green patina.  All other things being equal, a collector would pay more for an all brown copper coin than a green copper coin.  So, if you are planning to re-sell, I do not advise buying a modern coin with a green patina.

BUT … 

If you are just assembling your own collection, buy what you like.  As an example, I added this coin with a nice even green patina to my collection because I liked the look: 

My motivation for conserving these coins is twofold.  One, it is a learning process that may come in handy if I ran across a more expensive coin that would benefit from conservation.  Two, it may approve the appearance of the coin while arresting the corrosion.