Why does copper turn brown, not green?

Barry

I asked this in the middle of another thread. Either it got buried, or nobody knows, so thought I'd try again.

Question for any chemists out there:
Why do copper coins turn brown when they oxidize (oops, I mean tone), and not green like a copper roof?
As I recall from college chemistry, a long time ago, copper is multi-valent (is that the right word?) and can combine in the cuprous or cupric form. I presume it has something to do with that. Can anyone explain in some detail?

MadMarty

Excuse me?

Cam40

hmm... lovely emerald green periphery toning.

northcoin

Hey that's no fair. Baking the coins in a potatoe to achieve toning is a natural process, but dropping it into the blue cheese dip qualifies as AT.

ColonialCoinUnion

I think if you leave a penny on the roof of your house for 13 years it will turn light green. Yes, I'm sure of it.

KlectorKid

Well rain and heat might have something to do with it.

LincolnCentMan

acid rain

David

melikecoins

This is brown and green


Glen

Conder101

Just off-hand I can see two possibilities. One the toning on copper coins tends to usually be Copper Sulfide which is brown or black while the copper roof probably has other copper compounds such as Copper Chloride which has a strong green color. Most copper compounds are greenish. Second could be that it is dependant on the thickness of the compound layer (Much as the thickness of the sulfide layer on silver coins determines the color that you see.). On the coins the layer is usually very thin while on the roof it will tend to be much thicker.

Shamika

I believe this is a case where it depends on what is actually reacting with the copper.

Copper oxide (cuprite or Cu2O) and copper sulfide (chalcocite or Cu2S) ranges in colors from brown, gray, and black.

Copper sulfate (antlerite or Cu3SO4[OH]4) is emerald to blackish green.

Copper carbonates (malachite Cu2CO3[OH]2 and azurite Cu3[CO3]2[OH]2) range from bright green to intense blue.

Another form of copper sulfide (covellite of CuS) is known to be indigo to dark purple.

JohnZ

Mad Marty: that's a serious case of PVC. Wait. You knew that already. Never mind.

Libertad

The lovely green-blue patina from weathered copper is mostly due to the formation of copper acetates.
Acetic acid is a pretty common organic compound, formed by lots of living creatures as they go through the business of converting food to wastes.

It's commonly known as verdigris.

It comes in three different compounds, developed by contact with the acid and depending on various conditions, including contact with organic matter (like dust, bacteria, algae, other one-celled and multicellular small organisms.
1. blue verdigris, Cu(CH3COO); green verdigris,
2. 2Cu(CH3COO);
or neutral verdigris,
3. Cu(CH3COO)2;
or a mixture of them.

There is a basic form of verdigris, somewhat transparent, and relatively unstable.

Verdigris on coins eats the coin surface as it oxidizes, usually locally and often appears to be started from spit droplets. Removal of long-term verdigris reveals pitting of the surface from the oxidation reaction.

I think the reason that copper left out in the elements to weather turns to the green patina is due to the presence of organic material and it's acetic acid content, accented by weather (rain/heat).

If you wear the copper and don't put in back in your pocket for 'cleaning', the green color will appear on your skin as well.

Skipper53

So, in layman's terms, would that mean green toning on copper comes from organic compounds
and brown toning comes from inorganic compounds ?