Why Do Copper Roofs on Old Buildings Have a Bluish Green Patina?

Those copper roofs on old churches and city halls have been out in the weather longer than a borrowed lawn mower, all those years that have passed since people could afford to cover roofs with that durable and beautiful red metal.

Today copper is too expensive to use to shelter even the heads of politicians and priests. It is even too expensive to make pennies out of; a penny’s weight of copper is now worth more than one cent. Ever since 1982 pennies have been made of zinc, with just a thin coating of copper for old times’ sake. But if you really want to, you can still leave a penny out in the weather for fifty years or so, and it will turn roof-green.

There’s no fast and easy way to do it.

That’s the reason, in fact, that copper is such a good material for covering roofs: It corrodes very slowly, much more slowly than iron rusts. Within a few weeks, bright, shiny copper will darken because of a thin layer of black copper oxide.

Then, as the years go by, it reacts slowly with oxygen, water vapor, and carbon dioxide in the air to form the bluish-green patina that chemists identify as basic copper carbonate. In addition to roofs, this patina colors the Statue of Liberty, which is made of three hundred thick copper plates bolted together, and which has been exposed to New York City air since 1886.

Incidentally, the green color that you see on pennies in the bottoms of fountains, tossed in by people who believe that one cent will bribe the Fates into granting a wish, and that are somehow overlooked by the midnight scavengers, is not the same, chemically, as the green color you observe on the roofs. It is due to other compounds of copper such as copper chloride and copper hydroxide that don’t have the same blue-green color and that don’t adhere very well to the metal.

You can try to duplicate the patina of copper by buying some cheap jewelry made of brass, which is an alloy of copper and zinc. Wear an unlacquered brass ring or bracelet for a few months and the copper will react with the salt and acids in your skin to produce copper chloride and other compounds. Your skin will turn as green as Miss Liberty’s. But it still won’t be exactly the same shade as hers.

Many outdoor statues in pubic places are made of bronze, which is an alloy of mostly copper and tin. When the statues weather, they develop a dark-green patina similar to copper’s. (The white splotches on the statues have quite a different origin.)

An interesting sidelight on copper is that instead of the red; hemoglobin in human blood, which has an iron atom in its molecule, lobsters and other large crustaceans have blue blood containing hemocyanin, which is similar to hemoglobin but contains a copper atom in place of the iron. There may be some truth, after all, in the claim of revolutionaries that the world’s blue-bloods are among the lowest forms of life.

What about those copper bracelets that are supposed to cure arthritis?

Nonsense. The thinking (a generous term for it) behind these voodoo baubles appears to be (1) that copper is a good conductor of electricity (which it is), (2) that there is “electrical energy in the air” (whatever that means), and (3) that a copper bracelet will therefore attract that “energy” and conduct it to your aching bones. And, of course, we all know that “energy” is good for us.

The only energy that the bracelet will generate, however, is the energy that you will have to expend in scrubbing the green stain off your wrist. (Try vinegar.)

Take a file to a penny and you’ll find out that the copper is only skin deep. Underneath, you’ll see the silvery color of zinc.

Oddly enough, the penny is the only American coin that is not made from an alloy of copper. Nickels, dimes, quarters, and half dollars are all made from copper alloys, usually with nickel. Even nickels are only 25 percent nickel; the rest is copper.

There is only one American coin that is not currently being made of a copper alloy. (Side bet: It’s the penny.)