Why do Copper Coins Change Colors?

If you’re a coin collector or just happen to handle change a lot, you’ve probably seen old copper coins in various colors. From white to green to blue! Why do these coins take on so many different colors? One may think it’s because of something that got stuck to it, maybe a candy wrapper or some sticky food but these colors are actually naturally occurring!

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pile of various cull wheat cents

There are two common forms of attack upon these older metals. In the milder case, a metal may tarnish. “Tarnish” is a thin coating on the surface of a metal and is usually very uniform and does not often destroy the intended purpose of the metal. “Corrosion,” on the other hand, is often not uniform, but may cause pits and may reach such proportions as to destroy the metallic object so that it cannot be used for its intended purpose.

In dry air, even tarnishing takes place quite slowly; however, with the usual atmosphere around us, the humidity accelerates the tarnishing process. The lowest oxide level of copper is cuprous oxide, or cuprite. Its color is pink. Barely noticeable at first, a penny becomes darker over time due to the tarnish layer thickening, as well as the continued oxidation to the black cupric oxide, tenorite.

Over time, and upon repeated or prolonged exposure to moisture in the presence of dissolved acidic substances, such as carbon dioxide and the polluting substances found in acid rain, tarnished copper turns green. Among these acid substances are the oxides of sulfur and the oxides of nitrogen. Reacting with moisture, they form dilute solutions of strong acids.

Copper that is exposed to open air will corrode and undergo a series of chemical reactions that lead to the development of a patina – a coating of copper oxide molecules which actually protects the metal beneath. Over time, copper transitions from its shiny brown color to a darker brown shade.

After many years it transitions into blues. At an even later stage the formation of copper sulfate, carbonate and chloride salts in varying concentrations turns the surface green. There are several factors which affect the amount of time these processes take including moisture, temperature, and the level of pollution. The formation of the natural green patina seen on copper roofs and statues takes a very long time, but methods have been developed to speed the process up using chemical reactions.

Statue_of_Liberty_7Coins aren’t the only place we often see this chemical reaction take place; we’ve all seen the greenish blue Statue of Liberty, but did you know Lady Liberty was once a copper color? That’s right, the famous statue was once covered in a thin layer of copper and was bronze when she first arrived in the United States from France. Acid in rain covers the Statue of Liberty whenever storms hit New York, and her exposure to oxygen from being in the middle of the ocean gradually turned her blue over the years.

The Rare Mauritius “Post Office” Stamp

Among the rarest stamps in the world, the Mauritius “Post Office” stamps have some of the most rumor surrounding them.

Mauritius, a small island in the Indian Ocean, has made a name for itself in the world of philately. And this name started with the “Post Office” stamp of 1847.

The orange-red one penny stamp.

The orange-red one penny stamp.

It all starts with a stowaway on a ship. At age 22, a man named Joseph Osmond Barnard left his home in England and stowed away on the ship Acasta to Mauritius.

Barnard had luck on his side – he was allowed to disembark and live in the capital. As it so happened, Barnard specialized as an engraver and painter.

Barnard designed the stamps for Mauritius based on the Great Britain stamps at the time that showed the profile of Queen Victoria. They printed the stamps in two colors of one penny red brown and two pence blue. They are characterized by their primitive design.

Postage stamps were still very new at the time.

The printer made five hundred stamps of each value, printed in 1847. The wife of the Governor of Mauritius used many of them on invitations for a ball.

A set of two cancelled Post Office stamps, with the penny red brown and the two pence blue.

A set of two cancelled Post Office stamps, with the orange one penny and the two pence blue.

These stamps had the words “Post Office” printed on the left side. On the next printing, however, “Post Paid” replaced the phrase, making the stamps with “Post Office” rarities.

One particular legend says that using “Post Office” on the stamps had originally been a mistake. The book Les Timbres-Poste de L’Ile Maurice claims the mistake, and rumors surrounding the tale expanded to say that Barnard was a half-blind watchmaker and a forgetful old man who forgot what he was supposed to print on the stamps.

The two pence blue stamp.

The two pence blue stamp.

Anyone who pays attention knows that this can’t be true, since Barnard designed the stamps at 31 years old.

In 1864 the wife of a Bordeaux merchant found some of the stamps in her husband’s collection. She traded them with another collector, starting the ball rolling on the fame of these increasingly sought-after stamps.

In 1904 King George V paid roughly today’s equivalent of $190,000 for an unused two pence Mauritius stamp. And in 1993 a cover with two of the stamps sold for about $4 million, the highest price ever paid for a philatelic item.

 

Sources:

Engraved memory

Wikipedia