Saturday Science: Pretty Pennies
Metallurgy is a fascinating science. There are so many different ways to mix various metals together in different proportions, and even add in non-metals, and seemingly small changes can drastically affect the physical properties of the resulting mixture.
Any metal that’s intentionally a combination of metals and/or non-metals is called an alloy, and alloys are all around us. Basic steel? Iron + copper. Stainless steel? Iron + copper + chromium. Brass? Copper + zinc.
The alloy that we’ll be talking about today is bronze, a combination of copper and tin. Typically, modern bronze has about 12% tin to 78% copper, but it’s been in use for millennia, often with different amounts of each, or even other metals thrown in for good measure.
Many ancient cultures went through a Bronze Age, a period when Bronze was the hardest metal in wide use. Typically a Bronze Age is followed by an Iron Age, when metallurgists and smiths and miners put their heads together to create easy, cheap ways to make iron, which is better than bronze for things like plows and weapons. Bronze, however, doesn’t go away. It just gets moved into other uses.
Ancient Greek culture followed that Bronze Age-Iron Age path, and during their Iron Age they found a use for bronze in art. There are many famous bronze statues from Classical Greece that still exist today, like the Charioteer of Delphi. The Children’s Museum’s Ancient Greece temporary exhibit has a replica of a famous statue of a god, either Zeus or Poseidon (we’re not sure because he could have been holding either a lightning bolt or a trident, but the weapon is lost to time).
Now, click on that link and look at that picture of the Charioteer. See how he’s covered in a greenish-blackish gunk? That coating is one of the downsides of working with copper and copper alloys. You may recognize it if you’ve ever seen an old penny with similar coloration. Well, today we’re going to do a really simple experiment that Greeks, faced with a bronze covered in patina, might have loved. Too bad they didn’t have Heinz tomato ketchup.
- Several old pennies
- Ketchup (or anything with vinegar: hot sauce, mustard, salad dressing, or just plain old vinegar)
- A plate
- A sink
- A drying cloth of some sort
- Squirt some ketchup (or whatever) onto a plate.
- Plop a few pennies into your ketchup (or whatever).
- Add a little more ketchup (or whatever) so that all the pennies are covered.
- Let them sit for a minute or two.
- It is now mess time. Pick up your pennies, one at a time, and rub the ketchup (or whatever) into them like you’re scrubbing them clean (because you kind of are).
- Rinse off the ketchup (or whatever) and dry the pennies.
- Bask in their shiny copper glory.
So this is definitely a weird effect. Typically getting ketchup or hot sauce or mustard on something makes it dirtier, not cleaner. So why does it make the pennies look perfectly new? It all has to do with the common ingredient between all of your ketchup-or-whatevers: vinegar. Which, incidentally, is why you can do this experiment with just plain vinegar.
Vinegar is made of 2 things: water (mostly) and acetic acid (6ish percent). That’s right: you’ve been eating acid your whole life and didn’t even know it! But it’s a safe acid, like citric acid (in citrus fruits) or ascorbic acid (vitamin C). You probably do know that acids are good at dissolving things, and what they’re good at dissolving depends on what kind of acid you’re using. As it happens, acetic acid is good at dissolving the crud that builds up on pennies over time.
That crud is called copper oxide, and it’s formed by the same process that forms rust on iron (which is also called iron oxide). When oxygen from the air combines with atoms of metal, an oxide is formed, a new chemical that’s different from both the metal and the oxygen. Copper oxide comes in 2 forms: Cu2O (two copper atoms and one oxygen atom) forms first and CuO (one atom of each) forms later. A penny with black gunk on it has some CuO on it (and probably other dirt, too). A penny that’s been lying around for an extra-long time will sometimes be green. That green coating is called the patina, and it happens when the CuO starts reacting with even more things in the atmosphere, like carbon dioxide and sulfur dioxide.
When either form of copper oxide comes into contact with vinegar, the copper atoms disconnect from the oxygen and connect with a part of the acetic acid called the acetyl group. This forms a new chemical called copper acetate. Copper acetate dissolves in water, so when you rinsed the ketchup (or whatever) off the pennies, it washed down the drain, too, leaving a nice shiny penny.
But it’s not good as new…Because think about it: copper from the penny forms copper oxide, then turns into copper acetate and washes down the drain. That means that when you did this, you removed a tiny, thin layer of copper from the penny. It sure is shiny, but it’s ever so much thinner and lighter now that you took away some of its copper atoms. Do that enough and Honest Abe will completely disappear!
So come to think of it, maybe the Greeks wouldn’t have liked the ketchup method of cleaning their bronzes. With enough cleanings all of the fine details in their exquisite sculptures would have been worn away. It’s weird to think about it, but it’s true: the oxide and patina on bronzes actually protects them over time. It’s why the Statue of Liberty is still green, even though it’s made of copper and so was originally, well, copper colored. A team of engineers studied the patina and realized that it was a protective layer around the copper. So don’t take your ketchup to the Statue of Liberty and definitely don’t go time-traveling around Ancient Greece throwing vinegar everywhere!