You’ve Got Some Science On You: Independence Day edition [Updated]

Fourth of July fireworks Independence Day fireworks

Happy 4th! (By me, under the name pappatom/Flickr)

I like Independence Day as much as the next guy. And believe it or not, there’s a lot of science that goes into the things that make the the Fourth the Fourth. Let’s take a look at the big three: fireworks, grilling, and the flag.

The fireworks

*flash* BOOM “Ooooo! Ahhhh!”

Everyone loves fireworks…which means everyone loves the chemistry of metals and heat! Because that’s how fireworks get their colors.

Fireworks rely on metals, or rather, metal salts — compounds that result from the binding of a positively charged metal ion and a negatively charged ion of some other element, like carbon or chlorine. The colors come from the fact that when heated, metal salts glow at a characteristic wavelength of light.

The metal atoms in the salts absorb the heat energy from the exploding gunpowder, but can only absorb so much (electrons prefer to stay in a low-energy state). The atoms then release some of that excess energy as light — very bright, crowd-pleasing light. The amount of energy an atom releases dictates the wavelength — the color — of the light we see.

So what metals do fireworks makers use to get all these pretty colors? Let’s start with the patriotic ones (I’ve linked out to more info on each of the metals themselves)

And then move on to the others:

  • Orange: calcium
  • Yellow: sodium
  • Green: barium
  • Purple: mixture of strontium and copper (red + blue, just like in your watercolor set as a child)

If you really want to dive into the chemistry of fireworks, check out this page at the University of Wisconsin.

Let’s also take a moment to give the BOOM its due. We all know we see the explosion of fireworks before we hear it because of the light travels at the speed of light and the boom at the speed of sound. But let me give you a sense of the sheer magnitude of that difference in speed.

The speed of sound at sea level (because the speed of sound changes as the air thins with altitude) is 741.5 miles per hour (331.5 meters per second for you metric fans). The speed of light, however, is roughly 904,383 times faster:  6.706×10^8 (670,600,000) miles per hour (again for the metric fans, 2.998×10^8 meters per second). That’s 186,282 miles per second.

The flag

American flag backlit waving

(Jnn13/Wikimedia Commons)

While we’re on the topic of colors, let’s talk for a moment about Old Glory. Back in Betsy Ross’ day, colored dyes like the red and blue were hard to come by, because they had to be extracted from natural sources.

Where would colonial weavers have turned to get the dyes needed to color the flag?

  • Red: It’s thought that the red dye came from the cochineal, a cactus-dwelling South American insect that, when dried and crushed, made a brilliant red dye called carmine dye. It’s said that Spanish conquistadors marveled at the brilliant red fabrics they saw among the Aztecs, and learned that Montezuma demanded at that part of his annual tribute be paid in cochineal bugs. (The folks who run Colonial Williamsburg have an excellent history on colonial cochineal.) Another possible source for red was madder root. Harvested from plants in the genus Rubia, madder root was put through a long and complicated process — involving, among other ingredients, soda (sodium carbonate), alum, lead acetate, olive oil, and sheep’s dung —  to produce another red dye called Turkey red.  
  • White: Undyed white cotton.
  • Blue: Most likely from indigo dye, extracted from plants from the genus Indigofera. Native to tropical climates, especially in Asia, the indigo used by colonial weavers used was often grown in South Carolina.

The grill

It’s a sizzle like no other, the sound of a good burger on the grill. But what is it that the heat in your Weber is doing to the meat?

There are a few things going on. First, at about 130° F, the heat causes proteins within the muscle fibers (because, as we all know, meat is muscle) to denature: to lose their normal shape and conformation, unravel, and glom together. (Glom: that’s a technical word.) Second, at about 160° F, it melts the connective tissue that holds the muscle fibers together.

Third, you’re getting a Maillard reaction. As Nathan Myhrvold points out on the Modernist Cuisine blog:

The Maillard reaction occurs in cooking of almost all kinds of foods, although the simple sugars and amino acids present produce distinctly different aromas. This is why baking bread doesn’t smell like roasting meat or frying fish, even though all these foods depend on Maillard reactions for flavor.

The Maillard reaction is a chemical reaction between the amino acids in the natured proteins and sugars present in the meat. It’s responsible for the super-tasty seared char on your burgers, dogs and chicken; that char is full of compounds that really are what give meat its flavor. The reaction requires really high heat — 300 to 500° F — which is why you see its results in the places where your food has touched your grill’s cooking grate.

And the Maillard reaction doesn’t just make meat tasty. It’s essential to the flavors in lots of different foods that are roasted, toasted, or grilled, and even some that aren’t.

Happy Fourth of July, everyone!

PS: Incidentally, Myhrvold and his crew behind the Modernist Cuisine cookbook advocate for lining a grill with tin foil or putting a tin foil insert into a Weber to get more even heat. They also did a lot of math to help find the grill’s sweet spot — the place in the grill where the heat is even on all sides. Check it out!

UPDATE: I knew I forgot to include something in here. While the char on a burger or chicken tastes great, go easy on it. Two of the families of chemicals found in charred meats, heterocyclic amines and polycyclic aromatic hydrocarbons, may increase cancer risk; high levels of both cause cancer in animal models, but no one knows for sure whether they do the same in people. This fact sheet from the National Cancer Institute has the details. Just remember: everything in moderation!


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