The science of fireworks

In recognition of the fact that tomorrow is the 4th of July, I'd like to spend today's post talking about fireworks. I love watching fireworks - the colors, the shapes, the sizes, and the different ways they twirl and burst and shimmer and sparkle are all entrancing to me. In thinking about fireworks, I wanted to investigate and see if what I think I know about fireworks is really true. In particular, I wanted to look into the science behind what makes fireworks different colors. I believe that I already know a little bit of the answer (as may you), but since it never hurts to have our knowledge expanded upon, here we go...

First, let's talk about the kind of firework that we are most familiar with in fireworks displays during the 4th of July - skyrockets. Skyrockets are projected into the air before they explode (unlike ground fireworks such as catherine wheels, which are like small, glowing ferris wheels that spin as they burn). These are built around a basic design - paper or pasteboard tubing filled with a combustible material, called pyrotechnic stars. Different tubes filled with different pyrotechnic stars can be combined in various ways to make the many shapes, sizes and colors that are seen when the firework explodes.

Pyrotechnic stars contain 5 basic components. First, there must be a combustible fuel to burn. Second, there is an oxidizer. This provides the oxygen required to start the burning process in the first place. (For a reminder about oxidizers, you can read my entry on flaming gummy bears.) Third, there is also something to hold the entire firework together, called the binder. And finally, there are chemicals which burn to provide the color, as well as another chemical to help strengthen the color of the flame produced.

So I was right - the color is provided by the burning of certain chemicals. Here's something I didn't know - a few of the chemicals that are used in producing fireworks displays:
Red: strontium or lithium salts
Orange: calcium chloride or calcium sulfate
Yellow: sodium salts such as sodium nitrate or cryolite
White: magnesium, aluminum or barium oxide
Green: Barium chloride
Blue: Copper chloride
Silver: Titanium or magnesium

Apparently, the most difficult color to achieve is blue. That's because copper is a tricky metal to burn. If it does not reach a high enough temperature, it will not emit enough light to be seen. However, if it gets too hot, it will fall apart before it produces any light at all. So consider yourself lucky if you wee a blue firework! Actually, all of the color-producing chemicals have to be handled carefully to achieve the right color. If there is a small amount of chemical impurity, thee metal will not burn properly. In particular, trace amounts of sodium burn so well that they easily overpower the intended color, producing yellow-orange instead.

I think my favorite kind of firework is called the willow firework. What's a willow firework? Well, your basic firework is called a peony. It makes a spherical burst of colored stars. Building off that, your next most common firework is the chrysanthemum, which is like a peony but with longer burning stars which leave a visible trail behind them. The willow firework is a variation on a chrysanthemum, but it has extremely long burning silver or gold stars. These burn so long that a long trail of sparks can be seen falling gracefully to the ground, just like a weeping willow tree.

Unfortunately, this year we probably will not watch a display of fireworks ourselves, but we may get to watch one on TV. Of course, it's not quite the same as in person, but it's better than nothing!