It's the 4th of July, and here in the U.S., that usually means fireworks.*
What could be better than explosions in pretty colors? Maybe a few details of how firework makers get those colors into the fireworks.
If you've taken a chemistry course with a lab, you may remember having done "flame tests" of compounds. You dipped you little wire loop in the compound you were characterizing, stuck it in the Bunsen burner flame, and watched what color the flame turned. (You had to pay attention; if you missed the color your compound burned, you were left looking at the plain old Bunsen burner flame.) Depending on the compounds you had to burn, you'd see nice flashes of green (barium containing compounds) or blue (copper containing compounds) or red (calcium containing compounds) or yellow (sodium containing compounds). You might guess, then, that the different colors in fireworks come from different compounds whose elemental components include particular metals (or what the USGS calls minerals**). Here's how a nice Chemical & Engineering News article on fireworks explains the colors:
The orangish hues of ancient fireworks are largely produced by black- or gray-body radiation--the glow of very hot solid particles. By contrast, the striking greens and reds in modern fireworks are the spectral emissions of excited gas-phase molecules. A few metal chlorides, which fluoresce strongly in the visible wavelengths, are the basis for almost all the colors in modern fireworks.
The challenge, of course, is getting your compounds containing metal components that will burn in pretty colors up into the air and igniting them. You don't have a team stationed up in the sky with Bunsen burners. So a crucial challenge in making fireworks is working out how to deliver skyward the compounds you want to burn and the appropriate conditions for their mid-air combustion. That's not the only challenge, either. As noted in the C & E News article:
Barium chloride produces green; strontium chloride produces red; copper chloride produces blue. The problem is, these compounds are so hygroscopic [i.e., they draw water out of the atmosphere] that they render any mixture damp, unburnable, and even unstable. The solution to this problem has been to bring metal and chlorine together in a vapor during the burning process, where the energy from the burning can then excite the molecules' electrons, producing the colorful emissions.
So, you need the compound that burns in pretty colors, you need a way to keep it dry and prevent it from reacting with something else before you're ready to burn it, and you need a way to propel it into the sky and set up a sequence of conditions where the burning of the pretty compound will happen in the middle of the sky.
As described by Nicholas P. Mueller:
a firework needs a pyrotechnic mixture that will generate the above molecules [that will generate the desired color], evaporate them into the fire and then put them at a consistent high temperature as quickly as possible. To achieve good coloration, a substantial amount of the emitting molecules is also necessary. In short, the emitters are not alone; a fuel-oxidizer mixture is also necessary for a good firework.
The heat of the burning fuel will affect how quickly the molecules that make the colors burn up -- and if they burn up too fast, the flashes of color are too fleeting for good fireworks.
Firework makers have choices about the compounds they use as their sources of potassium or barium, and lately they've been leaning towards the more stable (less reactive) ones. Although they are harder to ignite, they also result in fewer accidental (and sometimes deadly) ignitions.
In recent years, I've been noticing a higher proportion of purple fireworks in the local displays, and generally the colors have seemed more intense. It turns out, the increasing intensity is not a figment of my imagination, but the result of serious effort to reduce "washout". Mueller writes:
For a deep coloration, only the selected color's molecules must be present, and nothing else. To generate the emitting molecules at an adequate temperature, a fuel-oxidizer mixture is used. Most fireworks you can buy, as well as most homegrown fireworks, use organic fuels such as pitch and a variety of resins. These compositions cannot generate temperatures as high as those available from metallic fuels such as aluminum and magnesium. These metallic fuels are commonly used in "industrial-strength" fireworks, such as those used at shows and other large events sponsored by municipalities and corporations. However, molecular emitters such as CuCl are quickly burned up in these intense flames. As a result, one never sees blue stars that are as bright as red, yellow, orange and green stars. Another problem with metallic fuels is their generation metal oxides, which can easily washout colors.
Pure coloration in a firework requires pure ingredients as well. Elements which washout colors must be kept out of the firework, either by carefully selected components, or by using a little chemistry. For instance, red fireworks dependent upon strontium chloride or hydroxide to generate their color use as excess of fuel to soak up oxygen and prevent the formation of strontium oxide, which washes out color.
So you can have your bright reds, and greens, and yellows, and even purples. But what about blue? Remember that copper chloride (CuCl) doesn't survive the hot flames. But, the C & E News article notes:
a big advance in fireworks colors has come in recent decades, with the use of a magnesium-aluminum alloy known as magnalium. Stars made with magnalium burn electric, almost fluorescent, green, red, yellow, and comparatively decent blue and purple. By themselves, magnesium and aluminum make silvers and sparkles and act as a fuel. The high heat generated by metal fuels can also increase the intensity of the colored molecular emissions. But the incandescence from the metal particles is usually so brilliant that it overwhelms the color.
Magnalium, however, still gets the flame hot without washing the color out. How this happens isn't exactly known, [John A.] Conkling [technical director of the American Pyrotechnics Association] says. But one possibility is that the metal somehow forms vaporous species in the gas phase, and so doesn't incandesce.
So blue fireworks are getting more intense ... but the pyrotechnics experts are hoping it will be possible to make them better still.
*Here in California, where conditions have been very dry and are shaping up to be very hot today, that also means that fire departments will have their hands full with yahoos setting off fireworks, whether illegal or legal ones, in places where they could spark fires. Will this be the year people will be guided by common sense in their enjoyment of pyrotechnics? I sure hope so.
**Metals in the form of aluminum filings also produce the element of professional fireworks that I could live without: the loud bangs.