On a Publication Finally About Explosives

Tesla is a pizza tosser

I made this pizza, start to finish. Multi-talented. That’s me.

Learning to professionally toss pizzas is difficult.  The variables to good pizza tossing are not just limited to the professional skill of the tosser; the variables quickly become a function of the way the dough was prepared.  How long did the dough proof?  How much yeast was used?  What was the protein (gluten) content of the flour the dough was made from?  Is the tosser right- or left-handed?  If the dough is over-proofed, it tends to be tacky and full of big bubbles – these lead to rips and tears and weak spots in the final pizza.

Explosives aren’t the perfect slice of pizza, but the idea stands.  Which oxidizer was used?  How sturdy is the reaction vessel?  Where the oxidizer and reducer added at the proper ratio?  Of course, if you mess up a pizza you can simply start over.  You only mess up an explosive once, or so I’m told.  Being America Day, I feel that this is an excellent time to discuss something very dear to my heart: fireworks.

The colors in fireworks are largely produced with very specific metal halides.

Fireworks were originally invented by the Americans in 1776.  Anyone who tells you differently is wrong, misinformed, or lying.  I do not recommend this source to anyone, as it contains a different story from the one I’m choosing to tell.  Regardless of who invented fireworks, the ingredients have been the same since the beginning: sturdy paper, black powder, and colorants.

Just what is black powder anyway?

Black powder (or gunpowder) is a mixture of two fuels (charcoal and sulphur) and an oxidizer (saltpeter).  Sulphur is a yellow solid that can be found pretty readily across the world; it’s also known as brimstone and it has a pretty terrible smell.  Charcoal is produced by taking carbonaceous organic material (read: wood) and subjecting it to extreme temperatures to drive off non-carbonaceous materials (read: burning it).  Saltpeter was often found in the water of mines – when the water was boiled off, a white residue was left behind and able to be mixed with the charcoal and sulphur.

The trick – as with any good recipe – is understanding the ratios you mix these ingredients in.  This ratio isn’t just a weight basis of the materials, but it includes a mean particle size and a distribution ratio of fuel to oxidizer (read: you have to get it perfect for it to be good black powder).  Many people on the internet can tell you how to produce gunpowder in your backyard – I don’t recommend it for the obvious safety concerns.  And it’s far easier to buy chinese firecrackers.

How does the firework get into the air?

basic diagram

A basic diagram of an aerial shell.

The type of firework that the general public loves the most are the aerial shells.  These are simple devices which contain three major sections: the propellant, the break, and the show.  The propellant is usually a mixture of finely ground black powder and usually sand or clay.  The sand and the clay are used to slow the reaction rate to produce a steady thrust rather than a violent explosion.  Once the shell has reached the design height, a secondary fuse (lit by the propellant) is ignited and this, in turn, ignites the break.  The break contains a much, much more finely ground mixture of black powder and perchlorates.  This increases the overall reaction rate and, thanks to the stiff cardboard shell, causes a detonation (the awesome boom).

Talk about the show!  What makes fireworks so pretty?

energy diagram

The energy diagram for the excitation of a valence electron, in general.

As it turns out, fireworks aren’t so different from a high school chemistry demonstration: The Flame Test Lab.  This was always one of my favorite labs to show students because it demonstrated that I could take an unknown solution and potentially determine which solute was in the solution and – with the help of an Atomic Emission Spectrograph – the concentration of the solution.

The colors in fireworks are caused by the excitation of an electron in the valence (outermost) shell.  The energy from the heat, which is generated by the combustion of gunpowder, is enough to bring the valence electron to a higher, excited energy state (ε1) from the ground state (ε0).  Because the excited state is unstable, the electron “falls” back to the ground state and releases energy in the form of a photon of light.  When you’ve got lots of atoms undergoing this same process, you get higher and higher levels of intensity of color and light produced.

The flame test lab teaches us that these valence electrons have very specific, defined energy differences (∆ε) which produce very specific wavelengths of light every time.  For example, if you excite the valence electron of a sodium atom, you will expect the light to have a wavelength of 589 nanometers (yellow).

The hardest color to produce in fireworks is blue.  One of the few compounds that produces a photon in the 475-500 nm range is copper chloride which doesn’t usually survive the high temperatures of fireworks.  According to ACS, magnesium-aluminum alloys have begun to be used because they produce a much better blue, but not always because the incandescence (glowing from heat) of the particles can overwhelm the color.

Halloween 018

Methanol with a sprinkle of boric acid produces a vibrant green flame.

My favorites are greens.  Coppers produce different greens than borons, but the industry standard is barium greens.  They’re said to be deeper and more intense than either copper or boron.

The sparkle shower has long been one of my favorite effects.  This is created by adding iron and magnesium dust to the stars – when the iron ignites, it crackles and sparks producing a beautiful shower.

Conclusion

Fireworks are awesome and require a ton of chemistry to understand and produce.  I really hope that this taste of firework chemistry interests you.  This is of course a very basic test, but if you’ve got questions, post a comment and I’ll give a more in depth answer.

This year, when you’re looking up at the sky in awe of the show put on the by the pyrotechnicians, see if you can determine which compounds were used to produce the colors you love.  If you need a little help, here’s a great cheat sheet.

-N.Tesla

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