Fireworks - Chemistry & Physics

It's the fourth of July and everyone loves a good fireworks show.  Lots of chemistry and physics (and even some geology) behind these awe-inspiring shows of hot summer evenings.

To launch of fireworks shell, you first have to light the fuse.  Fuses for fireworks are generally formed from cords with a black powder core.  They burn at a controlled rate and are used to ignite the shell's slow-burning black powder propellant - a combination of 75% saltpeter (potassium nitrate or KNO₃), 15% charcoal (basically C), and 10% sulfur (S).

Black powder has been around a long time and was invented back in 7^th century China.  It's a "low" explosive that burns at slower subsonic speeds rather than the fast detonation of  "high" explosives making it ideal as a propellant.  The charcoal in black powder acts as a fuel, the sulfur increases the rate of combustion by lowering the temperature of ignition of the mixture, and the saltpeter breaks down to provide the oxygen necessary for combustion (4KNO₃ → 2K₂O + 2N₂ +  5O₂).

Professional fireworks are launched out of mortars

unlike bottle rockets which use sticks as shown above

As the shell ascends, a time-delay fuse is still burning toward the upper compartment of the shell where the "stars" are located.  The stars are 3-4 mm clay-like masses which contain the chemicals necessary for the pyrotechnics show when the shell explodes from its charge of high explosives.  It's all a matter of timing.  You want the fireworks to explode at the apex of the shell's climb.

A 6" fireworks shell.  Fuse is at left, black cubes are the "stars",

a gray explosive charge is to the right, and the black powder

propellant is at the far right.  All encased in a cardboard shell.

Setting up the mortars and shells for a show

The high explosive charge ignites the stars and tosses them outward.  The packing of the explosive and the stars determines the type of effect- the spherical peony or chrysanthemum, the expanding ring, the drooping willow, etc.

 

There's a whole art and science behind the packing of fireworks shells with different configurations, formulations, and shapes.

 

Colors, of course, are due to the chemical composition of the stars.  Reds are formed from strontium (SrCO₃) or lithium carbonates (Li₂CO₃), orange from calcium chloride (CaCl₂), yellow from sodium chloride (NaCl), green from barium chloride (BaCl₂), blue from copper chloride (CuCl), etc.  Every manufacturer has their own secret formulations.

The explosive boom heard when watching a fireworks display is caused by the high explosive charge pushing air outward at faster than the speed of sound (340 m/s) causing a sonic boom. There is a noticeable delay between seeing the explosion and hearing the explosion since light travels much faster at 300,000,000 m/s (almost a million times faster!).  There's a 3 second delay between the flash and the boom for every kilometer of distance you are from the exploding shell.

Another funny thing about fireworks is that they appear to be two-dimensional, like they're being displayed on a flat screen, unless you're right underneath them.  This is because your eyes and brain can't determine which way the burning fragments of stars are moving since they're so bright against a black background.

You've seen the chemistry and physics, where does the geology come in?  Over 2/3 of the world's strontium, for example, comes from China where the strontium sulfate (SrSO₄) mineral celestine, also known as celestite, is mined.  Barium comes from the barium sulfate (BaSO₄) mineral barite.  Lithium comes from minerals like spodumene, a lithium aluminum silicate - LiAl(SiO₃)₂.  People tend to forget that virtually all the chemical elements we use in our modern industrial society have, as their origin, minerals dug from the Earth.

Science, as always, enhances our appreciation of beauty by giving us a deeper understanding of what we see and experience and how everything is connected in the world around us.

Enjoy the show and Happy Independence Day 2013!