Fireworks!

Have
you ever been to an aerial fireworks show at an amusement park, baseball game,
Fourth of July celebration, or on New Year's Eve and wondered about how all the
impressive colors and sounds are produced? People everywhere enjoy the fantastic
explosions and the brilliant light displays of fireworks. However, these
spectacles are much more than just a form of entertainment. Each firework
launched into the sky is a precisely formed assembly of chemicals and fuel,
carefully calibrated to produce a particular effect – a red chrysanthemum spray
accompanied by a powerful explosion, or a blue strobe, for example.
Understanding how the contents of a firework produce the impressive variety of
colors, forms, and sound intensities requires only a simple understanding of
chemical reactions.

Fireworks
generate three very noticeable forms of energy: a tremendous release of sound,
bright light, and heat. The tremendous booms heard at ground level are the
result of the rapid release of energy into the air, causing the air to expand
faster than the speed of sound. This produces a shock wave, a sonic boom.

The
colors are produced by heating metal salts, such as calcium chloride or sodium
nitrate, that emit characteristic colors. The atoms of each element absorb
energy and release it as light of specific colors. The energy absorbed by an
atom rearranges its electrons from their lowest-energy state, called the ground
state, up to a higher-energy state, called an excited state. The excess energy
of the excited state is emitted as light, as the electrons descend to
lower-energy states, and ultimately, the ground state. The amount of energy
emitted is characteristic of the element, and the amount of energy determines
the color of the light emitted. For example, when sodium nitrate is heated, the
electrons of the sodium atoms absorb heat energy and become excited. This
high-energy excited state does not last for long, and the excited electrons of
the sodium atom quickly release their energy, about 200 kJ/mol, which is the
energy of yellow light.

The
amount of energy released, which varies from element to element, is
characterized by a particular wavelength of light. Higher energies correspond to
shorter wavelength light, whose characteristic colors are located in the
violet/blue region of the visible spectrum. Lower energies correspond to longer
wavelength light, at the orange/red end of the spectrum.

The
colors you see exploding in the sky are produced by the elements with the
characteristic emissions listed in the table.

Color

Compound

Wavelength
(nm)

red

strontium salts, lithium
salts
lithium carbonate, Li2CO3 = deep red

strontium carbonate, SrCO3 = bright
red

650

orange

calcium salts
calcium
chloride, CaCl2

670

yellow

sodium salts
sodium
chloride, NaCl

610-620

green

barium compounds + chlorine
producer
barium chloride, BaCl2

590

blue

copper compounds + chlorine
producer
copper(I) chloride, CuCl

500-535

purple

mixture of strontium (red)
and
copper (blue) compounds

420-460

silver

burning aluminum, titanium, or
magnesium

In
making fireworks, the metal salts are put into stars, small clay or dough-like
lumps or cubes 3 to 4 cm in diameter. Stars consist of a blend of oxidizing
agent, reducing agent, coloring agent (metal salt), and binders. When ignited,
the stars produce both sound and light effects. The appearance of a firework is
determined by its stars, which are made by hand and carefully packed into
cardboard compartments within the firework shell, where they await ignition by a
time-delay fuse.

From
lift-off to color release, a carefully choreographed sequence of events takes
place, producing the desired effect. The power needed to lift each firework into
the air is provided by the highly exothermic combustion of black powder, a
slow-burning combination of 75% potassium nitrate, 15% charcoal, and 10% sulfur.
Said to have first been used in China about 1000 years ago, the recipe for black
(or coal) powder has undergone little change since then. This formulation
explodes at a rate of about 3 meters per second, classifying it as a low
explosive. In fact, when it burns in the open air, black powder’s heat and gas
dissipate quickly. The key to fireworks’ success is to trap the heat and gas in
the bottom of the shell, which is positioned in a launch tube or mortar, until
the trapped gas pressure builds to such a force that, when it escapes, it hurls
the firework high into the air.

A
firework is ignited by lighting the main fuse. That simultaneously starts both
the fast action fuse, which quickly carries the flame down to the lift charge,
and the time delay fuse, which continues to burn upward toward the cardboard
compartments containing the stars, even as the firework is hurtling skyward.

Fireworks
are classified as both a low and a high explosive. The initial lift charge that
sends the firework into the sky is a low explosive. The burning charge undergoes
rapid decomposition, but not detonation. The firework can be thought of as
flying through the air powered by a fast burning wick. Where the wick ends, it
meets the high explosive components of the firework. In this second stage there
is an instantaneous detonation producing both a loud explosion and a bright
flash of color.

The
black powder lift-charge is calculated to exhaust itself precisely when the
slow-burning, time-delay fuse reaches the first compartment packed with
light-producing stars and black powder. This happens when the firework is at the
very apex of its upward flight. Simultaneously the fuse sets off sound-producing
explosives and detonates the stars, initiating color emission. If the timing of
the fuses is off, however, the firework may detonate early, too close to the
ground, or late, when the firework is falling back to earth.

Chemistry of Fireworks

The
sights and sounds of each explosion are the result of several chemical reactions
– oxidations and reductions – taking place within the firework as it ascends
into the sky. Oxidizers produce the oxygen gas required to burn the mixture of
reducing agents and to excite the atoms of the light-emitting compounds. Various
oxidizers are used in both the black powder and the stars. The most commonly
used oxidizers are nitrates, chlorates, and perchlorates. The reducing agents,
sulfur and carbon, combine with the oxygen from the oxidizers to produce the
energy of the explosion.

The
most commonly used oxidizers are nitrates, the major component of black powder.
Nitrates are composed of nitrate ions (NO3-) with metal ions. The most common
oxidizer is potassium nitrate, which decomposes to potassium oxide, nitrogen
gas, and oxygen gas.

When
reacting, nitrates release two of their three oxygen atoms. Because the
oxidation does not result in the release of all available oxygen, the reaction
is not as vigorous as that of other oxidizers and is more controlled. This is
why nitrates are used as the major component of black powder. In fireworks their
main purpose is to provide the initial thrust to power the package into the sky
and to ignite each bundle of stars. Nitrates are usually not used in star
explosions, because reactions of nitrates do not produce a temperature high
enough to energize many of the more colorful metal salts.

In
the 1830s Italian fireworks makers found a group of more explosive oxidizers,
which produced temperatures of 1700 to 2000°C and made possible the creation of
much more intense colors. These oxidizers are the chlorates, which contain the
chlorate ion (ClO3-), and they give up all their oxygen upon reaction.

This results in a much more intense and spectacular reaction.

These
chlorates have the disadvantage of being less stable mechanically than nitrates,
and therefore more dangerous to handle. Chlorate compounds sometimes can be
detonated just by dropping them on the ground! This instability results from the
fact that although the chlorine atom has the potential to bond with four oxygen
atoms, in chlorates it bonds with only three, leaving the chlorine atom
unsaturated and reactive. The complete release of its oxygen atoms makes
chlorate a better oxidizing agent than nitrate. Unlike nitrate-containing
compounds that produce a relatively slow burning rate, the oxidation by
chlorates produces a much faster detonation – an explosion. In recent years,
fewer fireworks manufacturers are using chlorates. Instead, perchlorates are now
more commonly used because of their increased stability and oxygen
release.

Perchlorates
contain the perchlorate ion (ClO4-), in which each chlorine atom is bonded to
four oxygen atoms. The chlorine is bonded to its maximum number of oxygen atoms,
and so perchlorates are more stable than chlorates. Yet, perchlorate is able to
release all four of its oxygen atoms.

So,
perchlorates are not only more stable, but more oxygen-rich than chlorates.
They, like chlorates, produce more vigorous reactions than
nitrates.

The
oxygen released by nitrates, chlorates, and perchlorates in the star
compartments immediately combines with the reducing agents to produce hot,
rapidly expanding gasses. The most common reducing agents are sulfur and carbon
(charcoal) –standard components of black powder – which react with oxygen to
produce sulfur dioxide and carbon dioxide respectively:

The
reactions that produce these gases also release a great deal of heat energy, so
no only are the gases produced rapidly, they are hot and rapidly expanding
gases. This adds to the explosive force of the reaction.

Firework Safety

Fireworks
are used so frequently today in celebrations that it is easy to forget that they
are dangerous explosives. Every year more than 8,000 people in the U.S. suffer
injuries caused by the personal use of fireworks. Nearly half of the victims are
children. A third of the injuries are caused by illegally obtained fireworks,
and burns account for half the injuries. (An ordinary sparkler burns at a
temperature of more than 1000°C!)

The
National Fire Protection Association (http://www.nfpa.org/) enforces stringent safety
regulations for large fireworks displays. Spectators must be kept at least 840
feet from the launch area (that's based on the height and burst diameter of the
largest shells). Shells may not be launched if winds are stronger than 20 miles
per hour, because they could be blown off course. Nevertheless, many accidents
occur with unregulated, informal neighborhood displays, when spectators
attracted to the activities stand dangerously close to the launch area.

Fireworks
manufacturers also go to great lengths to ensure safety, but even so, more than
20 workers were killed in firework plants in the U.S. between 1970 and 1995.
Safety regulations require that buildings be separated by concrete blast walls
and that roofs be weakened to ensure that any explosion travels upwards rather
than outwards. In addition, most fireworks are still made by hand because metal
machinery could produce sparks or static electricity which would ignite the
explosives.

Origins of Black
Powder

Gunpowder
or black powder was invented in China by alchemists experimenting with a
naturally-occurring salt, potassium nitrate, also known as saltpeter.
Ironically, they were looking for an elixir of immortality. But in handling and
heating the sensitive substance they inevitably discovered its explosive
properties. The first known account of the use of gunpowder as a weapon dates to
1046 in China, describing a catapult-launched grenade, an incendiary bomb, and a
smoke bomb. The Song Dynasty Emperor in 1067 banned the sale of saltpeter and
sulfur to foreigners and nationalized the production of gunpowder.

Marco
Polo is sometimes given credit for bringing gunpowder to Europe but that is
unlikely. When Europeans invaded the Middle East during the Crusades, they
encountered gunpowder weapons used by Moslem forces. Despite government control
and attempts to keep the formula secret, gunpowder probably traveled the Silk
Road from China to the Moslem world far earlier than Marco Polo's trip in the
late 1200s. English philosopher Roger Bacon (1217-1292) is believed to the first
Westerner to describe gunpowder and fireworks. By the mid-1300s, European armies
were using crude cannons and other gunpowder weapons.

For more on fireworks and illustrations of their construction, see:

the NOVA website, http://www.pbs.org/wgbh/nova/fireworks/anat_nf.html

Chemistry of Pyrotechnics: Basic Principles and Theory, by John A. Conkling,
Marcel Decker, Inc., 1985.