Anatomy of a Firework
When most of
us think "fireworks," we think of brilliant bursts of light and color we've seen
paint a night sky. But such bursts are merely the spectacular end of fireworks that likely
took centuries of experience, weeks of planning, and hours of painstaking labor to fashion
and fire. In this feature, pull back the wrapping on a typical aerial display shell and
see what it looks like before its glorious denouement in the dark.
Break
In a multi-break firework, stars are contained in
separate cardboard compartments within the shell. Each compartment has its own bursting
charge, which ignites and throws out the stars. In order to spread these decorations over
a wide area of sky, the break must burst open with tremendous force. The more the
compartment can resist the explosion and bottle up its force, the bigger the display will
be. Resistance comes from the break's heavy wrapping, which momentarily keeps the gas and
heat from reaching the bursting charge.
A firework's breaks may also contain sound charges,
which result in the cracking bangs and thunderous booms that thrill audiences. To make
these loud explosions, which are usually accompanied by a bright white flash, firework
manufacturers use mixtures of perchlorate, a different kind of explosive than black
powder.
Time-delay fuse
As the firework ascends through the air, the time-delay
fuse continues to burn. By the time the shell nears its apogee, the fuse has burned low
enough to ignite the black powder in the first break (or compartment). Colored stars
ignite in every direction. But the show isn't over yet. The fuse keeps burning, making its
way toward the stashes of black powder in the second and third breaks.
Timing is critical. In a three-break firework, the
middle break needs to ignite at the highest point in the shell's trajectory. Thus, the
first break should blow a little before and the third break a little after. If the timing
is off, the firework might detonate too close to the ground. Great care is used in
designing the fuses and calculating their lengths.
Stars
Stars are the precious cargo carried by
"aerial" fireworks like the one depicted here. An unlit star isn't much to look
at -- just a dull black lump about the size of a jawbreaker. But appearances can be
deceiving. When ignited, stars create the breathtaking flashes of color and light that
elicit "oohs"and "ahhhs" from even the most jaded spectators.
Fireworks masters manufacture their creations by
hand, including the hundreds of stars that go into a single firework. They mix carefully
measured ingredients like perchlorate and black powder with binding and coloring agents:
magnesium or aluminum for white, sodium salts for yellow, strontium nitrate or carbonate
for red, barium nitrate for green, copper salts for blue, and charcoal or other forms of
carbon for orange. The result is a huge slab of dough, which is then cut like a tray of
brownies into half-inch cubes; these are then set out to dry.
Stars can be extremely dangerous if not handled and
stored with care. A sharp blow can detonate one. Oil from nearby machines can combine with
certain chemicals to create an explosive gas. Even synthetic clothing, which generates
static electricity, can create sparks capable of detonating the fragile shells. Firework
makers must stick to wearing cotton -- all the way down to their underwear.
Black powder
The
recipe for black powder, or gunpowder, the basic material in all fireworks, has remained
the same since it was discovered in China about 1,000 years ago: 75 percent saltpeter
(potassium nitrate), 15 percent charcoal, and 10 percent sulfur. Black powder lends itself
to fireworks because it's a "low explosive," meaning its detonation velocity is
less than about 100 yards per second. ("High explosives" like dynamite have a
velocity of detonation greater than 1,000 yards per second.) Fireworks makers can also
control the powder's rate of burn in several ways. One way is by manipulating the size of
its grains: Fine grains burn more quickly than coarse grains.
Launch Tube
Most
fireworks are launched from rows of tubes secured in troughs of sand. The tubes, or
"mortars," are three times as long as the firework shells but have the same
diameter. If a firework doesn't fit snugly into its launch tube, the pressure created by
the lift charge will escape, and the firework can misfire.
Main Fuse
During
the Renaissance, when fireworks as we know them were invented, pyrotechnicians lit their
creations with tissue paper rolled around a trail of black powder. Later, string embedded
with gunpowder was used. Today, electrical wires connect fireworks to a master control
board. With the push of a button, an electrical current rushes through each wire and
creates a spark at the point of contact on the main fuse.
The main fuse simultaneously lights two secondary
fuses -- a fast-acting side fuse that ignites the lift charge, and a time-delay fuse
buried inside the shell that leads to the heart of the firework.
Lift Charge
When
black powder burns in the open air, the heat and gas it generates quickly dissipate. But
if the black powder is confined, say in a pouch at the bottom of a firework cylinder, the
trapped heat and gas will push vigorously at the inside of the launch tube until an
explosion results. This explosion will free the heat and gas and hurtle the firework shell
as high as 1,000 feet into the air.
A
simple shell used in an aerial fireworks display. The blue balls are the stars, and the
gray is black powder. The powder is packed into the center tube, which is the bursting
charge. It is also sprinkled between the stars to help ignite them. Simple shells consist
of a paper tube filled with stars and black powder. Stars come in all shapes and sizes,
but you can imagine a simple star as something like sparkler compound formed into a ball
the size of a pea or a dime. The stars are poured into the tube and then surrounded by
black powder. When the fuse burns into the shell, it ignites the black powder, causing the
shell to explode. The explosion ignites the outside of the stars, which begin to burn with
bright showers of sparks. Since the explosion throws the stars in all directions, you get
the huge sphere of sparkling light that is so familiar at fireworks displays. 
Sphereical shells used by professionals are very similar to the smaller
versions sold on the consumer market. The most obvious difference is the larger
size, which lets the shell hold more stars. In the diagram on the left, the
internal time fuse goes out through the bottom of the shell and is ignited by the lifting
charge as is the case with consumer shells. On large shells, however, the blast of
the lifting charge could literally blow the fuse away without igniting it, so the time
fuse is made to go out through the top of the shell. If this method is used, the
time fuse is coated with a priming compostion and is ignited by the hot gases of the lift
charge. Smaller display shells can usually be held by the quick match fuse
when being loaded into the mortar, but larger ones have a string attached to them which is
used to lower it down - otherwise, the weight of the shell would rip the quick match out
of the lift charge.
Cylindrical shells were invented in Italy and are more common in Europe
and America than spherical shells. Although they don't give perfectly sphereical
bursts of stars like round shells, they are easier to make and store. The time fuse
is ignited by the quickmatch as the fire goes down to the lift charge. The time fuse
is nearly impossible to ignite from the end, so holes are punctured in the sides and
strips of black match fuse are inserted. This is known as a "cross match"
assembly. Black match takes fire very easily, and transfers it into the powder
core of the time fuse. The time fuse gives off no "side spit", to the
burst charge doesn't ignite until the black match on the opposite end is ignited by the
time fuse. The cutaway diagram shows a more detailed look at cross-matched fuse.