The shell is launched from tubes called mortars. Mortars can be constructed from heavy cardboard, plastic (HDPE=High Density PolyEthylene), GRP ( Glass Reinforced Plastic) or metal sunk into the ground or mounted in racks. The shell must fit snugly inside the mortar to allow proper thrust. Shells measure from two inches to three feet in diameter and can weigh up to 700 pounds. Dozens of mortars of various sizes are used at large professional fireworks displays.

The lifting charge of black powder explodes in the pipe to launch the shell. When the lifting charge fires to launch the shell, it lights the shell's fuse. The shell's fuse burns while the shell rises to its correct altitude, and then ignites the bursting charge so it explodes.

ANATOMY OF A FIREWORK

              


Launching Tube
Most fireworks are launched from rows of steel tubes which are secured in troughs of sand. The tubes or "mortars" are three times as long as the firework shells, but are the same size in diameter. If a firework doesn't fit snugly in its launching tube the pressure created from the lift charge will escape, and the firework won't become airborne.

Lift Charge
When gunpowder burns in the open air, the heat and gas it generates quickly dissipates. But if the gunpowder is confined, say in a pouch at the bottom of a firework cylinder, the heat and gas are trapped and will push wildly 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 1000 feet into the air.

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 the wire and creates a spark at the point of contact.

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.

Black Powder
The recipe for black powder, the basic material in all fireworks, has remained the same since it was discovered in China 1000 years ago: seventy-five percent saltpeter (or potassium nitrate), fifteen percent charcoal, and ten percent sulfur. Black powder explodes at the relatively slow rate of 1/10 of a second per foot -- making it a "low explosive."

Stars
Stars are the precious cargo carried by "aerial" fireworks, like this one. 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 breath-taking flashes of color and light that elicit "ooohs" and "ahhhs" from even the most jaded spectators.

Fireworks masters, like the Grucci family of Brookhaven, NY, manufacture their creations by hand, including the hundreds of stars that go into a single firework. Carefully measured ingredients like perchlorate and black powder are mixed 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, that 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.

Time-Delay Fuse
As the firework shoots through the air, the time-delay fuse continues to burn. When the shell is close to its apex, 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 -- 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.

Breaks
In a multi-break firework, stars are contained in separate cardboard compartments within the shell. Each container has its own bursting charge which lights and throws out the stars. In order to spread these decorations over a wide area of the sky, the container must burst open with tremendous force. The more the container can resist the explosion and bottle up its force, the bigger the display will be. Resistance comes from the container's heavy wrapping, which is designed to momentarily trap the gas and heat from the bursting charge.

(Information courtesy of Fireworks by Grucci and NOVA Online.)

MULTIBREAK SHELLS
More complicated shells burst in two or three phases. Shells like this are called multibreak shells. They may contain stars of different colors and compositions to create softer or brighter light, more or less sparks, etc. Some shells contain explosives designed to crackle in the sky, or whistles that explode outward with the stars.

Multibreak shells may consist of a shell filled with other shells, or they may have multiple sections without using additional shells. The sections of a multibreak shell are ignited by different fuses. The bursting of one section ignites the next. The shells must be assembled in such a way that each section explodes in sequence to produce a distinct separate effect. The explosives that break the sections apart are called break charges.

The pattern that an aerial shell paints in the sky depends on the arrangement of star pellets inside the shell. For example, if the pellets are equally spaced in a circle, with black powder inside the circle, you will see an aerial display of smaller star explosions equally spaced in a circle. To create a specific figure in the sky, you create an outline of the figure in star pellets, surround them as a group with a layer of break charge to separate them simultaneously from the rest of the contents of the shell, and place explosive charges inside those pellets to blow them outward into a large figure. Each charge has to be ignited at exactly the right time or the whole thing is spoiled.