Feature

Lighting the Sky

Remember, remember the fifth of November, gunpowder,
treason and plot.

by Janine Griffin

Fireworks have fascinated people for centuries and have been used to herald everything from royal weddings to presidential inaugurations. Although most people have waved a sparkler in the air or fired off a rocket, few people are aware of how they work.

A thousand years after its discovery by the Chinese, gunpowder is still the basic ingredient of most fireworks. Fireworks effects are based on the combustion of fuels with metal nitrates or chlorates, most commonly the potassium nitrate used in gunpowder. Gunpowder, or black powder, also contains sulphur and charcoal. Potassium chlorate and potassium perchlorate are also widely used in place of the potassium nitrate of gunpowder.

When ignited, this mixture burns briskly but does nothing remarkable. Confined, though, it can propel a projectile for considerable distances, giving a bright flash and a cloud of smoke. Adding different chemicals produces a range of colours.

Fireworks come in many shapes and sizes from the common sparkler to the 700-kilogram Universe I Part II exploded at the Lake Toya Festival in Japan in 1988. This giant -- the largest firework ever exploded -- covered an area 1.2 kilometres across.

A basic firework, like a star shell or a rocket, has two charges of gunpowder. The first, the lifting charge, is used to launch the container into the air; the second, the bursting charge, ignites the contents, producing the familiar kaleidoscope of colours and patterns.

What produces those colours and patterns are small, hard pellets, or stars, in the bursting charge. Intricate patterns can be produced by arranging the stars inside the shell. Some Japanese stars are so complexly arranged that they can produce the intricate characters used in Japanese writing.

The different chemicals in the stars produce different colours. Strontium is commonly used for reds, barium salts for greens, calcium for orange and sodium for yellow. Blues require copper, but are more difficult to produce because they also require chlorine donor atoms, which are very specific in their temperature requirements. Today, the chemicals used tend to be less toxic than those used in the past, such as copper arsenic salts.

Other Considerations

Chemistry is only one of many considerations in making fireworks, says Anthony Lealand, owner of the Fireworks Company which imports and manufactures fireworks for displays throughout New Zealand. Other, probably more important considerations are how the chemicals are confined, the charge of the contents or the amount of friction.

"A big star shell, if it's not properly confined," he says, "will just dump stars out in an untidy heap. If it's over-confined, the stars will detonate and one will just end up with a bang and a bright flash of light instead of stars cascading through the sky."

The ground-based fountain fireworks also have their tricky points.

"The chemistry is absolutely bulk standard, but the critical factors are making sure it doesn't have voids, cracks or separates from the container. Because if it separates from the container, the flame front can travel down the side, so instead of burning like a candle from one end, the flame travels down the side, the pressure builds up and the whole lot detonates," says Lealand.

Chemistry, Lealand insists, is not the main problem in designing fireworks -- static electricity is also a factor. Confetti tends to pick up a charge, making it clump together rather than float to the ground in individual pieces. It can also be extremely hard to get off carpet, causing problems with cleaners.

Firing shells are fired from mortars, which are made of very thick cardboard, high density polyethylene or steel. A mortar is simply a tube sealed at one end into which the shell is placed. The shell is ignited by either a fuse or an electric igniter that runs alongside the shell and is inserted in the bottom. The fit of the shell inside the mortar needs to be tight enough so the gases produced when the shell is ignited propel the shell upwards instead of escaping around the sides.

If a manufacturing defect causes the shell to detonate inside the mortar, the resulting explosion is very dangerous. People have been killed when a steel mortar splinters, and steel mortars are increasingly being replaced by safer mortars that shatter into smaller, safer pieces or, in some cases, disintegrate, says Lealand.

Large fireworks shows may involve a series of shells being fired off to a particular piece of music or an elaborate depiction of a significant event using a set piece.

One of the more elaborate types of fireworks used in shows is the set piece. These make extensive use of lances, which are made of a paper tube filled with a colour composition to give a coloured flame. When ignited, the lance burns steadily until its contents are exhausted. Road flares are a variation on the lance theme. Different-coloured lances set on a frame can spell out a message or portray an event. Some set pieces also incorporate moving parts and can be incredibly complex.

One produced at the turn of the century portrayed the Austrian Alps with a train moving in and out of tunnels along the base. An "avalanche" of snow then cascaded down the mountains, just missing the train as it disappeared into a tunnel. More common, these days, are the public fireworks shows where a series of shells are set off, often to music. Here, electrical firing is used to control how many shells are fired and when they're fired.

"One of the most embarrassing things with pyrotechnics is that they don't go off," says Lealand. The techniques used need to be both reliable and safe.

Electrically igniting shells is much safer than using flares, as the person firing the shell can be a considerable distance away. Lealand uses electrical firing extensively, not only because it is safer, but also because it allows a high level of control. Electrical igniting systems have changed the face of fireworks shows and choreography is rapidly becoming a standard part of these shows.

"It's very difficult to choreograph fireworks closely to music, and we're being asked for choreography to within a tenth of a second," says Lealand. "With the techniques of the past, this was extremely difficult."

Choreography can also be made more precise by using a computer. The Fireworks Company is currently testing software that allows a computer to control the show. It's a far cry from using a flare to ignite each shell individually.

Fireworks for use indoors require more precise techniques than outdoor fireworks and use smokeless powders rather than gunpowder.

"Indoor fireworks have a very accurate flame production," says Lealand. "If I say the height of our fireball is two metres, and they put a three-metre ceiling, and then the fireball slaps into the ceiling and catches the dust on fire, we've got a serious problem on our hands. So the critical thing with the indoor fireworks is that they're very precisely made and they always operate exactly as specified."

A glitter cannon that shoots 35-mm squares of glitter 20 metres high needs to be made so that the glitter comes apart rather flying across the venue in a clump. The streamers in a streamer cannon need to be arranged so that they unwind correctly.

"A whole bunch of streamers hurtling from one end of a venue to the other and clouting someone is not a particularly funny incident," says Lealand.

The face of modern fireworks continues to change, and the art shows no sign of dying out. Next February, the International Pyrotechnics Society will hold a week-long conference in Christchurch. Researchers from around the world will be presenting papers on topics from new ignition techniques to safety. Naturally, a fireworks show will feature.

Janine Griffin is a freelance journalist specialising in science issues.