Sparklife
Bonfire night is a few weeks behind us now, and many of us are now looking ahead to the Christmas holidays. For those not from round here, we in the UK and a few other places like to let off fireworks to commemorate a historical (1605) failed bombing attempt and to scare everyone’s pets. The long winter nights of Christmas can be a good time to dig out some unused sparklers, set light to the ends and enjoy the snowball of sparks as they burn. To misquote the great Bruce Springsteen, “You can’t start a firework display without a sparkler.”
But how does this little length of wire produce its sparkling magic? Spoiler alert: it’s not really magic, it’s mainly chemistry. You need fuel, oxygen and a means of sticking it all together.
First and foremost, you need sparks. The sparks from a sparkler are little particles of burning metal: literally burning, as in reacting with oxygen and producing heat. Some metals do actually burn in air. Aluminium powder, with a suitable oxidant, has been used as rocket fuel. Those of us old enough to have lived through a certain conflict of the early 1980s might remember the carnage caused after missile strikes, when the aluminium superstructure of a ship could ignite and burn fiercely. The heart of a sparkler consists of fine particles of metal. Iron gives orange sparks; aluminium, magnesium and titanium give white sparks; the iron-titanium alloy ferrotitanium gives golden yellow sparks. Each spark lives as long as it takes for all the metal in the particle to be burned and fully oxidized.
To sparkle and burn vigorously, a sparkler needs more oxygen than can be delivered by our atmosphere. Sparklers contain an oxidant, an oxygen-containing compound that can release some or all of its oxygen. Typical oxidants are potassium perchlorate (KClO4), potassium chlorate (KClO3) and potassium nitrate (KNO3). Heat from the combustion releases the oxygen, which then feeds into the combustion, creating more heat which releases more oxygen… the cycle is self-sustaining until something runs out. Heating a gas increases its pressure. Adding additional oxygen has even more effect, and this pressure helps expel the sparks so they fly outwards.
A sparkler may also contain reducing agents, which – by definition – like to react with oxygen. But why would you want to use up the precious oxygen that is released by the oxidants? It might be simply to burn more fuel, so there is more combustion going on, so more heat and pressure to keep things sparking. Also, the right combination of reducing agents can help regulate the combustion rate of the other fuel components for the perfect compromise between spectacle and duration.
We’ve got our metal particles, our oxidant and our reducing agents. How do we persuade these components to stick to our sparkler wire? We simply make up a slurry with a binder material. Typically, the binder is dextrin, a short-chain polymer of glucose molecules. Once coated and dried, dextrin will burn along with the other fuels. This helps to release the metal particles into the spark shower.
A sparkler is simply the result of some smart chemistry. The sparks themselves are particles of burning metal. An oxidant releases oxygen to assist the burn and the heat and pressure help eject the sparks. A combustible binder holds it all together on the wire then releases everything as it burns. What’s not magical about a bit of Christmas chemistry?
