Almost magical, fireworks have been entertaining us for hundreds of years with their loud bangs, bright colours, and incredible effects, but how do they work? Here we take a closer look at the science behind our favourite pastime.
So what is gunpowder? Gunpowder consists of an oxidiser, fuel, and binders. The basic ingredients of all gunpowder’s are saltpetre, sulphur and charcoal. When it burns, it releases a large volume of hot gases which expand quickly giving the propellant force. In fireworks, gunpowder is made to perform in various forms and sizes such as rockets, bangers, Roman candles, and Catherine wheels.
To understand the science of the colours, we must first understand the nature of light. Light is a form of energy that behaves like a wave. Visible light of different wavelengths is detected by our eyes as a range of colours. Of the light that is visible to us, violet has the shortest wavelength and red has the longest.
Fireworks create lights and colours due to the physical and chemical attributes of the specific compounds used. When these compounds are heated and combusted they release energy, when the energy released coincides with the wavelengths in the visual region of the spectrum we see the corresponding colour.
The main colours seen in modern fireworks are blue, orange, red, green and yellow. Although advancements in pyrotechnics are happening all the time and combinations of these basic colours are cleverly used to create an almost infinite palette to entertain us. Yellow is created by atomic emission. The other colours are produced by a combination of atomic and molecular emissions.
As a substance is heated, the heat energy can enter the electrons of the atoms or molecules. This in turn raises the energy of the electrons. When they return to a lower energy they emit photons. It is these photons which our eyes see. The energy of these photons, and therefore their wavelengths, varies from substance to substance and so we see a range of different colours depending on what substance is used. Molecules absorb and radiate energy differently. It is possible for energy to be stored by the vibrations in the different parts of the molecule. Groups of atoms can vibrate relative to each other. The end colour that we see from an excited substance is a combination of the atomic and molecular energy changes and is manipulated by the pyro-technician.
Red is the lowest-energy visible light, so in a red hot object the atoms are just reaching the point of having enough energy to begin emitting a light that is visible to us. If you apply enough heat the electrons generate all the colours of the spectrum and will appear white. This is why we have the terms red hot and white hot.
The charcoal in the gunpowder mixture behaves like a black body. As it is heated to sufficiently high temperatures, it glows with a faint red colour. As it is heated further, the colours change from dark red to bright red to orange to yellow and finally, white. By then it is radiating photons of every wavelength that our eyes can detect. For materials like charcoal, incandescence (the emission of visible light by a hot object) occurs from about 500 degrees to 1400 degrees.
Metallic fuels, such as magnesium, aluminium, and titanium achieve very high levels of brightness when burned, and are used to make the bright sparks that are given out by many fireworks.
So that’s the colours, now how do we make a firework?
The most important component of a firework is the black powder. By 300AD a scientist of the Chin Dynasty called Ge Hung had actually written the ingredients of gunpowder and described the effects. He made gunpowder by mixing Sulphur, charcoal and saltpetre (also called Potassium Nitrate).
Sulphur is found naturally in our environment as a yellow rock, it is mined and processed to create sulphur that can be used in gunpowder. Saltpetre can be made from animal manure, and this can be drained off by washing the manure through with water. The three separate powders are then mixed together using roughly 15 parts of saltpetre to 3 parts of charcoal and 2 parts of sulphur.
However, today, these mixtures vary according to the individuals making them and it depends on what form or shape you wish to make with the fireworks - rockets, Roman candles, bangers etc.
Let’s have a look at rockets now. The basic principle behind every rocket is Newton's Second Law: "To every action there is an equal and opposite reaction".
A rocket throws mass in one direction and is propelled in the other direction.
The mass comes from the weight of the black powder that the rocket engine burns. The burning accelerates the mass of fuel so that it shoots out of the rocket nozzle at high speed. The fact that the fuel turns from a solid into a gas when it burns does not change its mass, because all the oxygen needed for combustion is contained within the rocket's fuel.
One of the most crucial things about making a rocket is establishing how much force it needs to take off and rise to the required height to explode safely above the ground. For the rocket to be propelled, it is necessary to generate a force greater than gravity. First, we need to understand Velocity, Acceleration and Force.
To find out what the rocket's velocity is, the following equation is used-
v = d/t
Where v = velocity, d = distance, t = time
Acceleration from a stationary launch would be as follows:
a = v/t
Where a = acceleration, v = velocity, t = time
The force would then be:
F = m x a
Where F = force, m = mass, a = acceleration.
We can work out how far the rocket travels above the spectators' heads based on the following information.
The rocket produces a force (F) of 25N, its mass (m) is 0.6kg and it travels for 3 seconds (t).
To work out the distance travelled, we first must to calculate the velocity and in order to do that we must first determine the acceleration.
F = ma
25 = 0.6 x a
a = 25/0.6
a = 41.7ms-2
Now that we know the acceleration, we can work out the velocity:
a = v/t
41.7 = v/3
v = 41.7 x 3
v = 125.1ms-1
Finally, we use this information to work out the distance travelled:
v = d/t
125.1 = d/3
d = 125.1 x 3
d = 375.3m
So the distance travelled by the rocket is 375.3m which is, of course, approximate due to wind conditions and minute variations in the gunpowder.
"Roman candle" is the traditional name for a firework that has existed for many centuries. In its simplest form, it's just a tube with a shell inside. A fuse runs into the tube and ignites a lifting charge, forcing the shell out of the open end and into the air where it explodes, giving the effect.
The lifting charge is made of black powder which when ignited produces a lot of hot gas which in turn pushes the shell upwards.
Roman candles can have multiple shells inside, stacked one on top of the other, separated by lifting charges. The fuse runs down the inside of the tube, igniting each charge in turn. The shells are launched one after the other, with a pause between each one. This is known as a barrage, repeater or multi shot candle.
A banger is probably one of the most powerful fireworks because the whole contents explode at once. Whereas the contents of most other fireworks go off a bit at a time, in a banger all the explosive is tightly packed into a single tube.
The fuse ignites this and the explosion shoots out of the end of the tube and into the air. The effect is powerful and sudden which is why we get such a loud bang. The same amount of black powder in a barrage or multi shot repeater would not have the same volume or force.
Bangers as single tube fireworks have been banned in the UK since 1998 due to a large number of complaints from the public over nuisance noise around Guy Fawkes Night. However many fireworks still contain loud bangs and we don’t mind.
The way fireworks are made has changed a lot over the years, and the range of effects and colours has improved massively just in the last decade. Many more advancements are sure to come. However the fundamental chemistry of pyrotechnics has not changed in a millennium and the same basic principals apply today that did when they were first invented.
So if you love fireworks as much as we do, give a little thought to the hard working pyro-technicians out there that are constantly working to bring us the next generation of great fireworks.