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Publié : 9 mai 2012
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How do fireworks work ?

How do fireworks work ?

Black powder and the art of making explosions have been known by the Chinese since at least the eighth or nineth century. In Europe, black powder was brought back by Marco Polo from China. It has been used for firearms since 1346. The first real firework was fired at Anne of Austria and Louis XIII’s wedding in 1615. It contained black powder, gold dust and silver dust.

Fireworks have been a traditional part of New Year’s celebrations since they were invented by the Chinese almost a thousand years ago. Today fireworks displays are seen on most holidays. There are different types of fireworks. Firecrackers, sparklers, and aerial shells are all examples of fireworks. Though they share some common characteristics, each type works a little differently. Firecrackers are the original fireworks. In their simplest form, firecrackers consists of gunpowder wrapped in paper, with a fuse. A sparkler consists of a chemical mixture that is molded onto a rigid stick or wire. I will focus on the aerial shells through my presentation because when most people think of fireworks an aerial shell probably comes to mind as it is the most used at fireworks display.

Aerial shells

Aerial shells are one of the most beautiful and certainly one of the most complex types of fireworks. A shell consists of those main parts : a container, a lift charge, a time fuse, a burst charge, and stars/effects. The container, or shell casing, is a strong wall that protects the contents. The lift charge propels the shell out of the tube, and the time fuse ignites the burst charge at the right altitude. Then, the blast of the burst charge ignites the effects.

Shells are launched from a tube known as a mortar. A string loop is often attached to consumer firework shells so it can be lowered into the mortar by the fuse. When the fuse enters the lowest part of the shell – which contained the lift charge -, it ignites the lift charge, creating an explosion that ignites the time fuse and shoots the shell high into the air.

As the shell ascends, the time fuse burns towards the burst charge. At the precise altitude - which is determined by the length of the time fuse - usually where the shell is briefly hanging in the air - the time fuse ignites the black powder burst charge, causing the shell to explode. The powerful explosion blasts apart the shell casing and ignites the stars, scattering them in all directions across the sky. These stars burn brightly and give off sparks, creating a huge spherical pattern in the sky.

Black powder

A major component of fireworks is the black powder which is a combination of about 20% charcoal, 10% sulfur and 70% potassium nitrate or sodium nitrate. Black powder burns rapidly and produces a lot of gas. If this gas is confined in a tube it reaches very high pressures. This can be used to launch a firework into the air from a « mortar » which is a tube with one open end ; like a gun or cannon. It can also be used to make a rocket by allowing the gas to escape through a small hole at one end of at tube.

Redox reaction

Redox (portmanteau for reduction-oxidation) reactions refer to all chemical reactions in which atoms have their oxidation state changed.

Like acid-base reactions, redox reactions are a matched set, that is, there cannot be an oxidation reaction without a reduction reaction happening simultaneously. A redox couple consists of two chemical compounds which are the main involved during a redox reaction.

  • An oxidizer is an atom, ion or molecule able to capture one or more electrons.
  • A reducer is an atom, ion or molecule able to release one or more electrons.

The oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction.

  • Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
  • Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

Combustion

Fireworks are based on combustion. Combustion is the sequence of exothermic chemical reactions between a fuel and an oxidizer (redox reactions) accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame. The reaction starts with a transfer of electrons between the fuel and the oxidizer. Then the fuel atoms bind to oxygen atoms released by the oxidizer and release at the same time energy such as heat and light. In black powder, the fuel is charcoal and sulfur, the oxidizer and oxygen-producer is potassium nitrate.

Rate of reaction

If black powder is confined in a closed tube or container while it burns it will burst and spread the contents of the firework out in a large ball. Black powder does not technically explode : it just burns very fast. However if it is confined while it burns it can generate enough pressure and heat to do just as much damage as an explosion. The rate at which the powder burns can be controlled by how it is made, how much powder is used, and how it is confined. That helps manufacturers control how high fireworks go when they are launched from mortars or by rockets and how large a display they produce when they burst and spread the bright burning metal compounds inside.

Light and colors

Creating firework colors is very complex and requires considerable art and application of physical science. Excluding propellants or special effects, the points of light ejected from fireworks, termed ’stars’, generally require an oxygen-producer, fuel, binder (to keep everything where it needs to be), and color producer. There are two main mechanisms of color production in fireworks, incandescence and luminescence.

Incandescence

Incandescence is light produced from heat. Heat causes a substance to become hot and glow, initially emitting infrared, then red, orange, yellow, and white light as it becomes increasingly hotter. When the temperature of a firework is controlled, the glow of components, such as charcoal, can be manipulated to be the desired color – determined upon the temperature reached - at the proper time - when the stars are ignited by the blast of the burst charge. Metals, such as aluminum, magnesium, and titanium, burn very brightly and are useful for increasing the temperature of the firework.

Luminescence

Luminescence is light produced using energy sources other than heat. Sometimes luminescence is called “cold light”, because it can occur at room temperature and cooler temperatures. To produce luminescence, energy is absorbed by an electron of an atom or molecule, causing it to become excited, but unstable. When the electron returns to a lower energy state the energy is released in the form of a photon (light). The energy of the photon determines its wavelength or color.

The energy exchanged between an atom and the external environment is quantified. That is to say it can take only certain discrete values of energy. To each atom corresponds a unique set of discrete energy levels. The level of lowest energy of an atom is associated to its stable state, called the ground state. The levels of higher energies are associated with the excited states of the atom. Thus, to each atom corresponds a characteristic spectrum and therefore a specific emission color.

For example : Strontium and Lithium are used to create a red burning firework. Calcium is used for orange while Sodium is used for yellow. Green is created by adding Barium while blue is created by adding Copper halides. A white color is emitted when Aluminum, Titanium, Beryllium or Magnesium. A gold color is created by adding charcoal or iron.