Fire art: a guide to the science behind fireworks, the grandest display of chemistry in the sky.
THE ANATOMY OF AN AERIAL SHELL:
Aerial shells are the most common type of firework. Cylindrical or spherical in shape, they act as packaging for black powder and "stars" the key ingredients for a fireworks show.
Black powder, also known as gunpowder, is a low explosive typically composed of 75 parts potassium nitrate, 15 parts charcoal and 10 parts sulphur and is a staple in all fireworks. It is packed both in the centre tube of the firework and in between the black lumps known as stars, helping them to ignite. The finer the powder, the quicker the fireworks will burn. Using a coarser grain and not blending the ingredients too much creates a display that hangs longer and brighter in the sky. This is because when the ingredients are not blended well and the grain of the black powder is coarser, the fuel and oxidizer do not combine or burn as easily. For fireworks that are really sparkly, big grains approximately 1,000 microns in size are used.
Unlit, the show's headliners appear to be simple black lumps. Hundreds of these "stars'" are packed into the average firework and once lit, they create the dazzling colours and lights seen in the sky. Firework experts make them by hand by combining an oxidizer, fuel, binder and compounds that produce colour (usually metal salts).
The oxidizer is included to speed up the burning time. Potassium nitrate is the most commonly used oxidizer. When decomposed it becomes potassium oxide, nitrogen gas, and oxygen gas.
2 KN[O.sub.3] [right arrow]) [K.sub.2]O + [N.sub.2]+ 2.5 [O.sub.2]
Small amounts of chlorates--which are highly volatile--are sometimes used as the oxidizer, creating very explosive fireworks. Since this presents a manufacturing hazard, perchlorates, which have a much higher ignition temperature, are more commonly used.
The fuel in stars is most commonly charcoal and the binder used is most commonly dextrin. Very temperamental, stars can easily explode if they are hit hard and workers must wear cotton head to toe because synthetic materials may generate electricity which could set off the shells and stars.
Multibreak shells provide more variety in the show and have a more complicated shell design. They can be constructed with multiple sections, each with their own fuses. One section bursting triggers the next one to burst. Each section produces a distinct effect such as different colours or levels of brightness and sparkle.
There are at least two legends behind tile invention of fireworks. One story takes place roughly 2,000 years ago in China, when a cook happened upon the recipe for black powder. Mixing charcoal, sulphur and saltpeter, all common ingredients in the kitchen at the time, an amazing explosion occurred.
Another story credits a monk named Li Tian, who lived in China 1,000 years ago, with the invention of the firecracker. Legend has it that he filled bamboo shoots with gunpowder and the result was a loud sound and explosion. At the time the loud sound produced was the more desirable effect as it was believed to ward off evil spirits. A temple in Liuyang, China is built in Tian's honour.
Marco Polo is believed to have brought fireworks to Europe in the thirteenth century, after bringing the black powder recipe back from China. In Europe, fireworks were being used for military purposes by the fourteenth century and were a popular form of entertainment by the seventeenth century. Both Germany and Italy took the lead in innovation. Germany focused on scientific innovation, and Italy on elaborate fireworks shows.
Fireworks came to North America as early as the seventeenth century when Captain John Smith (from the story of Pocahontas) set off fireworks in the Jamestown Colony (present day Virginia) to try and impress the Powhatan tribe.
THE END PRODUCT
Brightness of the firework is dependant on how high the temperature gets, and the characteristics of the materials used. Sparks happen when the metals found inside the mixture or at the burning surface are heated to temperatures at which they incandesee. The size of the fuel particles is what determines how big the sparks will be.
The pattern in which the stars are packed into the aerial shell determines the shape you see in the sky when the firework explodes.
It is hard to control the orientation of the shell when it shoots into the sky. To correct this, experts are trying to control the tendency of the fireworks to rotate in the sky once launched. One expert proposes securing a light rope onto the bottom of the shell in order to keep the firework down. This might make spelling something in the sky possible.
SHAPE : Willow
DESCRIPTION: Long burning stars fall into the shape of willow branches.
DESCRIPTION: Sphere-like shape appears in the sky resembling the flower and leaving a trail.
DESCRIPTION: This looks like the chrysanthemum, but with a middle that is different colours from the outside.
Bursts into small tubes that make random paths, may have exploding stars at the end.
The variety of bright colours that are seen at fireworks shows were not always available. Fireworks once came in very basic orange hues because they were made with charcoal. Once other elements were added to fireworks, a variety of colours started appearing in firework displays. Fireworks produce colour through both incandescence and luminescence.
THE ILLUSIVE DEEP BLUE
There are medium quality blues, but there is no chemical species discovered that emits an intense blue colour. The element copper produces blue but not an intense one because it's not stable enough at the high temperatures that produce intensity of colour. At a high temperature, copper continues to burn but washes out and becomes white.
Traditionally, shells are launched from steel tubes called mortars, into which the fireworks are snugly packed. The steel tubes, which are usually placed in sand, are the same diameter as the firework but three times longer. Inside the mortar is black powder, known as the lifting charge. When lit it explodes, causing the shell to launch while at the same time lighting the shell's fuse.
The more black powder packed into the mortar tube, the higher the firework will launch. The larger the shell, the coarser the grain of powder used for the launch, since larger grains help to build the pressure up slowly to burst the shell. Obtaining the desired altitude requires a lot of trial and error in order to develop the appropriate type and amount of powder.
Now, fireworks can be launched using compressed air and then detonated in the sky using an electronic timer. This eliminates the need for a fuse and allows the explosion to be more exact, and as a result, shows can be coordinated to music.
COLOUR COMPOUND RED Strontium, Lithium ORANGE Calcium GOLD Iron with carbon YELLOW Sodium GREEN Barium BLUE Copper PURPLE Strontium and copper SILVER Aluminum, titanium, magnesium
Fireworks have been linked to elevated levels of perchlorates in the environment. A 2007 U.S.
Environmental Protection Agency study examined perchlorate levels in an Oklahoma lake that held fireworks every fourth of July. The study found that fourteen hours after the show, the perchlorate levels had risen 1,000 times past their natural levels. This is problematic because perchlorates have been linked to thyroid problems. Perchlorates have also been viewed as a threat to aquatic ecosystems, as fish living in waters considered contaminated have been found with several thousand parts per billion of perchlorates in their heads, and hundreds of parts per billion in their fillets. These fish have also been found to have thyroid conditions including follicular hyperplasia, hypertrophy, and colloid depletion.
Another environmental problem chemists are trying to reduce is smoke emissions from fireworks. Some success was achieved using carbon compounds instead of salts which produce a lot more smoke. There is ongoing research to produce bright colours that are a lot less smoke-producing than current technology.
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|Title Annotation:||CHEMISTRY: PYROTECHNICS|
|Publication:||Canadian Chemical News|
|Date:||Jul 1, 2010|
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