BEFORE YOU LIGHT THE FUSE

I cannot help but begin with a personal touch to this article. I am from India, and not a lot of days remain before the entire country starts to revel in the festivities of Diwali. Diwali is almost synonymous with the festival of lights, which includes modern electric lights, terracotta lamps, diyas, candles, and so much more. But when it comes to light, the child in me is still excited to hear the loud booms of the firecrackers, the sparkling, the crackling, immediately followed by brilliant flashes of red, green, blue and silver light. 

Fireworks are an enigma. Although they cause a lot of pollution and severely harm the natural biome, there is no denying that the best way to revel in the festivities of Diwali is to light some crackers. Being a science student, I can vouch that the true beauty of fireworks lies in their making, for it involves a drop-dead-gorgeous amount of chemistry, which the common people can be scarcely aware of. Only the perfect mixture, not a little more, and obviously, not a little less, goes into creating the spectacular show of light and sound. The sole purpose of any firework is to make some noise, a dazzling show of light, a cloud of smoke and an awe-striking display of burning stuff flying apart from ground zero. 

The principal ingredient for all kinds of fireworks is gunpowder. Yes, you heard it right. Gunpowder, as the name suggests, is a very specific explosive chemical mixture that goes down the barrel of a gun and propels the bullets toward your enemy. Gunpowder has a very interesting history, from its discovery in ancient China during the Tang Dynasty around the 9th Century AD and how it changed the nature of warfare for what we know. Over time, the original Chinese recipe has undergone various modifications. Modern-age gunpowder means a dry mixture of potassium nitrate (KNO₃), elemental sulfur (S) and carbon (C), better-called charcoal. This mixture, when ignited, undergoes a combustion reaction in presence of oxygen (O) and forms the compounds potassium sulfide (K₂S), dinitrogen (N₂) and carbon dioxide (CO₂). Gunpowder is basically a low-yield explosive, meaning it will not undergo a violent chemical decomposition nor cause any severe damage of the magnitude of bringing a building down. When gunpowder ignites, it forms a lot of gases. If confined in a small space, like inside the barrel of a gun, then the burning gunpowder and the resultant gases will exert tremendous pressure on the inner wall of the barrel and expel the projectile (bullet/cannonball in case of a cannon) with high velocity. 

Be it Roman candles, Catherine wheels, or a Kalashnikov rifle, the principal ingredient that operates them is gunpowder, also known as black powder. 
Image Credits: Photo by Wikimedia Commons 

 Gunpowder is essentially a fine granular mixture (dry) mixture of 75% by weight potassium nitrate, 15% by weight carbon/charcoal and 10% by weight sulfur. The finer the grain size, the greater the efficacy of rapid chemical decomposition. In one of my previous articles, I said that a successful chemical combustion reaction needs a sufficient supply of fuel or the combustible material that is to be burned, a good supply of oxygen to facilitate the combustion reaction, and an initial source of heat to supply the activation energy needed to jump-start the combustion reaction. In the mixture of gunpowder, sulfur is an interesting element. All elements of the periodic table have a fair share of entrancing properties. A small amount of sulfur, which has a melting point of about 115℃, can be effortlessly burned with the help of an ordinary matchstick that can reach temperatures as high as 600-800℃. When elemental (yellow) sulfur ignites, it readily melts and turns into a red viscous liquid, burning away with a brilliant blue flame in an oxygen-rich environment and producing various oxides. Sulfur undergoes an exothermic decomposition releasing a lot of heat as it burns. When the mixture of gunpowder is exposed to a naked flame supplying the necessary activation energy, sulfur ignites at first, and the heat released in the process goes on to decompose KNO₃ and detach the three oxygen atoms. This free oxygen further aids the decomposition of sulfur, which releases more oxygen from potassium nitrate and eventually sets off a sustained combustion reaction. But gunpowder is a three-phase system where the third phase is charcoal, which is nothing other than cellulose (C₆H₁₀O₅) in its pure form. When gunpowder ignites, this cellulose, viz., carbon, becomes incandescent and the three-phase powder just burns unhindered. The decomposition reaction is itself quite complex and forms a variety of compounds such as potassium carbonate, potassium sulfide, potassium sulfate, elemental sulfur, potassium nitrate, potassium thiocyanate, carbon and ammonium carbonate as solid byproducts and oxides of nitrogen, carbon and sulfur, including hydrogen, methane and water as gaseous byproducts.

In standard scientific jargon, gunpowder deflagrate. That is, instead of exploding suddenly and violently, gunpowder burns at subsonic speeds where the burning flame propagates steadily through the mixture. It only explodes if confined in a very small place and then ignited. Burning gunpowder generates a lot of gases, and by the laws of physics, a typical container can only withstand a certain amount of pressure depending on its material. If the pressure gets too high, then the container will explode. The gases will rush outwards in all directions resulting in a rapid compression of the surrounding air medium and a characteristic bang based on the strength of the explosion. In a gun or a cannon, a small amount of gunpowder is stuffed behind the projectile (bullet or cannonball), where the gases develop, and the only way for them to escape is to push the projectile forward. This is also the underlying principle behind the skyrockets. All we need to do is bring some gunpowder in a case, attach a fuse and leave behind a small opening at the place of the fuse for the gases to escape. Next, this fuel head is attached to a suitably hard stick. As the fuse burns through the hole, it ignites the gunpowder inside the fuel head. The downward exit of the gases generates an upthrust which lifts the rocket from the ground following Newton's third law.  

But our fireworks are not just booms and bangs. We see various colours of light - brilliant red, bright green, flaming yellow, fiery orange, peacock blue, golden rain, and so much more. This is where the magic happens. What goes into fireworks is a tweaked and complicated mixture of fuel, oxidizer, and other additives called the pyrotechnic mixture⁽¹⁾. The pyrotechnic mixture contains the principal fuel to be burned, i.e., various kinds of metal salts, metalloids, metal hydrides and chlorides, followed by carbon-based fuels or organic chemicals and polymers. The pyrotechnic mixture further contains oxidizers since fireworks can not avail oxygen in the air. These oxidizers typically include perchlorates, chlorates, nitrates, permanganates, chromates, oxides and peroxides, sulfates and some organic compounds. In addition to fuel and oxidizers, the pyrotechnic mixture also needs additives like binders to hold keep the dry mixture together, colourants for the red-green-yellow fires, coolants to bring down the temperature of the burning mixture and also to slow down the reaction rate, catalysts to ensure an efficient burn, stabilizers to control the burn, etc. 

Flame tests of different metal salts. From left to right: LiCl, SrCl₂, CaCl₂, NaCl, BaCl, trimethyl borate, copper chloride, cesium chloride and potassium chloride
Image Credits: Wikimedia Commons

The colour of fireworks depends on the metal salts they contain. When metals get hot enough, they start to lose their valence electrons and form metal ions. These ions recombine with the oxygen in the air to form metallic oxides. When a typical metal atom is heated, the individual electrons in the atom gain thermal energy and are transported to higher energy states known as excited states. Since it is the natural tendency of a dynamical system to stay in the lowest-possible energy state, the electrons return to their original energy level and release the excess thermal energy in the form of visible radiation. Depending on their position in the periodic table, different elements emit different frequencies of visible light, characteristic of their electronic configuration. Sodium emits a bright yellow light corresponding to the D1-D2 transitions at the wavelengths of 589 and 590 nm. In this same manner, Strontium chloride (SrCl₂) emits a brilliant crimson to scarlet red indicating presence of Sr-ions. Calcium chloride (CaCl₂) emits a brick-red light indicating the presence of Ca-ions. Cesium chloride (CsCl₂), copper salts, lithium chloride and barium chloride, to name a few, respectively emit blue-violet, blue-green, carmine red and pale apple-green colours. 

The green stars which we see in a typical star-shell firework come from a very specific mixture of barium salts and copper salts, while the red/orange sparks are due to fine particles of some solid incandescent material being burned in the air. To those who are familiar with standard physics, all of our fireworks are possible because of quantum transitions of electronic energy states that results in visible luminescence.  
Image Credits: Photo by pixnio

Fireworks come in various types, from aerial rockets and the big intimidating shells to ground based pinwheels, flower pots, fountains, shots and you all know the type. Due to a limit of space and time, I have no other option but to describe only the following three kinds. 

Two burning sparklers 
Image Credits: Photo by pexels

• Typical sparklers contain granules of aluminium, magnesium or their alloy magnalium (5% Al, 95% Mg) to produce the signature ''sparks'' that are usually bright white. Iron and titanium are separately used for orange and white sparks, respectively. Their alloy, ferrotitanium (10-20% Fe, 40-55% Ti and carbon), is used for golden-yellow sparks. The sparkler mixture has to have sulfur and charcoal as fuel followed by necessary oxidizers such as KNO₃, Ba(NO₃)₂, Sr(NO₃)₂, KClO₄ and NH₄ClO₄. Chlorides and nitrates of barium, strontium, sodium and others may are added for various colours. Finally, all this dry mixture needs a combustible binder like dextrin or nitrocellulose to mould them along the sparkler wire. 
• Fountains contain a hard cardboard casing, generally cylindrical and filled with a pyrotechnic mixture. The cylindrical case is sealed with clay at the base, and on the top, there is a similar seal fitted with a fuse, a nozzle or a choke. In India, these fountains are made from clay terracotta pots conical to spherical in shape (like a frustum) with an opening at the top and sealed with clay at the bottom. The pot is then filled with a desired pyrotechnic mixture. A fountain behaves very much like a solid rocket booster. As the fuel burns, gases try to exit through the nozzle and lift some of the burning material out in the air to get the desired fountain effect. Fountains come in many forms, including coloured ones and crackling types. The crackling comes from the presence of little granules that explode with a crackle. Here I must say that making the Indian fountains are a little tricky. If the mixture is pressed too hard, if any air leaks through because of a small crack in the pot or if the seal is not perfect, the whole thing will explode midway with a deafening bang. 
• Star shells or aerial shells are a five-stage arrangement. It includes a spherical shell that contains the pyrotechnic stars. This shell is placed at the bottom of a long cylindrical housing unit called the mortar shell. The base of the cylinder is sealed off tightly and hardened to hold down everything. At the base, the shell sits over some gunpowder with a fuse sticking out from the side. The gunpowder acts as the lift charge to propel the shell towards the sky. The shell itself contains a delay fuse which is ignited as soon as the shell launches, and the delay fuse burns for a few seconds, during which the shell reaches its desired height which may be over a hundred feet, depending on the specifications. The aerial shell contains a burst charge that explodes the shell in the air and causes the pyrotechnic stars or dragon balls to burn and explode in a specific pattern. 

Now that I have, at length, talked about the chemical composition and the making of fireworks, it is time I sound the clarion bell. I must say there was a time when I was totally crazy about fireworks. I used to burn a lot, and in one particular year, I got so many fireworks that it took almost three days to burn everything. Back then, I never had the faintest idea regarding the massacre I was doing by lighting dozens of skyrockets, shells, fountains and whatnot. I knew nothing about the health hazard and the toxicity of barium or lithium chloride. When I got into high school, I took physics and chemistry. That is when I realised my reckless act. Since then, I have vowed not to touch a single firework for the rest of my life. Instead, I would like to raise awareness about the stuff that goes into our fireworks.

 Being a science student, I have had the opportunity to learn that no matter how fascinating they look, fireworks are not all glory. Every time we light up the fuse and our rockets go boom in the night, we sound the death sentences of countless species of birds, insects, small or big animals, and not to mention our future generations. With every single skyrocket, we fly, we are endangering the microbiome and making ourselves vulnerable to partial deafness, restlessness, and breathing disorders. Firecrackers create a deadly smog that can blanket an entire nation for days. Fireworks are never good. And in light of climate change and anthropogenic global warming, we must renounce our old habits. We can always come up with a hundred different ways to celebrate a festival or the winning of our favourite soccer team. Instead of shouting for a ban, I would humbly request all to take your hands off of fireworks. If our one minute of joy kills a million birds, let me ask a direct question, is this what we are supposed to do? 

Bibliography: 

1. https://blogs.scientificamerican.com/observations/what-makes-them-go-boom-our-favorite-explainers-on-the-science-of-fireworks/
2. https://wwwn.cdc.gov/TSP/PHS/PHS.aspx?phsid=325&toxid=57
3. https://www.forbes.com/sites/grrlscientist/2019/12/31/festive-fireworks-create-harmful-pall-of-pollution/?sh=51b1924f2853
4. https://theconversation.com/explainer-the-science-of-fireworks-21293
5. https://www.nationalgeographic.com/environment/article/the-hidden-toll-of-july-fourth-fireworks?loggedin=true