The showy science of fireworks

Dazzling displays of skyward explosions have delighted crowds for centuries, eliciting collective oohs and aahs and spontaneous applause at celebrations worldwide. From the window-rattling booms of bright chrysanthemums to the crackling of cascading willows, fireworks are a crowd-pleasing blend of science, art, engineering, and technology.

Here are four facts you might find surprising.

1. Fireworks far predate the printing press.

The first firecrackers — fireworks that produce a loud bang — are widely attributed to ancient China, most likely appearing in the first or second century BCE. Bamboo was tossed into fires to ward off creatures and evil spirits because, when heated, the buildup of steam inside the hollow areas caused the stalk to explode with a loud crack.

Firecrackers received an upgrade in ninth-century China with the invention of black powder, the earliest form of gunpowder, which contained naturally occurring saltpeter (potassium nitrate, an oxidizer), charcoal (the primary fuel), and sulfur (an ignition and burn-rate enhancer). When ignited, the flammable trio generated rapidly expanding gas, leading to powerful explosions. Packing bamboo and, later, paper with black powder, then throwing it into the fire, created even louder bangs and flashes of light.

The next few centuries brought several innovations, including twisted paper fuses, which meant firecrackers didn’t have to be thrown into fire; intricate chain reactions; and shows combining fireworks and colored smoke. Gunpowder spread into the Middle East, India, Europe, and beyond, and monarchs commissioned elaborate fireworks displays for celebrations and military victories. Around the 13th century, the Chinese invented gunpowder-propelled rockets, and aerial fireworks joined the festivities.

2. Blue fireworks are the most challenging to produce.

The next major leap in fireworks technology occurred during the Renaissance. Artisans, especially those in Italy, began experimenting with additives and packing designs, achieving different colors and burst patterns. The vibrancy of their colors might not wow audiences today, but modern fireworks are colored in essentially the same way — by adding small bits of metals or other compounds, now called stars, to the shells.

During combustion, the stars heat up, forming excited gas-phase atoms and small molecules. As electrons drop back down into a lower state, they emit photons at wavelengths corresponding to specific colors. Today, red is typically produced by adding strontium salts, orange by calcium compounds, yellow by sodium compounds, and green by barium compounds.

Of all the colors, vibrant blue is the most difficult to achieve. Blue fireworks are generated by adding copper compounds that form the excited gas-phase copper chloride species during combustion. The process, however, requires a Goldilocks temperature zone. If the combustion temperature is a little too low, the additives don’t fully vaporize and the color is dim. If it’s a little too high, the gas-phase compounds start decomposing and other emissions wash out the blue light. The narrow range is hard to hit given the high temperatures at play. Even today, you can judge the quality of a fireworks display by the purity and brightness of its blue effects.

3. The core elements of fireworks are the same now as they were hundreds of years ago.

By the 16th century, the main components of today’s aerial fireworks were in place: lift systems such as mortars and rockets that could propel fireworks into the sky, fuses, bursting charges (the center explosive in a shell), and additives that enhanced light and color.

As alchemy transitioned into chemistry and industrialization and automation took root over the next few centuries, fireworks became more stable and predictable. Novel displays relied more on engineering than craft as combustion was better understood and color additives became more predictable and refined.

The fireworks displays of the late 1800s would look and sound familiar to audiences today, although they lacked the speed and precision of computer timing and automated ignition. Colors and sounds are more vivid now, fireworks produce less smoke, and shells burst into more shapes — achieved by patterning stars around the bursting charge at the center of the shell — but the key components have long been in place.

4. Today, sounds are engineered for show.

The characteristic booms and crackles of modern fireworks displays aren’t just the byproducts of explosions. Like colors, the sounds have also been engineered for entertainment. Large shells and tightly wrapped fireworks containing flash powder, which burns much more quickly than gunpowder, are used to produce reverberating booms. Layering boom upon boom also builds drama, as seen in many grand finales.

The whistling of a climbing rocket is another technique for building anticipation. To create the sound, a special “whistling” mixture is densely pressed into one end of a long, narrow tube or rocket. Above the packed powder is an empty cavity with a small opening at the top. The mixture, which burns inward from the cavity, has a burn rate that oscillates thousands of times per second. This excites the tube’s resonance, generating the distinctive “phweeee” sound. Listen closely and you can hear the pitch decrease as the empty cavity lengthens.

Another fan favorite is the crackling that often accompanies fireworks when they rain down from the sky. This is the sound of many tiny explosions. Crackling fireworks contain encapsulated micro-particles or granules that rupture when heated, producing flashes of light and popping sounds. The abundance of these reactions is key to this soundscape.

As colorful bursts and thunderous booms fill the sky this season, consider the stories behind the spectacle: centuries of discovery and innovation by people seeking to understand — and celebrate — their world.