As the legend goes, it all began with bamboo shoots and evil spirits.
When fireworks originated in China, they were not much more than tubes of bamboo packed with black powder. Still, their blast was so ferocious that firecrackers were thought to dispel demons.
Now, 1,000 years later, throngs of visitors are expected to line the banks of the Charles River Wednesday to see pyrotechnicians blast shells into the night, painting the Boston skyline with resplendent sparks.
The art of fireworks has evolved dramatically in the last millennium, from the fields of Asia to Renaissance workshops in Italy. Modern displays like the annual Boston Pops Fireworks Spectacular have benefited from years of advances in chemistry and circuitry.
“It’s truly a synthesis of thousands of years of effort,” said John Steinberg, former president of the nonprofit Pyrotechnics Guild International, which brings together amateur and professional fireworks enthusiasts.
About 80 percent of the shells used in the Boston show are spherical, 3 to 10 inches in diameter, with a design that dates back hundreds of years, said Jon Berson, pyrotechnics line producer for Pyro Spectaculars by Souza, the California company that organizes the display.
Though technology has allowed the art of fireworks to progress, it remains rooted in history and tradition. Just as in early China, most pyrotechnics still contain black powder, with the shells made by hand.
“If you go to China and go look at the factories, it’s a lot of people sitting there, pasting things by hand, and it’s not super high-tech,” Berson said.
Time-honored classics like elegant weeping willows and effulgent peony blasts are still featured in the Boston performance.
But even traditional fireworks now use purified chemical compounds that burn in richer color than ever before. Dazzling sparks of federal blue and scarlet red shower the Charles each year in shades fit for an American flag. And every shell is fired by computer, in fluid sequence with song.
“Between the sophistication in the shell manufacturing and the sophistication in the firing, that’s really where the technology has moved the art,” Berson said.
The fireworks are shot from mortars on barges floating in the river. During the week and a half before the Fourth of July, each shell is carefully arranged on deck and attached to circuits that fire in time with the music that envelopes the Esplanade.
Show designer Eric Tucker coordinates a roughly 25-
minute pyrotechnic display with pop hits and patriotic standards. This year, when “Higher” by Taio Cruz and Travie McCoy plays midway through the celebration, fireworks will arc ever more steepply into the night sky.
It was, of course, not always thus. In times past, torches or flares lit fuses, an inexact ritual offering no guarantee that the sharp crackle of fireworks matched the concussive strains of the “1812 Overture.” But the invention of electric matches and computer firing systems has allowed the seamless melding of the visual and the aural.
“If there are any issues that they can’t light something or they light something off early accidentally, that could cause issues with the timing of the entire show,” said Chris Mocella, coauthor of “Chemistry of Pyrotechnics: Basic Principles and Theory.” “But with electronics, if it’s set up right, it should flow as intended.”
Pyrotechnicians marry fireworks and music to produce a transcendent experience.
“What you want the person to do is emotionally move beyond, ‘I’m watching a fireworks show,’ to be sort of emotionally engaged with what’s going on and sort of forget in some senses that it’s a fireworks show and just think of it as spectacular,” Berson said.
And what could be more spectacular than the colors? Until the 19th century, fireworks were just flashes of light with an orange hue from the charcoal in black powder.
That changed in 20th century chemistry labs, where pyrotechnicians studied wavelengths and perfected elements that burn in vivid colors, like the radiant green of barium and the deep red of strontium.
With big shows like the Fireworks Spectacular (it uses about 17,000 pounds of explosive material), Tucker has a wide palette that ranges from magenta to mint green and includes textured finishes, such as gold trails that shed sparks like comets.
That palette will be on full display Wednesday night with variegated blasts of green, yellow, and purple during Henry Fillmore’s march, “The Footlifter.” When Jason Mraz’s delicate ballad, “I Won’t Give Up” plays, Tucker said shimmering waterfalls will coat the sky like a brocade of gold.
Revelers along the Charles this year will also see patterned fireworks in shapes such as hearts, Berson said. The first pattern shells were purple hearts and yellow bows that welcomed troops home from Operation Desert Storm, said Julie Heckman, executive director of the American Pyrotechnics Association.
Such devices explode according to how stars, which burn visibly as sparks in the sky, are laid out around the bursting charge in a shell. The stars are actually clusters of chemicals that cling to a seed at their core. Berson said compounds are rolled around a seed similar to how cotton candy is rolled around a stick.
The Aqua Jellyfish, which contains silver and aqua stars in the shape of a dome and legs, epitomizes the pattern shells that will pirouette across Boston’s night sky, Berson said.
“You get the dome of the jellyfish in those silver stars, and then you get these big legs sticking down like tentacles off the jellyfish,” he said.
Yet, even after centuries of transformation, a fixture of any good show remains unchanged: an adoring audience. Wednesday night, more than half-a-million people are expected to sit on blankets on the Esplanade and crowd roof decks to marvel at explosions in the sky.
“It just hits so many senses,” Mocella said. Fireworks are “so mesmerizing that even with advancements in technology, I think people are always fascinated, and they have been since the first fireworks came out, since it was just black powder.”Zachary T. Sampson can be
reached at zachary.sampson@