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The rhythm of the rockets

A master pyrotechnician synchronizes the dazzling fireworks with music for Boston’s Fourth of July celebration on the Esplanade

By Laura J. Nelson
Globe Correspondent / July 3, 2011

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Eric Tucker can sing the rhythm of fireworks.

After 25 years as a pyrotechnician, he knows firecrackers well enough to know their rhythms and noises.

A Portuguese Cor de Fugir shell shatters in a series of rat-a-tat pops. A Spanish Relampagos, which blooms into a bouquet of celestial colors, releases a piercing wail as it rockets skyward.

In shows across the world, including tomorrow’s 10:30 p.m. fireworks display on the Esplanade, Tucker arranges these shells and thousands more into intricate launching and firing sequences, forming a dance of sparks intended to match the pace and tone of the music. This choreography requires months of exacting work that combines design, musicianship, engineering, and chemistry.

“Doing fireworks is a one-time event, like when you turn your light bulb on and it blows up,’’ said Tucker, who works for California-based Pyro Spectaculars by Souza. “After all of that work, you destroy it.’’

The stage for a fireworks show is the night sky. Now in his ninth year choreographing Boston’s show, Tucker knows exactly how much space he has on the Esplanade: nearly a third of a mile up and across, fired from a string of barges as long as a city block.

The shells accompany a soundtrack that is hashed out months in advance by sponsoring company Liberty Mutual and members of Boston 4 Productions, the company that coordinates the fireworks display and concert that draw hundreds of thousands to the Esplanade. Last winter, the group pared down hundreds of marches, movie scores, and classic rock ballads to 11 songs.

Not every pop anthem or patriotic chestnut makes a good fireworks song. The key, Tucker said, is hearing songs visually: understanding how tempos, volumes, and tone translate to firing speed, height, and color.

“It can’t all be about Sousa marches and 4/4 time,’’ Tucker said. “We’re trying to design an arcing story that’s going to carry the audience to the end, but also surprise them.’’

Tomorrow, Yanni’s orchestral “Santorini’’ will ease the audience into the show, followed by the Cincinnati Pops’s “Yankee Doodle,’’ and Katy Perry’s appropriately named “Firework.’’

Most of the shells Tucker selects from Pyro Spectaculars’ storerooms, an hour north of Los Angeles, originate from overseas. Since trade regulations encouraged pyrotechnic production in China in the 1970s, the bulk of shells for American shows have been produced abroad, pyrotechnics specialist John Conkling said.

Now, Tucker shops at centuries-old fireworks factories in Italy, Spain, and Portugal. He visits China to create custom fireworks and watch as test displays illuminate rural rice paddies.

One of Tucker’s signature pieces, a golden sphere of sparks called a chrysanthemum, can only be made from black powder mined from a specific Japanese island. Italian fireworks are loud - “If you don’t feel it in your belly, it’s not a good firework,’’ Conkling said - and Asian fireworks focus on color, patterns, and symmetry.

Importing a firework to the United States typically costs as much as buying it, with a price tag sometimes in the thousands, said Julie Heckman, executive director of the American Pyrotechnics Association.

The choreography begins once the rockets are in the storerooms of Pyro Spectaculars. Some fireworks Tucker uses right away; others sit for years.

Since the late 1990s, pyrotechnic designers have used computer programs such as Finale or ShowSim to construct their shows, Heckman said. The programs convert music files into audio waves that make crescendos, rests, and other key musical points easier to see.

“You see what type of firework should go with what music,’’ said Conkling, a chemist who teaches at a Maryland college during the school year and gives lessons in pyrotechnics during the summer. “When the music is at its maximum, you can see where you want huge bursts going off, and when it decrescendos, you see where you want the sky to be quiet.’’

Then designers like Tucker, who has a musical theater and engineering background, can drop simulated fireworks into the program to test locations and angles.

A sense of one-upmanship pushes internationally known pyrotechnicians to seek new patterns, colors, and techniques, Conkling said. That includes the hunt for the perfect blue, the hardest color to make. A perfect indigo is the blue ribbon for firework production and design.

A cocktail of chemical compounds determines a firework’s color: sodium burns yellow, the elements strontium and lithium burn red. But the rule of thumb - essentially, the hotter the flame, the brighter the color - does not apply to the copper compounds that create blue. Copper is heat-sensitive and incinerates at a high temperature, Conkling said.

“You basically have to play ‘The Price is Right’ to get high enough in temperature without destroying the color entirely,’’ Conkling said.

Pyrotechnicians primarily choose shells based on the song’s personality and pace, Tucker said. During tomorrow’s third song, “Firework,’’ the colors will be as brassy as Katy Perry’s singing, with explosions timed to the song’s crescendos. And as 20 sound towers blare Natalie Cole’s sultry jazz hit, “Orange Colored Sky,’’ dripping weeping willow shells will gild the sky.

“Anyone can shoot fireworks and play music,’’ Conkling said. “But when you see a choreographed presentation, and the fireworks are at a peak as the music crescendos, with tiny rhythmic fireworks catching every ba-da-bump of the music, that’s a great show.’’

For example, during “The Star-Spangled Banner,’’ pyrotechnicians should ignite the requisite red fireworks three to four seconds before the line, “the rockets’ red glare.’’

“You say to yourself, ‘OK, I want the shell to unfold with the music, so it needs to be launched 15 to 20 frames earlier,’’’ Tucker said.

Databases provide information on how long fireworks take to fly and explode, and Tucker’s studio has a digital rendering of every firework. The programs compress that information into a spreadsheet, or script, that details the ignition, firing delay, and angle of each firework. This year’s script is 28 pages long.

Once the fireworks have been shipped from California to Boston, a team of 10 pyrotechnicians spend eight days following the script, installing more than 20,000 pounds of fireworks on the barges in what is one of the world’s most sophisticated firework launching systems.

Each shell has a slim micron wire that uses a computer network to talk to a central control box near the Hatch Shell. When a pyrotechnician presses play, an electric match attached to each wire gets white-hot, sparking the firework’s gunpowder and thrusting the shell out of its casing. At its apogee, the shell bursts into a familiar shower of sparks.

The night of a show, nerves run high. A dress rehearsal for a show of 15,000 shells is out of the question.

Tucker will watch from near the Hatch Shell and listen for the oohs and ahhs as blooming sparks light the Esplanade and the Charles River.

“As good as TV is about broadcasting the show, nothing beats being there live and watching it,’’ Tucker said. “Boston fireworks are a shared emotion and a shared memory.’’

Laura J. Nelson can be reached at lnelson@globe.com.