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A burst of colour from Sunday evening's display in Edinburgh

The first festival of its kind took place in 1947 and for the past 27 years, a firework display from Edinburgh castle has impressed crowds craning their necks towards the ramparts, which rise from a central promontory of rock.

On Sunday 2 September, 250,000 sets of eyes and ears experienced the combined effect of over 100,000 fireworks and more than 50 musical instruments. But how was the show choreographed to satisfy both senses?

"With synchronicity, sympathy and symmetry", said pyrotechnician Keith Webb, 46, whose company Pyrovision designed and executed the 35-minute display.

As pyrotechnical director of the show, his role was to scrutinise his team's timing and that of the Scottish Chamber Orchestra, and to make any immediate adjustments to ensure the fireworks exploded in time to the music.

THE ANSWER Codes are called out by a score reader, alerting a 'firer' to send electronic pulses which ignite the fireworks

Festival ends with a bang

His was one of four essential timekeeping roles: a conductor to lead the orchestra, a score-reader to call out a series of firework codes, a "firer" to ignite the fireworks and a director to compensate for any timing differences.

"The challenge is to call the code fractionally before the cue, to take into consideration the time it takes for my call to register with the person setting off the firework and for him to press the button," says score-reader Andrew Moore.

He had to call several of the 198 codes, each corresponding to a particular set of fireworks, two or three seconds before the cue, to allow for their "rising time" and "burst delay".

It's a job that demands lightning-quick reflexes, says firer Gary Dunn, who compares it to a "sprinter setting off on the gun".

Mr Webb explains his role thus: "If, for example, the conductor speeds up, I might overrule the score-reader's call, telling the firer to kill a set of shells; otherwise you could end up with loud banging in a quiet section of the music."

Silver waterfall

Several tricks of the trade enable him to deal with such situations: lighting a firework at both ends will decrease its duration, as will igniting a second firework before the first has finished, creating a layering effect.

The display was 'scripted' over 24 pages

"We used Roman candles to mimic the military beats of the snare drum in Gershwin's Strike up the Band," says Mr Webb.

Fireworks were also used to enhance the tone of a piece. During Barber's Adagio for Strings, which was poignantly used in the film Platoon, silent flashes of colour flared from the castle battlements, echoing "the emotional pull" of the piece.

A silver waterfall, which has become the centrepiece of the annual show, was also supposed to cascade down the north face of the castle during this piece. However, a technical fault meant that the 1,250 volt charge failed to ignite the length of 70 upside-down fountains.

In order to cover the visual gap which would have been left, Mr Webb overruled the score-reader's call, setting off the next set of fireworks early. The fault was repaired and the waterfall was ignited during the last minute of the finale.

"Each year, I hear people saying the waterfall was their favourite, and it gives me a secret buzz," says Warren Griffiths, who erected the specially-designed effect.

Funny fireworks

And there's even scope for humour. A series of hummer mines, which make a "sliding" sound were used to tell the story of a cowgirl slipping from her horse in Copeland's Buckaroo Holiday. A firework, which exploded into the shape of a horse's tail, was also used for a "literal visual image".

THE SHOW IN NUMBERS More than 100,000 fireworks, equalling four tonnes 35 minutes of music 250,000 spectators 56 musicians 24-page firework script 198 ignitions Finale: 133 fireworks in 3 secs

The pyrotechnical interpretation of Ives' Three Places in New England however was punctuated by a series of seemingly random flares erupting from the battlements to mimic the "disjointed" nature of the music.

"It's often difficult pieces of music like this which are our most successful. A fresh challenge pushes you to come up with the best ideas," says Wilf Scott, of Pyrovision.

A choreographed firework display takes several months' planning. Having laboured for four weeks preparing the material, a team of 14 spent seven days installing the fireworks at the castle.

After listening to the music "about 100 times" and sourcing specific firework effects over a period of months, it took a fortnight to design the show.

While pyrotechnicians can prepare for many eventualities, they cannot predict the weather. Conditions are monitored every hour during the week before an event and on the day of the show, test shells are exploded and the debris monitored to determine the wind strength and direction.

These findings are passed to the police, ambulance service, and council health and safety officials, who may decide to cancel the display, relocate spectators or remove some of the larger fireworks.

Fortunately this year's choreographed display was not hampered by the weather and the famous waterfall, though somewhat delayed, was yet again on everyone's lips.