At first glance, the stacks of cardboard boxes and little brown paper parcels stuffed into the re-inforced steel storage space belonging to Pyrovision might look innocent. But the names emblazoned across the packaging – silver dragons, red coconuts, glittering fish – send a tingle of excitement down the spine. When the fireworks concert approaches, these unassuming packages will be loaded onto a truck and driven hundreds of miles before being placed with precision around Edinburgh Castle, their fuses finally lit, allowing them to explode, crackle and fizz into the sky, all in perfect time with the Scottish Chamber Orchestra.

Until then, they remain on standby at Pyrovision’s HQ at Chestnut Farm in Lincolnshire, where ducks and geese wander through the yard, unconcerned by their proximity to vast quantities of explosives. It’s up to the company’s creative director Wilf Scott and his pyrotechnics partner Keith Webb to offer a visual interpretation of the score (this year including works from Gershwin and Barber), punctuating notes and phrases, changes of mood and key with carefully chosen arrangements of fireworks.

But they only start on one condition. ‘We work when the conductor agrees that he will play it exactly the same as the recording we’ve been given,’ explains Scott. ‘If he’s busy he will lie through his teeth when he won’t actually have listened to it. And that’s when it all goes a bit topsy-turvy on the night.’ For Scott and Webb, timing is everything. If either the orchestra or the fireworks are out, the co-ordinated choreography they are aiming for won’t come off. They are full of tales of experienced conductors who suddenly got cold feet at the thought of more than 250,000 people watching from vantage points across Edinburgh or tuning in on the radio. ‘I’ve seen grown men shaking,’ says Scott, slyly.

Webb pulls out last year’s ‘fireworks score’, full of baffling squiggles, rows of boxes and lines, and points out the timings, configurations and positioning of the fireworks. ‘They’ll find this in years to come, and think it’s some kind of mystic religion,’ laughs Scott. This score was for Prokofiev’s Romeo and Juliet, a set of delicate suites which meant they opted for gentle flare effects. This year, the audience can expect a change. ‘American music tends to be livelier and that helps us to be inspired to create some more unusual things,’ explains Webb.

Scott, an artistically scruffy-looking 60-year-old who lives in Cambridge with his wife and three daughters, first got involved in fireworks as an art student looking for effects for a film project and progressed to providing pyrotechnics for Pink Floyd. Webb, who is 45, got hooked while employed as a teenager to help out at a Battersea Park show and has kept fireworks in the family with the elder of his two teenage sons joining him in Edinburgh.

A recent highlight for both men was the Queen’s Silver Jubilee Concert from Buckingham Palace. ‘One of the scariest moments of my life was about half an hour beforehand,’ recalls Webb. ‘I popped my head over the roof, saw all the people and realised what we were doing.’ Their hard work paid off, and Scott was made a Member of the Victorian Order in 2002.

Still, there have been near misses along with the successes. Scott remembers one Edinburgh when the casing accidentally blew up, cutting the power. In blind panic he improvised to save the show. ‘We were lighting fireworks with cigarettes just to get them up.’

Bank of Scotland Fireworks Concert, Princes Street Gardens, 0131 473 2000, 2 Sep, 9pm, £10–£25. More tickets released 23 Jul, 26 Aug.