The Bishopsgate Bomb: Deadly mixture that sent a wave of destruction: Tom Wilkie explains how a ton of solid fertiliser became a lethal weapon when adulterated and given a Semtex detonator with organic material

Nitrogen, nitrous oxide and steam - the most abundant gas in the atmosphere, the dentist's 'laughing gas' anaesthetic, and water vapour - were responsible for the destruction in the City of London at the weekend.

What started off as a ton or so of dense, solid fertiliser, adulterated with organic material to provide fuel for the explosion, was converted in an instant into a mixture of gases. As these expanded to fill more space than the material from which they had come, they produced a blast - a shock wave pushing the air in front of it to shatter windows and damage buildings in the surrounding area.

The blast is usually attributed to the fertiliser ammonium nitrate, but manufacturers point out that, in the form in which it is sold to farmers in Britain, ammonium nitrate cannot explode. Barry Higgs, director-general of the Fertiliser Manufacturers' Association, said: 'Ammonium nitrate is extremely safe: you could put a match under it and it would not burn.'

The IRA has to adulterate the fertiliser with some organic material before it can be used in an explosive. It also has to detonate the explosion, most probably by setting off a small charge of Semtex in the middle of the load.

In an attempt to cut off supplies to the IRA, the Republic of Ireland prohibited the manufacture of ammonium nitrate in 1973, decreeing instead that farmers should use calcium ammonium nitrate, which was thought to be less explosive.

This idea was comprehensively disproved by the bomb at the Baltic Exchange last year, when calcium ammonium nitrate imported from Ireland (rather than British ammonium nitrate) was used as the raw material.

ICI, Norsk Hydro and Kemira are the three manufacturers in Britain and they make more than 1.5 million tons of ammonium nitrate a year. An additional 500,000 tons is imported from Eastern Europe. Almost all of it is used as fertiliser and the widespread application of ammonium nitrate has been responsible for the vastly increased yields Britain's farmers have secured from their land since the Second World War.

Farmers will buy 20 or 25 tons at a time, usually from an agrichemicals supplier who may have up to 200 tons of ammonium nitrate in stock. It comes in 500kg bags, or in 50kg sacks (50kg is about one hundredweight) stacked on pallets to make up a 1.5 ton consignment.

Don Martin, technical manager for Hydro Agri (UK), a subsidiary of Norsk Hydro, discounted the idea that men with, or without, Irish accents could go into an agrichemicals supplier and hand over a bundle of used notes to buy a ton of ammonium nitrate.

'The bulk of the market is through merchants who sell to account holders - to known people,' he said.