Explosives - don't try this at home

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Capture of an explosion
Image caption,
High-speed filming reveals patterns caused by the detonation of a "shaped charge" for the first time

From firecrackers to nuclear bombs, it is only recently that scientists are beginning to understand the complexities of explosions, and many of our big discoveries were more by mistake than design.

When it comes to explosions, I am generally no fan of accidents.

This history of accidental discovery started in a book dating from the 9th Century, where there is one of the first written recipes for something containing the chemical ingredients of black powder... or gunpowder.

The book is called "Classified Essentials of the Mysterious Tao of the True Origin of Things" but the interesting passage, when translated, is in a section clearly headed "do not try this at home".

Whether, as it would now, this was really meant to encourage popularity of the book is not clear.

Of course, I had to try it.

The recipe is from an attempt by alchemists to produce an elixir of life. It not only tastes horrendous but, when cooked as directed, it has one of the most surprising and unpredictable reactions I have ever seen.

There is an often-repeated myth that the Chinese, after inventing gunpowder, used it purely for entertainment for hundreds - even thousands - of years.

In reality, the military quickly saw the potential of what was then an incendiary weapon.

Written in 1044, the compendium Wujing Zongyao is believed to be one of the first documentations of gunpowder used by the military. It records three different "recipes" for gunpowder - two for bombs to be launched by catapult and one for a poison-smoke bomb.

Militaries soon developed it further - by putting it in a small space to increase the speed of reaction and propel projectiles out of tubes.

Image caption,
Strange things happen when you follow ancient Chinese recipes marked "don't try this at home"

Low explosives, like gunpowder, are pretty powerful but high explosives which were the next big accidental discovery are truly in a different league.

In 1846, European chemists were experimenting with nitration - a chemical technique which typically involves treating substances with a concentrated mixture of nitric and sulphuric acids.

They were trying to discover the chemical composition of everyday materials such as sugar and cotton through their reactions. But they discovered instead that when nitrated, many carbon-rich substances formed compounds with very surprising properties.

The Italian chemist who discovered nitroglycerine received a face full of glass when he dipped a hot wire into it.

He begged others not to repeat his mistakes but the explosive potential of these substances was too great a temptation and of course many other accidents were reported.

Nitroglycerine turned out to be so dangerous that its manufacture received an international ban and its use put on hold until somebody figured out how to tame it.

Eventually both nitroglycerine - in the form of dynamite - and rival guncotton were made safe enough to use as everyday explosives. And it was a commercial block of stamped guncotton that led to yet another accidental advance in the story of explosives.

Shaped charges

It is an accident that can be recreated with a rubber stamp and a sheet of modern high explosive.

When detonated next to a sheet of metal, the impression of the stamp is transferred into metal, but not in the way one might first imagine.

Instead of the metal following the contours of the embossing, the deepest impressions in the explosive make correspondingly deep impressions into the metal. Like a curved mirror, the shape of the explosive can focus the resulting shockwave.

A tiny quantity of modern high explosive, with its immense power concentrated and shaped in such a device, can achieve the most astonishing results.

"Shaped charges" are often used to devastating effect in warfare to penetrate armour.

Other applications range from the neat cutting of metal to safe bomb disposal.

It is amazing how much our knowledge and understanding of explosives have changed in the last 150 years but also fascinating how much we still have to learn.

Because the conditions in which things happen are so fast and at such extremes of temperature and pressure, it can be difficult to pin down exactly what happens in that tiny fraction of a second.

Now technological advances are allowing us to see - using high-speed camera footage - what happens in these explosions for the first time.

Explosions: How We Shook the World is on BBC Four, Wednesday 13 October, 2000 BST and will be available to watch again on iPlayer.

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