Explosions: How We Shook the World

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We see explosions all the time,

and during my career as an engineer, I've certainly made a few.

But actually understanding them and controlling all that power,

that's a whole different story and sometimes quite a surprising one.

It's a story that starts with the accidents of the medieval alchemists...

Don't try this at home.

Whoa!

..but eventually leads us to a fundamental understanding of the forces of nature...

..forces that we've mastered for good or evil.

Explosives revolutionised battlefields,

industry and engineering.

To uncover the story, I'll be reading the words of medieval scholars...

..going deep underground through ancient Cornish mines...

That looks like a lot of gunpowder to me.

..and making some of the most dangerous substances ever known.

It mustn't go above 18 degrees centigrade.

It's a journey that will take us right to the centre of matter.

- Is that a split atom?

- Oh, yes.

Wow!

And the power it can unleash.

This is the story of how we learnt to harness the forces that shook the world.

The very first record we have of people using explosions comes from

a Chinese document which could date from as far back as two centuries BC.

It describes how travellers in the mountain wilderness of the West

were threatened by shape-shifting creatures of the night.

To scare away these creatures,

they would lay lengths of bamboo on their campfires.

- BANG!

- The very first Chinese firecrackers.

CRACKING AND HISSING

The hissing noise we hear is moisture in the bamboo turning to steam,

but bamboo has a special structure to it.

It grows in sealed compartments.

Now, when the moisture in these sealed compartments

starts turning to steam, pressure builds up inside here.

It can't go anywhere.

- Water, when it turns to steam,

- BANG!

wants to expand hundreds of times, but there isn't room for it do that,

so pressure builds up.

Eventually the structure of the bamboo breaks down. Kcrrr!

It explodes,

scaring away shape-shifting creatures of the night.

Using simple natural explosions like this

was the first step of mankind's journey to harness explosive power,

starting to understand the process in order to control it.

It's easy enough to create an explosion.

Any explosion is simply the moment when gas tries to expand suddenly.

- LOUD BANG

- Oh!

And when that suddenly expanding air crashed into the air around it,

it created a pressure wave that then moves through the surroundings.

A sudden change in pressure forced a cloud of water droplets out of the air.

These allow us to see the wave.

The faster the gas is trying to expand,

the more powerful the explosion,

when that pressure wave hits your ear, you hear it as a bang.

An explosion relies on a lot of gas trying to expand.

Heat can make this happen, because heat, of course, makes things expand.

Introducing more gas can do the same thing,

but mankind discovered a way to create both heat and gas

by reacting chemicals together and this was the start

of our journey to really master explosive power.

In Europe, chemical explosions were unknown until the medieval period,

and the first time people came across them,

they were a bit shocked.

I've come to the Bodleian Library in Oxford

to see a manuscript that describes one of these early encounters.

It's one of the few copies of a book written in 1267

by the medieval scholar Roger Bacon,

who split his time between Oxford and Paris universities.

Now, this particular passage that starts "et experimentum"

describes his knowledge of man-made explosives at the time.

"There is a children's toy, something no bigger than one's thumb,

"made in many parts of the world, that is an example

"of how something can assault the senses with sound and fire.

"It is no more than a bit of parchment which contains a powder

"combining the violence of that salt called saltpetre

"together with sulphur and willow charcoal

"but the bursting of this small thing assaults the ear

"with a noise that exceeds the roar of thunder

"and a flash brighter than the most brilliant lightning."

Now, I suspect he might he may have been exaggerating slightly,

but this was the first time that anyone in Europe

had come across man-made explosives.

Roger Bacon was a Franciscan friar,

and the church at that time had envoys all over the world.

It seems likely that one of those envoys

must have posted a package back to him

containing these children's toys.

The big question is, though, where exactly did that package come from?

In the Middle Ages,

the most technologically-advanced region of the world was China.

A printed book dating from before the battle of Hastings

indicates that the Chinese were already deploying explosives

of a similar sort on the battlefield.

What we have here is a Chinese military manual

first printed in 1044

and in it, we find a recipe for a thing called

the "fire mixture" or the "fire chemical"

which contains the principal three ingredients

in Roger Bacon's recipe of 200 years later.

We start off with, here we have these two things here,

they're both forms of what we now call sulphur...

- Right.

- ..followed by various forms of organic matter

like pounded dried roots and twigs that produce the carbon,

followed by saltpetre, the next one.

The saltpetre is something you get from the decay of organic matter

in relatively warm conditions.

The Arabs refer to it is a Chinese snow.

Right, so China was just a good place at the time,

like, had the right climate for saltpetre to occur like that?

There's a lot of things that come together,

but the availability of the right climate is important.

Can I try and start assembling it in the right proportions?

Certainly. Well, roughly, you want to put in about 50% of saltpetre.

For every one of those?

Yeah, that's equal amounts of the powdered sulphur that we've got

and the powdered charcoal.

What were they trying to make at this point?

This is a mixture for parcelling up and throwing, basically,

into an enemy city using a catapult like this.

This thing here is called a hui pao, which means a fire catapult.

Now, from my knowledge of chemistry,

- saltpetre is what they call potassium nitrate.

- That's right.

And that's got kind of oxygen bound up with nitrogen inside it.

That's right. If we warm it up, it'll let the oxygen loose,

and that will aid the burning of the other ingredients.

The sulphur basically helps everything to happen at a rather lower temperature before

and ultimately, of course, the carbon is the main source of the stuff that burns.

And there we go, that's good, look at that!

Take the flame away now. See it goes. That's very nice.

That's fabulous!

I'd call that an effective incendiary, wouldn't you?

I can imagine once you get a bucket load of that landing in your camp...

- It's discouraging, isn't it? Makes you wish you hadn't come.

- Yes.

The black powder that the Chinese military were using in 1044 had got

grains of different chemicals close enough to react together

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