Practical Pyrotechnics (Happy Birthday, Good Omens!)

The novel, Good Omens, was first published on 10th May 1990.

Today (10th May*) is the thirtieth anniversary of the release of the book Good Omens, which is an old favourite of mine, and one I’ve found science-based excuses to write about before. In honour of the day, I’m going to do it again—but this time I’m going to talk about fire.

Fire plays an important role in both the book and the acclaimed television adaptation. Of course, fire is rather easier to do in a novel, since reading words like “fire” and “flames” are generally quite safe. In TV land, however, it’s a bit trickier. In particular (spoiler alert), at the start of episode five, the bookshop owned by the angel Aziraphale is burning when Crowley arrives and walks in. Crowley, after all, is a demon. From Hell. Fire can’t hurt him.

Except, of course, he’s actually the lovely David Tennant, who is a very much not-fireproof human being. Which poses a few questions: did the film crew really set the bookshop set on fire? Did they really make David Tennant walk into a burning building? How is that done safely? And what did they actually burn?

It turns out that they did, in fact, burn down the bookshop set. According to The Nice and Accurate Good Omens TV Companion, director Douglas Mackinnon “wanted a real fire” and “there were thousands of books, tapestries and beautiful grandfather clocks inside the shop that were real.”

Actual books were harmed in the making of Good Omens (photo used with permission).

Which… argh. Actual books. In flames. I might be a bit traumatised. Give me a moment.

Anyway. The thing is, if you’ve ever set fire to paper you’ll know it’s not very controllable. You can’t just burn books and achieve consistent and, more importantly, safe, flames. The Good Omens TV Companion goes on to explain that the set was rigged with gas lines and flame bars. It doesn’t say what the fuel was, but the probable candidate is propane.

This is where we get to the chemistry. Propane is a hydrocarbon—a molecule made of hydrogen and carbon atoms—and the “prop” part of its name tells us that it contains three carbon atoms. The “ane” part tells us it’s an alkane, and from that, handily, we can work out its formula without having to do anything so mundane as look it up, because the formulas of alkanes follow a rule: CnH2n+2. In other words, take the number of carbons, multiply it by two, add two, and you get the number of hydrogen atoms. This gives us three carbons and eight hydrogens: C3H8.

Propane’s boiling point is -42 oC, meaning it’s a gas at room temperature. You may be familiar with propane canisters which slosh when moved, suggesting liquid, and that’s because the propane is under pressure. The only real difference between a gas and a liquid is the amount of space between the individual particles. In a liquid, the particles are mostly touching one another, while in a gas there are large spaces between them. If you take a gas and squash it into a small volume, so that the particles are forced to touch, it becomes a liquid.

Propane is stored in pressurised canisters (photo used with permission)

But once the propane is allowed to escape from the confines of a pressurised container, at room temperature, its molecules spread out once again, into a gas.

The expansion is BIG. Theoretically, at room temperature, one litre of propane liquid (with a density of 493 g/litre) will expand to occupy roughly 270 litres of space. But, of course, the space it’s expanding into also contains air, so the volume of flammable mixture—approximately 5% propane to 95% air—is actually much higher.

Gases burn faster than either liquids or gases. We know this, of course: it only takes a brief spark to light the gas burner on the cooker hob, for example, but you’d struggle to light a liquid fuel with the same spark (unless it was warmed, and therefore starting to vaporise). The reason is those big gaps between molecules: each molecule in a gas is free, none are “buried” in the middle of a volume of liquid (or solid), so they can all mingle freely with oxygen (needed for combustion) and they all “feel” the heat source and become excited more easily.

Propane is a hydrocarbon with three carbon atoms.

Apart from being a gas at room temperature, propane is also chemically very safe in that it’s non-toxic and non-carcinogenic. It’s also colourless and odourless—although small amounts of additives such as the eggy-smelling ethyl mercaptan (ethanethiol) are sometimes added as a safety precaution, to make leaks more noticeable.

Mechanically there are more hazards. There’s a significant temperature drop when a pressurised liquid expands into a gas. The simplest way to think about this is to think of temperature as the energy of all the particles in a substance divided by its volume. If the volume increases while the number of particles stays the same, the energy is spread out a lot more, so the temperature drops. Potentially, a sudden release of too much gas near a person could severely chill their skin, and even cause frostbite. Plus, of course, although propane isn’t toxic, if it displaces oxygen it could cause asphyxiation, and it’s heavier than air, so it tends to accumulate in the bottom part of a room—precisely where people are trying to do pesky things like breathe.

Yellow flames, and smoke, are a sign of incomplete combustion (photo used with permission).

Then there’s the issue of complete combustion. Generally, when hydrocarbons burn they produce carbon dioxide and water as products, neither of which are too much of a problem for nearby humans (up to a point). However, when there’s not enough oxygen—say, because the fire is inside a building—other products form, in particular carbon monoxide, which is very toxic, and carbon particles, which make a terrible, terrible mess.

I mentioned earlier that a flammable mixture is about 95% air to 5% propane, and this is why. In fact, it’s even more precise than that: for propane to burn cleanly it should be 4.2% propane to 95.8% air. In industry terminology, if there’s not enough propane it produces a “lean” burn, where flames lift from the burner and tend to go out. If there’s more propane (and thus not enough oxygen) it’s called a “rich” burn, which produces large, yellow flames, soot, and the dreaded carbon monoxide.

They did burn the bookshop. But it’s OKAY, it was restored again at the end! (Photo used with permission.)

You might, of course, want a certain amount of yellow flame and smoke, to achieve the right look, but the whole thing needs to be carefully controlled to make sure no one is in danger. It’s all manageable with the use of properly checked, monitored and maintained equipment, but you can imagine that a big effect like the bookshop fire needs a very experienced professional to oversee everything.

For Good Omens, that was Danny Hargreaves (of Real SFX), who’s worked on all kinds of projects from War of the Worlds to Doctor Who. As he says in the Good Omens TV Companion, “everything is under control [but] we took it right to [the] limit.” At one point, he says, he turned off gas lines sooner rather than later and, when director Douglas Mackinnon asked why, had to explain that the roof was about to catch fire.

So, yes, they burned the bookshop set. But it’s all right, everyone. It’s all right. Because (another spoiler) thanks to the powers of Adam Young, everything was restored again afterwards. Phew. All the books were saved. Shh.


*Funnily enough, everyone thought the anniversary was 1st of May. Including the whole Good Omens team. So they made a brilliant lockdown video** to mark the occasion and celebrate. And then it turned out it was actually the 10th. Just an ordinary cock-up, as Crowley would say.

**Which proves the bookshop, with all its books, was fully restored, doesn’t it? Told you.


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Want something non-sciency to distract you? Why not check out my fiction blog: the fiction phial. There are loads of short stories, and even (recently) a couple of poems. Enjoy!

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Why you should buy a carbon monoxide detector, right now.

So after my April the 1st ‘warning’ about PHMEA, this one is for real.  I mean really.  Don’tco hazard ignore this and, more importantly, make your kids read it if they’re living away from home as well.  This might just save your life, or theirs.

As I write this tragic story is being reported by most major news organisations: Kelly Webster and Lauren Thornton die on Windermere boat.  It appears that they died of carbon monoxide poisoning.  Stories like this turn up with terrifying regularity.  The dangers of carbon monoxide are on the GCSE Chemistry syllabus, and every year before I teach it I search for ‘carbon monoxide’ on google news.  And every year a list of recent, tragic stories appears.

In fact doing it now, apart from the story mentioned above, there is:-

Stevenage man hospitalised with carbon monoxide poisoning (posted April 2nd)
Landlord fined over carbon monoxide poisoning (posted April 2nd)
‘Buckwild’ star Shain Gandee likely died from carbon monoxide poisoning (posted April 2nd)

And so on.  It’s always like this.  And the terrible thing is, these incidents are completely avoidable.

So what is carbon monoxide and where does it come from?  It’s a simple molecule, made of two atoms: carbon and oxygen, CO.  Like its more familiar cousin carbon dioxide, it is colourless, odourless and tasteless.  Unlike carbon dioxide, it is deadly.

Why?  Because it bonds, very strongly, to the iron in haemoglobin. Haemoglobin is thehemoglobin-carbon-monoxide molecule in your blood, specifically in your red blood cells, that transports oxygen around your body.  Without this mechanism, oxygen can’t get from your lungs to your muscles, or your brain, or any of those other important bits and pieces.

Without oxygen, everything stops working pretty quickly.

Normally what happens is that oxygen bonds to the haemoglobin, and this tidy little package (in the red blood cells) gets transported to wherever it needs to go.  Then, crucially, the oxygen is dropped off so that it can get to work doing useful stuff, namely, respiration.

The trouble is that when I say carbon monoxide bonds strongly to haemoglobin, I mean REALLY strongly.  So strongly that it’s a one-way process.  Oxygen can no longer attach to that molecule of haemoglobin, and ultimately that red blood cell becomes useless.  If that happens to enough red blood cells, your oxygen-transport system stops working, like a tube train network where all the train doors are permanently jammed closed.

And then?  Essentially, you suffocate.

If you’re exposed to a tiny amount of CO, for example from smoking a cigarette (bad habit, don’t do it), a few cells are affected but you survive – in the short term at least – because your body helpfully replaces your red blood cells with shiny new ones every 48 hours or so.

But if you’re exposed to a slightly higher level, and it doesn’t have to be that much, over a sustained period of time you will be poisoned.  It only takes 20-30 ppm (parts per million) over a period of a few hours.  2000 ppm for one hour will leave you unconscious.

Unhelpfully, the symptoms in the early stages are not unlike viral symptoms.  They include a headache, feeling sick, feeling tired and foggy-headed and having stomach pains.  I say unhelpfully, because  what would you do if you started getting symptoms like this?

You’d probably take a couple of paracetamol and go to bed.  And that’s where the trouble begins, because the CO is probably in your house.  And you’ve just shut the killer in with you.

How did it get there?  Well carbon monoxide forms when carbon-based fuels burn in limited oxygen.  Carbon based fuels include coal, wood and natural gas.  If you have a gas boiler, a wood stove, or even an open fire, you need to make sure you’re not in danger.  In the case of Kelly Webster and Lauren Thornton the problem appears to have been a faulty generator on their boat, which was probably burning diesel or petrol (gasoline).

If these fuels burn in plenty of oxygen, the only things that are produced are carbon dioxide and water.  As I mentioned, carbon dioxide is fairly harmless (obviously if you breathe in a LOT of it you’re in trouble – it will lead to something called hypercapnia – but this is very unlikely to happen under normal circumstances).

But these fuels burn in restricted oxygen, carbon monoxide forms.  Here are a couple of equations which make it quite clear.  I’m using methane, CH4, here – the primary ingredient in natural gas, because it keeps things nice and simple.

2CH4 + 4O2 –> 2CO2 + 4H2O

2CH4 + 3O2 –> 2CO + 4H2O

2CH4 + 2½O2 –> CO + C + 4H2O

Look down the list: the big numbers in front of the CH4 and the H2O are staying the same each time.  The numbers in front of the oxygen, O2, are going down, and as they do the products on the right hand side are changing.  We start at the top with just carbon dioxide CO2, and water.  If there’s a bit less oxygen we start to get carbon monoxide, CO.  If there’s less still, we also get a bit of pure carbon, C, which is essentially soot.

So carbon monoxide forms when fuels burn in limited oxygen.  Normal air is about 20% oxygen, so usually there should be more than enough.  That is, if your boiler is maintained properly and your chimney flue is clean and clear.  And that’s the key.  Make sure it is.  If you have an open fire, get that chimney swept by a professional.  If you have a wood stove, keep it clean, check the seals and, again, get the chimney swept.  Generators should be regularly serviced and checked.  If you have a gas boiler get it serviced yearly by a Gas Safe registered engineer.  Not your mate’s mate who knows a bit about boilers.  A proper Gas Safe engineer.  I realise this costs money, but this is your life on the line here.

If you’re renting it’s your landlord’s responsibility to get all gas appliances, including the boiler, inspected every year. By law they must also provide you with a copy of the CP12 Gas Safety Certificate.  Ask for it.  Make sure your recently-moved-out son or daughter has asked for it.  If they don’t provide it, insist.  If you’re not sure it’s legitimate, get it checked out.  If you have trouble, report the landlord.  They can be given a hefty fine for not following this procedure.

Regular servicing is important but it is possible that a fault could still occur.  The crucial thing, the thing that could simply and cheaply save all of these unnecessary tragedies, is this: get a carbon monoxide detector.

It should be marked with EN 50291 and also have the British Standards’ Kitemark or another European approval organisation’s mark on it. Carbon monoxide alarms usually have a battery life of up to 5 years, but it goes without saying that you need to check regularly as you do smoke alarm batteries.  A good alarm can be purchased for £15-£20.  It’s an expense, but it’s worth it – this piece of kit could save your life.  If your child is off to university and living in rented accommodation, buy them one and make sure they have a supply of batteries.  Amazon sells them.  Why not order one right now?

Don’t assume that just because it’s a new boiler (or other appliance) it’s safe.  New boilers can have problems too.  They can be badly fitted, and sometimes even a new boiler develops a fault.  Get that alarm.

If the alarm goes off:

  1. turn off the appliance(s)
  2. open the doors and windows
  3. call a qualified engineer before turning the appliance(s) back on
  4. if anyone has symptoms (headache, nausea, breathlessness) get them outside and seek immediate medical attention

In the UK 50 people die every year from carbon monoxide poisoning, and every single one of those could have been prevented if they’d had a working alarm and paid attention to it.  Every single one.  It’s really simple.

Don’t be one of those news stories.  Get a detector.