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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Five science facts we learned at school?

This week a post called ‘Five Science ‘Facts’ We Learnt At School That Are Plain Wrong‘ popped into my Facebook feed from a few different sources.

It led to more than one argument, and the unearthing of some interesting titbits. Most of these facts aren’t directly about chemistry, but hey, still interesting. Let’s have a look:

We’re taught we only have five senses: smell, sight, hearing, touch and taste
True enough that there are more than five, but I clearly remember being told in school that balance and pain were also senses, so I’m fairly sure biology teachers have been quietly trying to dispel this one for decades.

plastic paperclips

Non-magnetic paper-clips. Ha!

Which of the following are magnetic: a tomato, you, paper-clips? (Answer: all of the above)
I think this is a misleading question. What do you mean when you say ‘magnetic’? I think most people understand that to mean something that’s capable of being magnetised or at least is attracted to your everyday fridge magnet. In other words, the ferromagnetic materials: iron, nickel, cobalt and most of their alloys. True enough tomatoes and people interact with magnetic fields (this is the basis behind MRI scanners – check out these beautiful images) but does that make them magnetic? We-ell….technically…. (there are lots of types of magnetism) but it seems a bit mean to criticise an assumption by asking a less-than-clear question about it. Besides, if you’re going to be pedantic about it, what’s that paper-clip made of hmm? Plastic and aluminium (both generally considered to be completely non-magnetic) paper-clips exist. Bad question. Next!

CMYKThe true primary colours for paints and pigments are cyan, magenta and yellow
Broadly fair enough, look at your printer cartridge. Although we really ought to include black as well (which the original article didn’t mention; it’s the K in the CMYK model). You can make something pretty close to black by mixing the others, but it’s not the nice, crisp, blackest black that people want for text and outlines. All that said, to actually get red from a mixture of magenta and yellow you have to have pretty pure pigments. Grab a paint box and try mixing something that looks like magenta with something that looks like yellow, and you’ll actually get something that looks like orangey-pink (serious artists agree that if you want really bright red, you’re better off just buying some red pigment). Whereas if you mix blue paint with yellow paint you will, fairly reliably, get green of one shade or another. I just worry that attempting to clear this one up is going to cause a lot of children to mess up their paintings. That’s all I’m saying.

A little addition here: this question then led to a debate about the colour spectrum of visible light. How many colours are there, exactly? It’s commonly held that Newton invented the colour indigo because he felt, possibly for superstitious reasons, that there ought to be seven colours. As a result, some people will tell you the spectrum actually consists of six colours rather than seven: red, orange, yellow, green, blue and violet. But hang on. Look at a spectrum (here’s one):

600px-Spectrum

What’s that colour in between blue and green there? You might say turquoise, but in a return to the original question it’s more accurately named cyan. That band is pretty obvious. I’d argue that if you’re going to include orange in the spectrum, then you ought to include cyan. And, in fact, some people think that’s exactly what Newton was doing. Except he didn’t call it cyan, he simply called it blue. The bit we think of as blue is what he named indigo. In other words, the spectrum is, in fact: red, orange, yellow, green, cyan, blue, violet. Still seven colours, they just don’t quite fit with the whole Richard Of York Gave Battle In Vain thing.

Of course, those of us in the know are aware that there are actually eight colours. But you need to have octagonal cells in your eyes to see the other one. Or be a cat.

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Debunked in 1974. Still hanging around like a bad smell or, er, taste.

Tongue taste maps are nonsense
Yep. This one is unambiguous: there aren’t regions for sweet, salt, bitter etc. on your tongue. This was debunked back in 1974, but it’s still hanging around for some reason.

There are more states of matter than just solid, liquid and gas
Ah-ha, a chemistry one! Again, this is true. The strict states of solid, liquid and gas are fine when you’re talking about elements and pure, fairly simple, compounds (water, for example), but matter can indeed take other forms. There are ‘liquid crystals‘ – you’re probably reading this right now using some – and yes, there’s plasma. Once you get into mixtures all bets are off (no, you can’t melt wood, sorry). And colloids are a whole other kettle of fish.

But I think this is one of those times where you have to ask yourself why are we bothering to talk about solids, liquids and gases in the first place? Is it purely so that students can memorise three words? No. It’s so that they can go on to understand the concepts of melting and boiling, and their partners freezing and condensing. These ideas are critical to understanding ideas of measuring temperature as solid liquid gaswell as what happens to particles when they warm up (or cool down). Adding other technical terms in at this early stage is just likely to cause confusion. I don’t think that learning about the transition from solid to liquid to gas precludes later learning about liquid crystals, colloids and the like (hey, it’s how I did it). You’re just adding more information to a simple model, and someone studying A-level sciences and beyond ought to be capable of dealing with that. No harm, no foul, I say.

So there we have it: less “Five Science ‘Facts’ We Learnt At School That Are Plain Wrong”, and more one thing your teacher probably tried to correct you on, one misleading question, one thing you might have learned incorrectly at school, and a couple that might be technically untrue but it doesn’t really matter that much in the long run. But I suppose that IS less of a snappy title for an article.

Truth, Justice, Freedom, Reasonably Priced Love, and a Hard-Boiled Egg.