How do candles work?
Things I Mean to Know is a series of blog posts capturing my journey of getting to the bottom of things I’ve always wanted to know or understand, but never quite did. It’s inspired by the podcast episode Things I Mean to Know from This American Life.
In this first entry: Candles. How the hell do they work?
Back in middle school, I learned that any flame needs three components: fuel, oxygen and heat. I was also told that for a candle, the candle wax is the fuel, and for some reason I never quite believed that. If the wax is the fuel and therefore flammable, how come the candle doesn’t burn up all at once? And how can it be that the wax is melted by the same flame it sustains? For years I’ve gone on with my life, simply accepting that I didn’t quite understand the mechanisms behind these seemingly simple objects. But today, all of this is going to change. For myself and anyone else who doesn’t quite get it (god, I hope I’m not the only one), I’m going to lift the curtain on candles!
The basics
To my relief, I found that candles are actually quite intricate devices. For starters, they utilize all three states of the wax — solid, liquid and gas — in different ways. And surprisingly, the wick of modern candles has been designed specifically so that people don’t have to cut them off every five minutes.
But, first things first: what makes a candle burn? As stubbornly as I refused to believe it, the wax is indeed the fuel for the candle’s flame. And a candle is actually not much more than a solid mass of wax with a wick running through it. The top of the wick sticks out, and is usually covered with a thin layer of wax to get the flame started.
Once this happens, the fire will melt the top bit of wax, creating a puddle. This liquid wax will be drawn up through the wick, through a process called capillary action — the same mechanism through which sponges can soak up water from a surface, seemingly against the forces of gravity (more on this later). Once at the top of the wick — and thus basically inside the flame — the liquid wax is heated until it evaporates, at which point it can be ignited by the heat of the flame and become a part of it. (This is also why candles smoke when they’re extinguished: the heat of the wick vaporizes some of the wax without igniting it. You can verify that the smoke from candles is vaporized wax by igniting it.)
This process sustains itself and keeps going until one of the components of the flame (fuel, heat or oxygen) is removed — the fuel if you let the candle burn up, the heat if you decide to blow out the candle, and the oxygen if you use one of these fancy candle extinguishers.
That is, in brief, how candles work. And at this point you might be satisfied. If that’s the case, thank you for reading, and I hope you’ve learned a thing or two about the basic workings of candles. (If not, I’m sorry I wasted your time with this nonsense.) However, if you’re like me and still have a few questions, please read along.
Questions, answered
Q. If the candle wax is flammable, how come candles don’t burn up all at once?
Short answer: because the wax only burns in its vaporized state.
Long answer: once again I’ll have to put my ignorance in plain view, because I had no idea that only gases can burn with flames. If you’re anything like me, you might not even believe this at first. “So what about gasoline?” you may ask. “And what about wood?! Don’t tell me that wood doesn’t burn!” Well… (I’m so sorry) wood doesn’t burn. Not really. In both cases, it’s the gases from the substances that burn, not the liquid or solid substance itself. In the case of gasoline, it’s the naturally occurring gasoline vapor that produces the flame. And wood only starts producing a flame when it has been heated to such an extent that the molecules in the wood decompose into different elements, which in turn evaporate and burn. Without these flammable gases, the wood would only smoulder and not create a flame.
Q. How does this “capillary action” exactly make the wax go up the wick?
Short answer: the wick is porous and “absorbs” the liquid wax, replenishing the wax from below as it evaporates and burns at the top.
Long answer: candle wicks, much like sponges, are prone to absorb liquids. This is because they are made of braided cotton which makes them porous. Porous basically means “having lots of narrow spaces,” and this is important because liquid candle wax has strong adhesive properties, which means that it really likes sticking to certain surfaces. In narrow spaces, these adhesive forces between the liquid and the wall can become stronger than the forces of gravity acting downward on the liquid, causing it to move up into the narrow space. So in the case of our porous candle wick, it only makes sense that it can make liquids move upward.
Now usually, capillary action gradually slows down and eventually stops as the porous material becomes more and more full. But remember that in candles, the liquid wax evaporates and burns once it reaches the top of the wick. This creates depressurization at the top of the wick, and thus a way for the capillary action to keep happening at a steady rate.
Q. So if the wax is the candle’s fuel, what happens to the wick itself? Does it also burn?
Short answer: Yes, most candle wicks are designed so that they burn themselves up.
Long answer: in the olden days, candle wicks were significantly less smart. They required regular trimming to be kept at a somewhat equal length, a chore for which people designed specialized wick trimmers.
Nowadays, most people don’t need wick trimmers anymore, and that’s because candle wicks have become much smarter. They are braided with a flat braid, which makes them curve as they’re burning. This ensures that the tip of the wick dips outside of the flame, giving it enough oxygen to burn up and thus keep itself at a constant length.
Well, that’s about it. Okay, so I definitely didn’t tell everything there is to know about candles. But I did answer my own curious questions. I have to say that this process of digging into a subject and filling that gap of knowledge in my head has been really satisfying. There will definitely be more of these in the future, so feel free to suggest any questions of your own! Currently at the top of my Things I Mean to Know-list are:
- Why does human hair seems to grow forever while practically all animal hair doesn’t?
- What’s the deal with Israel?
- What exactly is “identity politics”?
I wonder.
Sources:
(Interesting sources for further reading in bold)
- “Candle” — Wikipedia
- “Find out What Happens to Candle Wax When a Candle Burns” — ThoughtCo.
- “Candle Science & Information” — National Candle Association
- “Comment about candle smoke by krystar78” — Reddit
- “If wax is the fuel in candles, why doesn’t it burn up all at once?” — Quora
- “Only gases burn with flame. But when you burn wood, it initially burns with a flame. Later, it only glows without flame. Why is it so?” — Quora
- “Ignition Temperature of Gasoline” — The Physics Factbook
- “Capillary action” — Wikipedia
- “Capillary action and why we see a meniscus” — Khan Academy
- “Candle wick” — Wikipedia
This article was originally published on 21–01–2018 on my own website.
