Have you ever wondered: what’s the mysterious process that you initiate by just lighting a candle? Where does the wax go, and why dripless candles don’t drip?! Well, I did ask that to myself more than a couple of times, and after having come up with totally naive conclusions, I deep dove into the science of candle making. Here are my findings to you.
When we light a candle, the wax is melted (liquified) then transformed by the flame’s heat into vapour. When the candle burns steadily with its iconographic teardrop-shaped flame, the combustion is extremely efficient. This combustion makes the solid wax dissolve into the air through a process of vaporisation.
The so-called “dripless candles” are made slightly different from the other candles because they have a thicker external layer made of higher temperature melt wax that becomes like a shell for the other internal layers of wax that will melt into liquid. This external shell of wax holds the liquid near the flame, which, consequently, keeps generating heat, making the melted wax slowly evaporate.
Let’s now dive into the nitty-gritty of the chemical reaction (soft-mode don’t worry) to discover where and how wax disappears.
What Does Actually Happen When I Light a Dripless Candle?
We’ve now understood that dripless candles have this external layer of wax that takes slightly more time to melt, allowing the liquid (lower temperature melted wax) to be held without dripping on its sides.
Yes! It’s a chemical reaction. But don’t worry I’ll make the explanation as simple as…burning a candle!
The flame’s heat let wax molecules evaporate, and they react with the oxygen in the air. As long as the wax doesn’t melt away from the flame, the flame will consume it completely and leave no ash or wax residue.
The flame radiates light and heat, but only 25% of the energy from the combustion becomes actual heat. It means that the flame has to maximise its combustion to keep the wax evaporating.
It’s the heat of the flame that keeps the reaction alive using the wax as fuel. The candle’s flame generates heat that continues to vaporise the wax. The chemical reaction is over when the heat is not warm enough to melt the wax or if there’s no more wax to burn for the combustion.
The vapour released in the air holds water letting the air itself feeling drier when a candle is burning.
It almost feels like we need some bullet points, what do you think? Here the step by step phases of what happens to the wax when we light a cande:
- The flame burns the wax-coated wick
- By its combustion, the flame generates heat
- The heat melts the internal layers of solid wax
- The melted wax is held into the external layer of wax
- The wick absorb a bit the liquid wax to fuel the flame
- The heat of the flame makes the wax evaporate, eventually
OK, that’s it, but what the majority of the time happens, to me at least, is that the so so-called dripless candles do drip! Shall we blame chemistry? or wax? Let’s find out.
Why Do Some Candles Drip Then?
We talked about the chemical reaction that happens when we light a candle, but chemical reactions do happen within the environment in which the elements are immersed, and sometimes, maybe most of the times, these environments aren’t appropriate to allow the chemical reaction to go as smooth as it should be.
The environment around our candle is what influence the drip of a “dripless” candle – if we consider that the candle is made properly, without any defects.
Before dive into the environmental causes, it’s worth mentioning, that the shape, the wick and the temperature in which the candle has moulded its key
As we have mentioned above, dripless candles have a higher temperature melting point for their external layer, to allow holding the liquid wax instead of letting it drip, if this step isn’t done properly it may cause the drip.
It’s also important to notice that different wax, melt at a different temperature, for example, paraffin wax melts way easier than bees or soy wax, letting a candle based on it more likely to drip down its sides.
If the candle is pretty narrow, the liquid that can hold within its external layer is less, so if the heat released by the combustion isn’t optimised for that small cup, the flame can ruin the balance letting the wax dripping on the candle sides.
Lastly, the wick plays a key role as well, and it’s important to have a balance between its thickness and the actual diameter of the candle.
Let’s now consider some environmental causes that may let your dripless candle, drip!
- Candle position: if the candle doesn’t sit straight up and it has some kind of angle, the liquid may easily drip down.
- Air Draughts: we know that everything is generated by the flame, but air draughts may compromise its burning performance. We want to have our flame in its teardrop-shaped form to have a nice 360-degree heat propagation. If a breeze moves it around, the wax is melted unevenly and the external layer may likely be melted before its time, causing the drip.
- Other heating sources: if we light several candles close by, the heat generated by all the flames may increase the melting chances causing more wax to melt and all the candles to drip!
We’ve been playing around with terracotta pots last summer and we found that those containers absorb the liquid wax causing potential risks, check out the article here.
Now, let me share with you a couple of things I discovered – slightly more general, but worth knowing!
Cool Fact: What’s the Difference Between Vapor & Gas
By studying the fascinating chemical reaction behind the simple act of burning a candle, we incurred a couple of topics worth clarifying if you want to have a deeper scientifically understanding of the whole process.
The most important one that I witness requesting many times related to candles is the difference between vapour and gas.
There is a common misunderstanding that sometimes we make, and it’s generated by the word “gaseous”: when melted wax evaporates, it produces water vapour that indeed has a gaseous form, but it’s not gas, it’s vapour.
A gas it’s something that remains permanent in its “gaseous” form while vapour can/will eventually condense changing its “gaseous” form into liquid one, and after a while when it cools down, solid.
If we can capture the vapour from a burning candle and let it rest somewhere, we will witness its transformation in liquid and then in solid wax again.
In this video below, Bill Hammack experiments with this concept.
Nerd Corner: the Chemistry Behind Wax Vaporization
OK, maybe I got too far with my research, but you know when I get into the binge research tunnel it’s always difficult to come out of the wagon.
Let’s get back to our beloved burning candle, and it is wax! So when the flame activates the wax, its molecules start interacting with the oxygen in the air around and below the flame.
The interaction, it’s a proper chemical reaction, the molecules of wax dancing in the air with the oxygen start producing three different things:
- water vapour
- carbon dioxide
- and of course heat & light
If you have ever tried to put a glass or a jar on top of a candle, you could have easily experienced that when the oxygen contained in that jar runs out, the flame expires.
Like us, burning candles needs air to keep nurturing the flame.
If you are into chemistry, I found an amazing blog that you can check out – it’s not about candles but chemistry in general – but there was a post that quite more complicated was explaining what I lay down above!
Below a neat visualisation of the molecules of wax and oxygen that take part in the simple yet mysterious reaction in which the wax evaporates under our eyes.