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I have an Ikea candle which has sat on my bookshelf in the sun for >5 years. Aside from an hour or two shortly after I bought the candle, I have not burned the candle regularly (in fact, the wick is broken off at the moment).

After sitting in the sun for a few years, the wax has begun to crawl up the sides of the glass jar the candle is sold in. How does the wax crawl up the jar? I know that the outsides are certainly growing up, rather than the centre falling down.

Originally after the candle was lit, the centre was depressed but no wax had begun to climb up the sides. It was only several years afterwards that I noticed the wax crawling behaviour. So it is not anything to do with candle being lit.

The candle looked like this when I bought it:

original candle

My candle now looks like this:

side view top view

How does the wax have such a strong attraction to the walls of the jar, and how does it flow, given that it is a solid? Where does it get the energy from?

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    $\begingroup$ Could it be due to the condensation of evaporated candle wax? $\endgroup$
    – Farcher
    Commented May 18, 2016 at 22:26
  • $\begingroup$ I believe you, but I think you should buy a new sample (if it's still available) and perform the initial burn to prove that your 5 years old observation is correct. $\endgroup$
    – Agent_L
    Commented May 20, 2016 at 10:59

7 Answers 7

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Candle wax expands considerably when hot and molten. So while burning the candle the level in the glass rises.

But when the candle is extinguished the outer region (nearest the glass) cools down quicker (candle wax doesn't conduct heat very well) and solidifies first, becoming immobile. The molten remainder then shrinks before solidifying.

So it's the temperature gradient (from outside to inside) and the preferential solidifying from outside to inside that causes the outside material to be higher up in the glass, after full solidification.

Here's a corroborating experiment almost anyone can carry out. Allow a cup candle (even a small tealight candle will work) to burn for a sufficiently long time, so a large molten puddle has formed. Now gently extinguish the flame and allow the candle to cool down and solidify undisturbed. The originally flat solid surface will have become convex.

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    $\begingroup$ I know that this didn't happen when I originally burned the candle. The marker spot on the side of the candle was the maximum mark from a few months ago, and the wax has risen above it now. $\endgroup$
    – hifkanotiks
    Commented May 19, 2016 at 7:13
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    $\begingroup$ @bearbin: You don't need to light the wick on fire to cause temperature changes in the wax. Your house changes temperature all the time! Can your candle see the sun, by any chance? $\endgroup$
    – Lightness Races in Orbit
    Commented May 19, 2016 at 10:21
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    $\begingroup$ @bearbin: to some extent the formation of a concave surface after burning a candle for some time and then extinguishing it happens each time. Candle rally expand quite a lot when it melts. I made some cup candles when I was young and obtained very concave surfaces because I poured the wax when it was too hot. Candle makers do indeed warn against that: pour the wax as cool as possible to avoid uneven solidification/shrinking on cooling and thus a very concave surface. $\endgroup$
    – Gert
    Commented May 19, 2016 at 13:59
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    $\begingroup$ @A.L The wax, as it heats up, but especially as it melts, expands considerably, much more than a typical ceramic or metal. This expansivity or coefficient of linear expansion is often written as ɑ and is very small for most materials. Metals have an expansivity in the region of 10-20 parts per million per °C, while the expansivity of ceramics is still smaller, between 5 and 10 parts per million per °C. Organic solids have higher expansion coefficients, around 100 parts per million per °C. Waxes have even larger a values, sometimes several times larger than this, ... $\endgroup$
    – Basic
    Commented May 21, 2016 at 16:20
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    $\begingroup$ @A.L (cont) ... and many expand several percent over 50°C. However, the expansion that results when a substance goes from a solid to a liquid state is greater than figures of this sort, being on the order of 10 percent or more for many waxes. From The Ultimate Book of Saturday Science (p179) $\endgroup$
    – Basic
    Commented May 21, 2016 at 16:21
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Yeah, I would guess that the alternate heating/cooling of the wax in the sun pushes it up the side of the glass. Presumably the surface tension between the wax and the glass is quite strong and holds the wax up once it's been pushed up. Subsequent cycles cause wax to "backfill" the wax that's been pushed up.

It would be interesting to design an experiment to learn more about the characteristics of the process.

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  • $\begingroup$ I suppose this would explain the asymmetry of the wax - it is significantly higher on one side and I assume that was the side closer to the sun? $\endgroup$
    – hifkanotiks
    Commented May 19, 2016 at 12:57
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    $\begingroup$ @bearbin If the asymmetry formed while the candle was burning, another explanation would be that air currents caused the flame to not be centered over the candle. $\endgroup$
    – Todd Wilcox
    Commented May 19, 2016 at 15:41
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The energy comes from the sun, that much is certain.

One possible mechanism is capillary action resulting in a meniscus, assuming the sun heats the wax to a (near) liquid state.

The other possible mechanism is a vaporization/redeposit cycle. During the day, heat from the sun creates wax vapor, which is heavier than air [citation needed] and therefore stays within the confines of the jar. At night, when the jar cools, the wax vapor is deposited on the sides of the jar.

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    $\begingroup$ @Gert You seem to questioning the OP's powers of observation. If what happened happened during that single burn, would not the OP have seen that? $\endgroup$
    – user3386109
    Commented May 19, 2016 at 2:06
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    $\begingroup$ Please don't put words into my mouth. OP is asking for an explanation. He obviously didn't observe any sun effects either. It's possible that he didn't take any notice and only later started asking himself what happened. Perception and memory are notoriously fickle. $\endgroup$
    – Gert
    Commented May 19, 2016 at 2:18
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    $\begingroup$ @gert I did observe some sun effects. Several months ago I put a marker on the side of the jar at the highest point of the wax, and the wax has risen above that point now. When I had the candle alight the wax did not flow up the sides. $\endgroup$
    – hifkanotiks
    Commented May 19, 2016 at 7:16
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    $\begingroup$ I would think capillary action is not in play here; if we compare gravitational forces $\rho g L^3$ vs capillary forces $\sigma L$ for $\rho\approx 1000\!\left[\mathrm{kg/m^3}\right]$ and $L\approx 10^{-1}\!\left[\mathrm{m}\right]$, it requires a surface tension of $\sigma\gg100\!\left[\mathrm{N/m}\right]$ which seems unlikely for wax. By comparison mercury-air interface only has a $\sigma\approx0.5\!\left[\mathrm{N/m}\right]$. $\endgroup$
    – nluigi
    Commented May 19, 2016 at 8:04
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    $\begingroup$ @gert, well at least one of us isn't reading what the OP has said. $\endgroup$
    – Lamar Latrell
    Commented May 19, 2016 at 23:41
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It is entirely possible that the entire candle has melted in the sun. If this happens the molten wax has a significantly greater volume than the solid wax candle and so the whole level will rise up the glass ie liquid wax takes up more volume than solid wax.

As it cools again the level will fall as it solidifies. Typically it will cool from the top and sides first as these are what is exposed to the air and can lose heat most easily. In fact wax will generally form a semi-solid skin on its exposed surface as it cools. This gradually sinks under its own weight as the bulk of the wax cools and contracts but will remain stuck to the sides of the glass at its edges at the level it was when it was completely molten.

To put it another way it is not so much a case of the edges of the wax 'climbing' the sides as the whole level rising and the edges being left behind as it re-solidifies.

You can actually watch this happening over a few minutes if you fill a container with melted wax and let it cool.

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    $\begingroup$ If the entire candle had melted, the central depression left by the first-and-only burn would not have been preserved. $\endgroup$
    – hmakholm left over Monica
    Commented May 20, 2016 at 15:46
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My hypothesis is this. When the Sun heats up the wax, the wax gets soften or melt. The surface tension between the glass wall and the wax then drag the wax up the wall.

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Once the wax is heated with solar radiation during the day, it expands in every direction in the cup. When it cools down during the nights, the center wax returns back but the wax adjacent to the cup faces another attractive force called adhesion. Adhesive force is the force between different molecules in contact. So, my explanation for the concavity of the wax is because of the existence of adhesive force between the wall molecules of the cup and molecules of the wax in contact with the wall, part of the wax facing the wall sticks to the glass cup wall (Click here for more on cohesive and adhesive forces). But the central wax doesn't face the adhesive force and hence returns back when cooled during the night. Repetition of this phenomena (sticking molecules to the wall) for a long time (day and night) results the shape we look at now.

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The heat of the sun probably caused the wax to expand resulting in it crawling up the walls

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    $\begingroup$ Indeed? You certainly earn points from your English professor for rephrasing the question... $\endgroup$
    – user113515
    Commented May 20, 2016 at 2:33

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