Summer Fun 2014 - Candle Science For Teens and Tweens - Why Can You Burn a Wrapped Crayon, But Not a Crayon Candle?

"A candle wick works by capillary action, conveying ("wicking") the fuel to the flame. When the liquid fuel, typically melted candle wax, reaches the flame it then vaporizes and combusts. The candle wick influences how the candle burns".  From Wikipedia, the free encyclopedia.

Late last week, we made a lovely crayon candle for Flag Day, that unfortunately didn't burn...

...prompting us to take extreme measures, borrow a non-Crayola recommended life hack, and light our actual crayons on fire.  Our crayons burned beautifully, but left us with the question - why do crayons burn, when crayon candles will not?

A little bit of Googling led us to a suggestion that a crayon candle might burn if mixed with paraffin wax.  We had a few left-over birthday candles on hand, so we melted them down with our crayon candle, fishing out the old wicks...

...and pouring our melted wax, and crayon into a Dixie cup, with a fresh wick.

Even with the additional candle wax added, our new crayon candle still did not burn for more than a few seconds. As soon as the wick burned down to the wax, the flame sputtered, sparked, and went out.

We weren't too surprised.  While our candle was hardening, we had watched a clip from How It's Made on crayons.  Crayons are made with paraffin wax, so adding more of the same was probably not going to help us.  However, crayons also have pigments, and other powders, and synthetic chemicals added in.

David Fisher at About.com suggested that the pigment in the crayon does not melt with the wax, but floats around in it, and clogs the wick.  That sort of made sense to us, but we still weren't sure.  We needed more information.

Navigating by Joy has a fantastic post all about candles, and why they burn.  We also found an excellent video by Bozeman, science teacher, Paul Andersen, explaining the same thing. Both suggest a similar demonstration to show that a candle flame is burning vaporized gas.  It's simple, and fun.

Take a lit candle, and blow it out.  Then, relight it, by lighting the smoke, rather than the wick.  In other words, blow out a candle, like the birthday candle below, and move it near to a flame.  It will relight...
 

...before the wick touches the flame.  What your doing is really igniting the gas from the wax, that is still in the air around the blown out candle.

We tried lighting the smoke from our crayon candle, immediately after it went out, by bringing a flame near to it, but nothing happened.  We had to touch the flame of our birthday candle right to the wick of our crayon candle before the crayon candle would relight.  There didn't seem to be any gaseous wax in the smoke.

If the candle making page of About.com was correct, and the pigments from the crayons were clogging the wick, then we wouldn't expect any gaseous wax.  If liquid wax can't get to the flame, it can't really be vaporized, or ignited.

That made sense, too.  Just to be sure, we decided to make a quick candle out of paraffin wax alone, to test our wicks.

That candle wasn't very pretty, but it burned bright, and steady to the last.

So, the problem was with the crayon, and not with our particular wicks.  At this we turned our attention back to the burning crayon.  This time we lit one wrapped crayon, and one empty crayon wrapper. 

 We had a well ventilated room, and a fire extinguisher on the ready, and felt safe - but we are NOT in any way recommending for anyone else to try this, and have read that the folks at Crayola do not recommend it either.

Okay, with that said, what we observed was really interesting.  First off, the empty wrapper barely burned at all.  We had expected it to go up in a poof of flame, and smoke.  There was a lot of smoke, but very little flame.

It wasn't so much burning, as smoldering.

The wrapped crayon however, burned brightly, with a large, rowdy flame, and a lot less smoke.

It also took about 5 minutes longer to burn all the way down, than the wrapper alone.

Inside of the wrapped crayon wrapper, we could see the crayon, melted, bubbling, and boiling (probably one of the reasons this is not a recommended use for a crayon).  The boiling wax got us thinking back to our homemade candles. 

When we had melted the paraffin wax on the stove, in prep for pouring it into our Dixie cup, candle mold, we had noticed how much faster, and easier it was to melt than the crayons.

According to the How It's Made clip on crayons, paraffin wax melts at about 143°F.  Crayons are made with paraffin wax, but we were starting to wonder if all the other things added - the fillers, strengtheners and pigments might, as well as doing all those things, also raise the melting, and ultimately the vaporization, temperature of the crayon.

So, we lit one more set of candles:  a store bought job from our mantle, another wrapped crayon, and one more empty wrapper, checking the temperatures they were burning at, with an infrared thermometer.  We're not that handy at using the thing, but from what we could gather, the candle was burning at somewhere around 250°F, the empty wrapper around 450°F, and the wrapped crayon at close to 650°F.

Our thinking is that the smoldering wrapper holds the melting wax in, essentially cooking it to a boil, allowing it to vaporize, and then combust.

With the crayon candle, the flame on the wick is not hot enough to melt all of the crayon, only some of the paraffin wax, and not all of the additives that might melt at a higher temperature.  So, instead of pulling melted wax up the wick to the flame, it creates a little puddle of melted paraffin on the surface, or maybe even melted something else. The flame burns down the wick to the little puddle, and is snuffed out.

Of course, we might be utterly, and completely wrong on all counts.  That's one of the things I like best about science at home - there's no one standing by with an easy answer;  just questions, research, observations, and answers that have to be tested, rather than checked at the back of a book.