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The Chemistry World team are going to have some fun with a few Christmas related posts this December. First up, Advent Candles, look out for more in the coming weeks…
— Advent candle
For me, candles are a huge part of Christmas. As the night draws in, there’s something about a cosy room full of flickering candle light to really make me feel all Christmassy, and I love singing in candlelit carol services (although I always worry a bit about synthetic fabrics and distracted children). This year, though, there’s another reason for me to enjoy Christmas by candlelight from Michael Faraday, the man who instituted the Christmas Lectures at the Royal Institution and general chemistry hero.
As part of an Open University course I’m doing in my spare time, I’ve been learning about the life, work and reputation of Faraday. As part of my study I’ve been reading Faraday’s The chemical history of the candle and so I’ve started thinking a lot more about what’s going on in that flame.
Most candles today are made from paraffin wax, a solid mix of hydrocarbons with around 20-40 carbons per chain. The match or lighter melts a small amount of the wax and that is drawn up the wick by capillary action where it combusts, giving a flame. That flame melts more wax and so the candle will burn and burn until it’s put out or it runs out of fuel. So, hydrocarbon and oxygen reacts exothermically to give water and carbon dioxide, just as Faraday demonstrated. So far so good. But what’s fascinating is that we still don’t know every reaction going on in that tiny flame.
On Earth, the candle’s flame is shaped by the convection of the hot compounds from the combustion of the fuel. The blue that you can sometimes see at the bottom of the flames is the molecular emission of carbon, and then after breaking and rearranging at the wick, the fuel creates soot particles that are carried up by convection to where they glow with heat to give the classic yellow flame, before moving to the outer surface for the actual combustion reaction. But what form does that soot take? That turns out to not be as stupid a question as it might sound. Earlier this year, chemists found diamond nanoparticles in the middle of candle flames – your advent candle flame could make over 1.5 million nanodiamond particles per second.
— A candle flame in microgravity
We’ve sent candles up into space to understand how gravity affects the flame (and Nasa suggest how to repeat the experiment by dropping a candle enclosed in a jar here). What you’ll see is that instead of a teardrop shape, the flame will burn round and blue (and only for a short while, as the CO2 produced doesn’t go anywhere and eventually snuffs out the flame, just like Faraday showed it does in his demonstrations). Another trick you can do with your candles is to relight the candle after blowing it out by lighting the gases coming off the wick. Although you see the soot particles rising up, there is also fuel in that column, and introducing a match to the smoke will light the fuel and travel down the column to relight the candle.
Over on twitter today, @neilwithers asked what would be the updated chemical history of the candle, I think I’d have to reply that while we can do others and that’s great, candle chemistry ain’t dead and at Christmas I don’t think it can be beaten. For example, only last week I wrote a story on using candle soot as a template for an omniphobic surface. The very reason, Doris Vollmer told me, that they used candles was because they were doing the work around Christmas last year and had a load of left over candles to hand. One caveat though, don’t mix candles and tinsel.