I'm a scented candle hoarder, and on my travels through the candle-buying world I often see claims that soy wax doesn't produce soot. This is completely false. No fuel can, in and of itself, be "soot-free". Here's why:
The technical term for burning is combustion - as well as fires, this includes the burning of petrol inside motor engines. You need a mixture of fuel (in this case, candle wax) and oxygen (from the air), and a source of ignition (a match).
For the most efficient burning of the fuel (i.e. getting as much energy out of it as possible), you want complete combustion - that is, you want the products to only be carbon dioxide and water.
This means you need the perfect ratio of fuel and oxygen. In reality, it's impossible to have the perfect ratio of fuel and oxygen, and generally it's oxygen in the air you run out of, so you end up with what's called incomplete combustion - that is, some of the chemicals produced can still be burnt to produce more energy.
Soot is made up almost entirely of carbon produced during incomplete combustion. So all you have to do to get any candle (even a soy candle) to give off soot is to simply not give it enough oxygen! Unless the candle manufacturer is in your house ensuring your candle is being burnt in exactly the right environment, they can't honestly guarantee you a soot-free candle.
You don't even have to do anything for soot formation to happen - unless you're actively introducing new oxygen, it's almost impossible to get exclusively complete combustion.
The easiest way to tell if you have complete or incomplete combustion is to look at the flame colour - complete combustion (and higher temperatures) is typically indicated by blue, while incomplete combustion (and lower temperatures) is associated with yellow. When was the last time you had a completely blue candle flame?
Of course, while you can't get rid of incomplete combustion entirely, there are some things that you should avoid:
- Too large a wick size - The smaller your wick, the less fuel will be burnt at any time, which increases the chances of new, oxygen-rich air circulating into the flame. Of course, if your wick is too small for the camdle, then you'll get tunneling. You can always control the wick length though - trim before you light.
- Too much scent - The scent chemicals are different to the relatively uniform wax - they contain lots of random compounds which might not turn so cleanly into carbon dioxide and water, even at the best of times. Most candle makers adjust the amount of scent accordingly.
- Drafts - Disturbing the flame (i.e. the combustion reaction) will generally result in a plume of smoke.
So if you ever see a small Asian girl shaking her fist at a soy candle description, this is probably why! There are lots of reasons to buy soy candles, but avoiding soot isn't one of them.