Wicks are vital in the candle making world. Not only do they provide a focal point for your candle, it is the tool which uses capillary action to draw up the oils to provide fuel to your flame. Choosing a wick too large will provide too much fuel and result in a shorter burn time, mushrooming of your wick in addition to a larger & hotter flame. A wick to small will the the opposite effect, not enough fuel will be drawn and your flame with suffocate.
Below is a handy chart, which guides you to the correct wicks to use. Different factors effect the size wick required, so your own testing is vital!
