When a candle is burning unevenly, the wax pool will often grow vertically and deeper (forming tunnels) rather than expanding outward. This can be caused by a number of things. Incomplete first burns are one common cause; this happens when a candle is extinguished before the top layer of wax has melted. A lack of proper wick trim also contributes to candle tunneling, as well as drafts and inferior quality candles.
The type of wax used in a candle can have a significant effect as well. High-quality waxes, such as soy or beeswax, melt more evenly than cheaper varieties. Other factors that can affect the evenness of a candle include the size of the wick, environmental conditions and the wick-to-candle ratio.
A few simple steps can help prevent uneven burns. The most important step is to allow a candle to fully liquify on the first burn. This will set the “candle memory” and ensure a full wax pool on subsequent burns. To do this, you should check the candle for a full melt pool every 30-45 minutes and only extinguish it once that has happened. Avoid blowing the flame out as this spreads ashy debris into the melted wax and reduces its lifespan. Alternatively, you can try using iodised table salt to slow the rate of melting and achieve a more even result. This method can be a little messy and tricky as the iodised table salt really heats up, so it takes some practice to get the timing right.