You've made some beautiful candles: they smell great: they look fantastic: Now it's time to burn them.
You light your first, walk away and come back some time later to find your candle is burning straight down the centre.
There are two main reasons for this.
Wrong wick size
Although this is quite often the 'go to' reason for tunnelling, it's only really one you can say with any certainty after quite a few hours burning.
You will need at least 5 hours of burn time, if not more, to decide whether the wick is too small or not. Sometimes it will be super obvious after 5 hours that the wick is just not going to create a full melt pool.
If this is the case then you will need to wick up one or two sizes and do another test burn. If the wax that is left on the sides of the glass is not too thick, then wick up one size. If it is very thick then wick up one or two sizes higher.
These pesky little critters are hard to spot until it's too late (on on most occasions.)
Sometimes however there are tell tale signs that you could have air pockets under the surface of your candle.
This candle for example, has a fairly noticeable crack on its surface. When this candle was lit and the very top surface burnt to liquid, this crack then opened up into a large crevasse. When this happens, the melt pool drops suddenly down into this hole causing the wick to quickly burn down to meet it. When you come back to check on your candle later you will find your candle tunnelling quite badly and usually with a very small flame.
It is usually quite easy to distinguish if this has indeed happened by the depth of the hole that has suddenly appeared. If this example on the right was a result of the wrong size wick, you would need quite a few hours of burning to even get close to such a deep melt pool. This sudden depth is very indicative of an air hole.
Air holes can be prevented by pour temperature and cooling environment. They are more common in winter due to the colder ambient temperature. If you encounter a lot of holes in your candles then pour your wax at least 5 to 10 degrees hotter. Make sure you room is not cold as this will cause the candles to cool too quickly. Candles should set from the bottom up. If your room is too cold your candles will set from the sides and across the top before the middle has a chance to cool down, resulting in air being trapped under the surface.