What is tunnelling in a candle and how to avoid it?

Have you ever had a scented candle and found that the wax does not melt to the edges of the container and leave a hole in the center or "tunnel" after the first burning? Then you have experienced what is called candle tunneling. Next time you burn the candle, the wax will not melt to the candle's edges, and the hole will only get deeper, not wider. 

Candle tunneling is a common problem that every candle maker faces during the candle-making process, especially during the design phase of making a candle when experimenting with candle jars, wick, wax, and fragrance oils. 

Four reasons why we should avoid tunneling?

There are at least four good reasons why we should design candles so that we avoid candle tunneling:

• Waste of candle wax - When the melt pool does not reach the candle's edges, there will be wax leftovers in the candle. Since the wax is a big part of the cost of a candle, it is simply wasteful not to utilize the resource. 
• Reduces the candle’s lifespan – since all the wax is not used, the burn time and life span of the candle will be less hours than they ideally could be.
• Negatively impact the scent throw - the smaller the melt pool, the less scent potential will be released to the room by the candle. 
• It does not look good - candle aesthetics is a big part of enjoying a candle, and when you have a candle tunneling, it simply does not look good or appealing. 

What causes tunneling in candles?

Typically this comes down to either how long we let the candle burn on the first burn cycle, the way the candle is designed, or a combination of the two. 

Not allowing the candle to burn for two to three hours the first time it's lit.

The most common reason for tunneling is that the first time you burn the candle - you will need to let the melted wax pool go to the edges of the candle.

A well-designed candle should take about 2 to 3 hours of burn time for a small to a medium-sized candle and maybe up to 4 hours for a larger candle.

If the candle is left burning too short a time the first time, the next time you light the flame, the candle will continue with more or less the same melt pool size and create the dreaded tunnel effect.

This effect is explained with the concept called wax memory, where previously heated wax will be softer than the wax area that did not get heated. So next time around, it will be easier for the flame to continue to melt the softer part of the wax than to expand the melt pool toward the harder part of the wax around the edges. 

A good rule is to let the candle burn until the wax is melted out towards the edge of the candle jar every time you enjoy the candle, but the first time is by far the most important. 

Not using the proper sized candle wick.

The other main reason for tunneling is a flawed candle design. 

A typical problem here is that the candle maker has used the wrong combination of candle container, wick, fragrance, and wax. The wick is simply unable to melt the entire surface even if we let it burn more than a couple of hours during the first cycle. 

For a candle maker, it is essential to find the right wick size that matches the other candle ingredients and will be able to create a sufficient melt pool. If the wick is too small, the flame will not be able to melt enough of the wax, but if the wick is too large, the flame may flicker, create soot and overheat the candle vessel. 

A well-designed candle should be able to create a full, or near-full melt pool within two to three hours at normal room temperature. However, sometimes the candle is designed so that it will leave a thin wall of wax at the edges. This could be a good thing as it is very likely that the thin wall of wax will eventually melt while it protects the vessel from overheating. 

Read more about candle testing and download our free candle-testing template here. 

How to avoid candle tunneling?

If you want to avoid tunneling, you need to make sure you burn your candle for at least two hours or until the wax has melted to its edge. Also remember that the room temperature could impact the candles performance. So you use the candle in a cold room it may be higher chance for tunnelling. 

If the wax is still not melted to the edge after about 2-4 hours, it could be that the diameter of the wick is too small. In this case, you will need to use a bigger-sized wick or several wicks to avoid tunneling. 

Is it possible to fix candle tunnelling? 

There are a few tips out on the interweb for fixing tunnelling after the harm is a fact. Let's quickly go through some of the methods:

1. The first method is to use an external heat source (like a heat gun or a hair dryer on low blow setting) to melt and level the wax. Trim the wick and heat the edges of the candle until the wax melts and evens out. 
2. Another trick is to use a candle topper that reflect and increase the heat so that the edges of the tunnel eventually heat and melt. If you don't have a candle topper a similar effect can be reached  with an aluminium foil with a hole that is wrapped on top of the candle. 
3. A third possibility that can be combined with the two above is to trim and remove the residual wax with a knife or spoon. If you have a candle warmer you can for example put the wax on the melter or use the wax and make your own wax melts. 

To summarize - for a candle-maker it is important to make sure that the candle is well designed to minimize the risk for tunnelling. It is also smart to provide instructions to the customer about the initial burn time. A customer needs to plan sufficient time for the first burn, but there are a few tricks that can be use to fix tunnelling if the damage is done.