There are 2 instances during a candle's life cycle where the candle wax will shrink or contract. The first occurrence is during the making of the candles, the other is while the candle is being burned. Here we will explore both of these scenarios and explain why they occur.

Why does wax shrink right after you make a candle?

When wax is heated and melted in order to produce a candle, it expands and opens up. This is an important part of the candle making process, because it makes space for fragrance/essential oils as well as candle dye, to be added to the wax. 

The hotter the wax becomes, the more space is created in the wax and the more fragrance oil can be blended with the wax. Ensuring that the wax is hot and open enough is also necessary in ensuring that the additives are dispersed evenly throughout the wax. 

Once you have added the scent and dye, the wax will immediately start to cool down. This cooling period is also when the wax will begin condensing. As the wax shrinks in around the fragrance/dye molecules, they become less able to move around and eventually are completely encased within the candle wax. 

If you don’t heat the wax enough to fully expand it before you add in your fragrance and dye, it can cause some major problems with your finished candle. The fragrance and dye likely won’t be able to distribute evenly throughout the wax and may end up pooling together in certain areas. This can cause the oil to be expelled from the wax as it condenses. Your candle will either look like it is sweating or it will have pockets of oil hidden within it. Hidden pockets of oil in a candle can be very dangerous if they are close enough to the flame to ignite.

To give you an idea of how it works, imagine that you have 2 balloons that you are going to fill with water. The first balloon is very small because it has only slightly been blown up which means that it will not hold very much water at all. It also has not been tied off, and as it contracts and gets smaller the water is pushed out of the balloon making a big mess. This is similar to how a candle reacts when the candle wax has not been heated to the ideal temperature before adding fragrance oil. 

The second balloon has been blown up to its maximum capacity and is able to hold a lot of water. After the water is added, it has been sealed which keeps the water securely inside the balloon. Eventually, any air that is in the balloon will be released causing the balloon to contract tightly around the water that is trapped inside. This is similar to what happens when you heat your wax to the proper temperature allowing the fragrance oil/dye to bind properly with the candle wax.

How different waxes contract

If you work with a variety of waxes, you will notice that different kinds of wax contract different amounts and in different ways. In general, harder waxes need to be heated more and will also contract more when cooled than softer waxes do. 

Beeswax is an incredibly hard wax with a melt point of between 141°F to 151°F. According to ScienceDirect, beeswax can shrink up to 10% when going from a liquid back to a solid state. This is why pure beeswax tends to be better suited for moulded candles as opposed to container candles. When poured into a container, by the time the candle sets there will be sizable gaps between the wax and the container walls. 

Another issue caused by this immense shrinkage is that it is not uncommon for a beeswax candle to crack after it hardens, especially around the wick. The best way to combat this issue is to:

1. Allow as much cooling (shrinkage) as possible to occur prior to pouring the candle.

2. Ensure that the cooling process takes place very slowly, because cracking is more likely to happen if the candle shrinks quickly.

Harder types of paraffin wax like IGI4786 and IGI4625 also experience a great deal of shrinkage when going from a liquid to a solid. It contracts differently from beeswax in that it will form a large sinkhole down the middle of the candle around the wick as the wax solidifies. That is why harder paraffin waxes require what is known as a second pour. 

A second pour is when your first pour has completely solidified leaving a huge crater down the middle of the candle. Please note that as your candle is cooling, it is recommended that you poke holes around the wick to allow heat to be released and that you wiggle the wick occasionally to keep the hardening wax from pulling the wick away from the centre of the candle. You will then need to melt some more wax and allow it to cool slightly before filling the crater and giving your candle a nice smooth top.

Soft waxes contract when cooled too

Even softer waxes will shrink slightly as it changes back to a solid from a liquid state. One of the biggest complaints among soy wax users is “wet spots” forming on the sides of their soy wax container candles.

Wet spots are a cosmetic issue that is caused by the wax cooling and contracting away from the container sides leaving an air pocket that looks like there is moisture on the side of the candle. While the problem is in appearance only, it can be incredibly frustrating for candle makers who are striving to make the perfect candle.

The bottom line is that when wax is heated and liquified, it expands which increases its volume. As wax cools and heat is released, the wax begins to contract as it re-solidifies and it takes up less and less volume. If you allow as much heat as possible to leave the wax prior to pouring your candle, you will reduce the amount that the wax will contract after it is in your container. Making it so the heat will leave the wax very slowly, will also help keep the wax from shrinking away from the container and leaving those less than desirable wet spots.

Why does candle wax shrink when you burn it?

The other point in a candle’s life cycle that you will notice the wax shrinking and even disappearing is when it is being burned. Some people wonder why wax shrinks and disappears when you burn a candle, but doesn’t when you are using wax melts in a melter. If you look at the main difference between a wax melt and a candle, you will discover the reason: the candle flame. 

When you are using wax melts in a melter, there is a heat source that warms the wax to the point that it becomes a hot liquid and then the fragrance oil in the wax evaporates and disperses throughout the room providing a lovely aroma. When the heat source is turned off, the wax will cool and condense as it re-solidifies. Over time, all of the fragrance oil will evaporate, but the wax will remain.

However, when you burn a candle, the heat source that is used to melt the wax and release the evaporated fragrance oil into the air is the wick’s flame. According to the National Candle Association, When you light a candle, the heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action.

The heat of the flame vaporises the liquid wax (turns it into a hot gas), and starts to break down the hydrocarbons into molecules of hydrogen and carbon. These vaporised molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapour (H2O) and carbon dioxide (CO2).

Approximately one-fourth of the energy created by a candle’s combustion is given off as heat radiates from the flame in all directions. Enough heat is created to radiate back and melt more wax to keep the combustion process going until the fuel is used up or the heat is eliminated.

In other words, in the same way that your vehicle uses gas as fuel in order to keep running and is no longer able to run once all the fuel has been completely used up; a candle utilises the wax, fragrance oil and whatever else you have added to your wax as fuel to keep the flame burning. Once all the fuel has been vaporised the candle will go out.

It is worth noting here that it is incredibly important that your wick has been primed with wax so that there is a small amount of fuel to get your candle going. If your wick isn’t primed, there is a good chance that the ¼” of wick that is above the surface of the candle wax will burn away before it has had the opportunity to create a melt pool. Without a melt pool to fuel the flame, it will go out.

Conclusion

There are 2 points during a candle’s life cycle that you will notice shrinkage. Both are caused by how candle wax handles heat, but under 2 very different circumstances. 

The first is during the making of a candle when the wax is heated, making it expand as it melts. Then it turns back into a solid after the heat is released, which causes it to shrink and contract. 

The second is when a candle is being burned. The flame heats the solid wax which turns it into a liquid that is then drawn into the flame to be used as fuel and vaporised. The longer you burn the candle, the more wax that will be vaporised and the smaller the candle will become until it is all gone.