What is the science behind a burning candle?

What is the science behind a burning candle?

A burning candle is a classic example of the chemical reaction between three major elements: oxygen, hydrogen, and carbon. The wax in a candle is composed of atoms of hydrogen and carbon. When the wick is lit, wax begins to melt, and the carbon and hydrogen atoms move up the wick to react with the oxygen atoms in the flame.

Actually, the flame you see on a candle is proof of a chemical reaction of carbon and hydrogen atoms with oxygen. The heat from the wick causes the carbon and hydrogen atoms to break apart and combine with oxygen to form a gas. This is called pyrolysis.

You will notice that the flame of a candle is different colors. The blue part is the hottest part of the flame. The flame is blue because of sufficient oxygen being provided in the reaction.


What happens when there is not enough oxygen available to complete the reaction?

You will have an excessive number of carbon atoms that group together to form soot. Since soot is black in color, it easily absorbs heat; and this heat allows them to glow. It is these carbon molecules that create the yellow, orange, and a red colored portion of the flame.

When soot particles glow because they are hot it is called incandescence. Ultimately, a burning candle gives off water vapor, and carbon dioxide (the very same gas we exhale when we breathe).

Over the past decade, there has been a steady climb in the sales of soy wax candles; claims being made that soy wax candles are somehow "better" for you than other candle waxes, such as beeswax, palm or paraffin. Putting aside all bias and focussing on science alone, the following conclusions were evident.

  • All waxes are primarily hydrocarbons, whether the wax is of animal, vegetable, or petroleum origin. The chemical composition of all waxes used for candle making is similar.
  • All waxes produce a yellow flame due to the presence of carbon.
  • No specific type of wax or wax blend is considered "best" for candle making. All candle waxes, when provided in a high-quality format, have been shown to burn cleanly and safely.
  • No candle wax has ever been shown to be toxic or harmful to human health.
  • There is no such thing as a soot-free wax. All organic compounds when burned will emit some carbon (soot) due to incomplete combustion. (
  • In fact, we all experience small levels of carbon in our air every day simply by cooking our food. It would be ridiculous to tell people to stop cooking their food to avoid carbon in the air.

How about carbon dioxide- the air we all exhale?

Should we all stop breathing to prevent putting this carbon in the air?  It is time for science and common sense to unite!

The science behind a burning candle is important to understand because for a wick to perform effectively, it is reliant on other components of the candle.

Some fuel sources will easily travel up the wick and produce the fuel vapor needed to keep the wick burning, such as fully refined paraffin wax.  Other waxes, including soy wax, sometimes require a larger wick to allow for pyrolysis to occur.

Other candle components that significantly affect the efficiency of the burning wick are fragrance and coloring.

Fragrances are composed of a combination of essential oils, resins, and other aromatic chemicals (ketones and aldehydes). Some of these fragrance ingredients will easily travel up the wick and react with the oxygen; others do not travel up the wick as easily.

Each component of fragrance oil has individual flash points (the temperature at which its vapor will evaporate). When fragrance oil contains higher levels of heavy base notes, such as musk, vanilla, amber, the candle may require a larger wick to allow for pyrolysis to occur.

Lighter, lower flash point fragrance components will easily travel up the wick and burn. Special care should be taken when using citrus type fragrances that have a low flash point.

If the wick size is too big, it may cause the citrus fragrance to give off a petroleum aroma instead of a citrus aroma. Using too big a wick with a low flash point citrus fragrance can also create a fire hazard; causing the entire top of the candle to ignite.

Coloring can also affect candle wicking.

Candles require the use of dyes specifically formulated for candle making. Any coloring that contains water or glycerin will not work in candles – so never use crayons, chalk, paints or food colouring! 

Color pigments, mica pigments, and titanium dioxide do not work well in wax. These pigments clog the candle wick, it breaks down the capillary action process, increases smoking, and reduces or eliminates scent throw.

If used, they should only be used for dipping pillar candles. Crayons are an example of color pigments suspended in a wax medium. It is never a good idea to color your candles with crayons (as suggested by some media sources).