Candles mainly blacken due to the accumulation of soot from incomplete combustion of the wax and wick, especially when the candle is poorly adjusted or when the ambient air is polluted.
A typical candle consists mainly of wax, usually paraffin, derived from petroleum products. This paraffin is made up of a mixture of hydrocarbons, which are chains of carbon and hydrogen atoms. There are also plant-based candles such as those made from soy wax, palm wax, canola wax, or even natural candles made from beeswax. Various components such as fragrances, dyes, or specific additives are often added to this wax to influence its color and burning properties.
When the candle burns, the melted wax rises in the wick and vaporizes due to the heat. Normally, the wax vapors react with the oxygen in the air to form carbon dioxide and water: this is complete combustion. But sometimes, there is a lack of oxygen, or the flame is not hot enough, which means that not everything burns as expected. The result is incomplete combustion, where small solid particles of carbon form, known as soot. These tiny black particles rise with the smoke and darken the glass or the surface around the candle.
The wick greatly influences the combustion of your candle. If it is too thick, the wax melts too quickly and doesn't burn completely. This leads to a larger and softer flame, which will quickly produce black soot. Conversely, a wick that is too thin makes it difficult for the flame, which can also result in imperfect combustion. The material of the wick also plays its role: some well-braided cotton wicks allow for balanced combustion, thus limiting the formation of that infamous soot.
Oxygen plays a crucial role in the combustion quality of candles. When a candle burns in a poorly ventilated space or low in oxygen, the combustion becomes incomplete, leading to the production of black smoke filled with soot. Conversely, when air circulates well and the oxygen supply is sufficient, the flame burns cleanly, producing little residue. A good supply of oxygen allows for a more complete combustion of the fuel, thereby preventing your candle from blackening visibly.
By regularly trimming the wick of your candle to about 5 mm in height, you limit soot production and enjoy a cleaner, more sustainable burn.
Scented candles can sometimes tend to soot more than regular candles; this may be due to the oils or fragrance additives that complicate and disrupt the complete combustion of the wax.
The color of a candle flame can indicate the quality of its combustion: a stable flame that is bluish at the base and light yellow at the top generally signifies good combustion, while a flickering flame that is dark orange often indicates incomplete combustion.
A candle flame generates enough microscopic soot particles to be detected by optical smoke sensors, which explains why candles that are too close to detectors can trigger false fire alarms.
The proximity of objects above or around the candle can promote their blackening, as soot is carried by the upward flow of warm air emanating from the flame. Always ensure to place your candles away from ceilings and light or fragile objects.
Candles made from vegetable waxes, such as soy or rapeseed wax, or those that exhibit optimal complete combustion, generally tend to produce less soot than candles made with cheaper materials and poorly suited wicks.
Yes, a flame that is yellow or orange indicates incomplete combustion, which is conducive to soot formation. A blue flame indicates optimal combustion with an adequate supply of oxygen, thereby limiting black deposits.
To reduce soot, choose a good quality candle with a suitable wick. Regularly trim the wick so that it doesn't exceed 6 mm, and ensure there is good air circulation around the candle to promote complete combustion.
If the soot generated by a candle is excessive, it could affect indoor air quality by releasing fine particles. Good ventilation and the use of high-quality, clean-burning candles significantly reduce any risk.
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