A person cups a candle in their hands

Carbon, candles and more: how soot became a wonder

Damage caused to paintings in churches by candle soot has been observed for years. Little did anyone know that this same soot would lead scientists to discovering some scientific wonders.

Grantham Scholar Manasi Mulay – who has a review paper about candle soot published – explains more.

woman putting a candle down during prayer
Candles are used in religious rituals all over the world

In celebration of the international year of the the periodic table I am going to talk about my favourite element – carbon. Carbon, essential for all organic life on the earth, provides the primary source of energy in the form of fossil fuels. Carbon also takes the beautiful form of diamond (which is hard) or comes to us as light and strong (as graphene).

I see ‘carbon’ as an emperor of the periodic table because it is powerful and strong. If an element has only one electron missing or remaining in its valence shell, it tends to form strong bonds with other elements easily, as it is quick to exchange one or two electrons with others. Such elements choose to take either the positive or negative side of the argument based on their electron donating or accepting nature. However, carbon with four valence electrons, instead of giving or receiving the electrons, demonstrates the concept of sharing and balancing. Its own kingdom of organic chemistry is established by networking carbon-carbon family and carbon with other friend elements. Carbon is a skilled team player, as it is useful for improving properties of materials. Carbon takes the beautiful form of diamond which is hard or comes to us as light and strong graphene. If employed correctly, carbon is the best functional material ever; but incorrectly employed, it can be the worst pollutant.

Candle soot – more than meets the eye

The carbon-based black particles produced by candles burning (known as ‘candle soot’) is an infamous pollutant. The typical size of candle soot particles observed under electron microscope is 50 nanometers. Paintings being destroyed by soot from church candles has been observed for hundreds of years. Candles are still employed as source of light for various decorative, traditional and religious purposes. Inspired from the blackening of paintings, candle soot even gave birth to a creative art called Sootoid. But until recently, no one imagined that this candle soot could lead us to some of the technological and scientific wonders of the world.

Can you guess the temperature a candle can reach? A typical candle is reported to reach 400-750 oC – in comparison, the boiling point of water is 100oC . Temperatures at the tip of the candle are higher than the bottom. Thus candle soot formation is desirable for radiative heat transfer.

Manasi Mulay

As a candle burns, melted wax reaches the top end of the wick by capillary action. The entire candle does not get burnt at once as the molten wax extinguishes the flame at bottom end of the wick – as famously demonstrated by Michael Faraday in his public talk in 1848. As the temperature at various regions in a flame varies, emissions from mid of the flame and tip of the flame differ in nature.

These days candles with beeswax or hydrogenated vegetable oil have replaced conventional paraffin. More research will be required to compare relative health hazards. Beautiful coloured candles, such as those used during religious festivals, get their colours from metallic additives. These additives result in different types of candle soot e.g. candle soot formed with iron (Fe+3) modified wax produces magnetic nanocarbons.

Nanocarbons from candle soot

In materials with dimensions in the range of nanometers (i.e. 10^-9 meters), surface area to volume ratio is higher relative to their larger counterparts. Higher surface area to volume ratio activates more surface reactions and supports faster electron transfer. Carbon-based nanomaterials (dimensions) such as carbon nanotubes or graphene exhibit excellent properties but involve expensive and complex processes.

However, candle soot is a cheap source of nanocarbon. The deposition process is also simple, just burning a candle – something humans have been doing for millennia, unaware that they were creating nano-carbons. This nanoscale nature of candle soot drives attention to its successful testing for various energy harvesting, storage and conversion applications. These include batteries for our cell-phone, laptops and electric vehicles.

Interestingly, carbon from candle soot is actually a fluorescent nanomaterial exhibiting different colours. This needs to be separated from unwanted organic material in the soot by post-processing, such as washing or treating soot with other chemicals. This candle soot derived fluorescent nanocarbon is useful in detection/sensing and cutting edge biomedical research. For example, these fluorescent nanocarbons are used to detect and highlight harmful agents inside the human body. Drugs for the effective treatment of certain diseases need to be delivered to specific areas in the human body without contact with other regions. Due to nanosize and the biocompatibility of candle soot derived nanocarbon, it is possible to employ it as a drug delivery vehicle.

How soot can be a smart material

Additionally, candle soot being superhydrophobic (strong dislike for water) creates possibilities for various self-cleaning surfaces. Examples include glasses, goggles, touch-screens, stain free fabrics, anti-corrosive, anti-fogging, anti-icing and anti-reflective smart textiles, as well as paints and coatings.

Just a pinch of candle soot in your graduation robe and you wouldn’t have to worry about whether it’s snowing or raining on your special day! A pinch more when manufacturing your favourite party dress and you would not cry over the spilled champagne. Public walls coated with candle soot superhydrophobic paints, can bounce back the liquids and keep cities clean. 

Similarly, the water repelling nature of candle soot also promises oil-water separation, which could address ocean oil spills. Furthermore, candle soot shows superior performance for dye adsorption and degradation from industrial wastewater, reducing long term ecological hazards.

 The deposition of candle soot is reported to be the easiest and most cost effective method for latent fingermarks visualisation.

A tasty looking cocktail
Would you like some soot with that?

My Review Article on Candle Soot

I wrote a review article about the research on candle soot, which is now published in Elsevier’s ‘Carbon’. The review talks about the journey of previous research papers on science of formation of candle soot and its real life wonders including energy, biomedical and environmental.

Candle soot was traditionally considered to be just a source of air pollution until the discovery of the fluorescent carbon nanoparticles from candle soot by Mao et al. in 2007, was followed by around 100+ research articles published on candle soot related investigations during the last decade, (based on Scopus).

There have been a lot of attempts to investigate the wide range of functional applications of candle soot, still, much scope remains for further research to tap the versatile potential of candle soot and its cost effective scale-up. I hope that this review article will act as a guideline for the future research on candle soot.

Maybe after reading this the next time you go to the pub you’ll ask the bar staff to remove the candle from your table!