'There is no better, there is no more open door which you can enter into the study of natural philosophy than by considering the physical phenomenon of a candle'
- Michael Faraday (1791-1867)
Faraday was a physicist and chemist, best known for his contributions in the areas of electricity and magnetism. Two units of measures are named for him: the Faraday (unit of electrical charge) and the Farad (unit of electrical capacitance). Faraday also spent time on projects such as lighthouse construction and operation, and protecting metal ship hulls from corrosion. He gave tips on the cleaning and protection of artwork. On top of these and other activities, he delivered a series of public lectures and wrote for the general public. Around 1860, Faraday gave a successful series of lectures on the chemistry and physics of flames called 'The Chemical History of a Candle', a transcript of which I stumbled upon recently.
On describing a method of making candles, he explained: 'The fat or tallow is first boiled with quick-lime and made into soap, and then the soap is decomposed by sulphuric acid, which takes away the lime, and leaves the fat rearranged as stearic acid, while a quantity of Glycern is produced at the same time ... The oil is then pressed out ... and at last you have left that substance which is melted and cast into candles'
Makes me tired just reading that! Candles can also be made by the dip method I've described before or from bees wax. In Faraday's day sperm candles were made from purified oil found within the head cavities of sperm whales and paraffin candles were made from paraffin somehow obtained from bogs in Ireland (how exactly this was done was not mentioned – I'm curious so I may look this up later).
A burning candle is a chemical reaction that turns wax and oxygen into carbon dioxide and water while letting off heat and light. Soot isn't a product of this chemical reaction, instead it is incompletely burned carbon. Once lit, how does a candle get fuel to sustain itself?
The heat of the flame melts a pool of wax. This wax is then drawn to the flame by capillary action – the wick just provides a way to get wax to flame. Capillary action is a process where liquid can rise, seemingly against the force of gravity, and it is common in the world around us. For example, the transport of fluids in plants uses capillary action. If you were to put a freshly cut celery stick into a cup of water that had purple food colouring in it, you would end up with purple celery. Capillary action occurs in thin tubes or within the weave of a candlewick as a result of inter-molecular attractive forces between the liquid and solid surrounding surfaces.
Molecules within a liquid are attracted to one another, this is called cohesion, which manifests as surface tension. Because of cohesion, the most efficient shape for a liquid is a sphere, which is why raindrops are round. When a liquid touches a solid material (like the wick) that attraction now occurs with the solid material – this is called adhesion. If adhesion is greater than cohesion the surface will curve up at the boundary like the meniscus formed when water is in a glass. Alternatively, if the adhesion is less than cohesion, then the surface will curve down. So, if the cohesion of water molecules and adhesion to a solid surface act together (which would happen in a thin space) the liquid would be drawn up – the thinner the space, the higher the liquid would rise. And that's how wax gets to the flame.
By the way, Faraday's idea of using a common thing like a candle flame to teach science is still used.