I’ve written about candles on only one other occasion, but it was in an entirely different context. While looking for ideas I found some photos of candle flames that inspired me to write this post. Do you know how a candle really works? If not, you’ll find out here.
Candles have been around for a long time. If you count wicks stuck into some flammable liquid substance as a “candle”, that was used by Egyptian and Roman societies as far back as 3000 BCE. The rolled types of candles you’ve probably used (including those on birthday cakes) were developed around 500 BCE. The Romans dipped rolled papyrus “wicks” into tallow (animal fat). It provided a long-lasting source of illumination, but had an unpleasant odor. In the 1500s beeswax was introduced as an alternative to tallow. It wasn’t as plentiful as tallow, but it burned with a neutral odor. It was also more expensive than tallow, so at first it was only used by the upper-class and clergy.
A candle will sputter and flicker a bit bit when you first light it, but once it’s burning at a steady state, what’s happening is shown in the main graphic. Here’s the key components of a candle flame:
- Radiant heat from the flame melts wax near the top of the candle. As the wax liquefies, it is drawn into the porous wick by capillary action. This melted wax is the fuel for the candle.
- The heat of the flame then vaporizes the gas, making it more susceptible to chemical reactions.
- The vaporized gas contains hydrocarbons from the wax which the heat breaks down into molecules of hydrogen and carbon.
- These vaporized molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapor (H2O) and carbon dioxide (CO2).
- The visible part of the flame — the part that gives off useful amounts of light, comprises incandescent carbon particles (soot) that glow for the same reason as an incandescent bulb filament — they’re hot.
- The glowing carbon particles are carried upward by convection, but at some distance they cool enough to lose incandescence. That’s the tip of the flame.
- As combustion products are convected upward, air is drawn toward the base of the flame providing oxygen to support continued combustion.
High-quality candles designed for illumination (not birthday cake candles), are very efficient combustion engines. Around 75% of the energy released is in the form of visible light with 60–80W intensity. The full visible spectrum is there, as you can confirm with a prism or spectrometer. But because the burning carbon emits light peaking in the range of 570–580 nm (nanometers) we perceive the color of the flame as pale yellow.
The other 25% of the energy goes into invisible infrared light (heat). That heat, radiated in all directions, is sufficient to melt the wax and keep the combustion going. It’s important to note that incomplete combustion of the carbon can produce carbon monoxide — so never use candles for heating, only for lighting in ventilated spaces.
At least that’s how a candle works on Earth under normal gravity. The convection that shapes the flame into its classic teardrop shape requires gravity to work. There has to be an “up” and “down” in order for hot air to rise. So what does a candle burning in zero gravity look like? The answer to that question will provide additional insights into the physics of candles.
In the late 1990s, NASA conducted space shuttle experiments to investigate combustion in microgravity. In microgravity, where convective flows are absent, the flame is spherical, soot-free, and all blue:
There’s still carbon being produced, but it diffuses by osmosis away from the wick. Likewise, oxygen diffuses in to keep the combustion going. But without strong convection, you don’t get incandescent unburned carbon. The combustion process is much slower and not nearly as much light is produced. Diffusion is way slower than convection for moving molecules around.
NASA doesn’t like to have open flames on their space flights for obvious reasons. But this experiment (which has been repeated on the ISS), was done inside a non-flammable transparent container in an area with fire suppression equipment at the ready. Oxygen was pumped into the container and combustion by-products were extracted, measured, and recycled. The experiment provided valuable insights into combustion in general, and a lot of that applies to candles.
Next Week in Sky Lights ⇒ Geoengineering the Arctic