The coldest part of a flame is the middle

The structure inside a flame, and where the reaction actually happens.

The coldest part of a flame is the middle because that inner region is filled mostly with unburned fuel vapour that has not yet met enough oxygen to react, while the actual combustion, and the heat it releases, is happening in a thin, much hotter shell wrapped around the outside of that core.

A hollow shell of burning

Most people picture a flame as a uniform blob of fire burning evenly throughout. In a candle or a gas jet, fuel vapour rises up the centre surrounded by more fuel vapour, and combustion cannot happen there because there is nothing for the fuel to react with yet. Only further out, at the boundary where the rising vapour meets oxygen diffusing in from the surrounding air, does the reaction take place, releasing heat and light in a comparatively thin shell wrapped around the unreacted core. The middle is where the fuel waits, a moment before it reaches the edge and burns.

The same layering explains the flame's colours. The pale blue region low down, close to the wick, is where enough oxygen still reaches the reaction for it to burn relatively cleanly. Higher up, where oxygen has to travel further inward to reach the fuel still rising through the core, combustion becomes less complete, leaving more of the glowing carbon particles responsible for the brighter yellow further up. A single candle flame is running through several stages of combustion completeness at once, stacked vertically.

It also explains why a thick pillar candle's flame differs from a thin taper's, even burning identical wax. A wider wick supports a wider column of rising vapour, so the cool core grows with it, and the oxygen at the shell has further to reach before the last of the fuel at the centre can burn.

Measuring the gap

Textbook measurements of a candle flame put the dark core at roughly 600 °C and the edge of the reacting shell at around 1,400 °C, more than twice as hot. The gap is easy to show. A fine wire mesh pressed down across a flame conducts heat away fast enough to stop the flame passing through it, and seen from above the mesh glows in a bright ring with a dark centre, a cross-section of the hollow shell. A card held briefly across the flame records the same thing as a charred ring around an unmarked middle. A genuinely solid flame would leave a solid disc.

The core can even be tapped. A thin glass tube with one end poked into the dark centre of a candle flame will carry a stream of unburned wax vapour out through its other end, where it can be lit as a second, smaller flame some distance away from the first. The vapour was travelling through the middle of a flame without burning, because there was no oxygen there for it to burn with.

Why the clear gap at a kettle spout is hotter

A kettle at a rolling boil has a short, clear gap right at the spout where nothing can be seen, before the steam has cooled enough to condense into the visible white cloud further out. Most people assume the white cloud is the hottest part, since it is the part on display, when the clear gap is hotter still and invisible only because it has not yet condensed.

A flame's core plays a similar trick on the eye. The bright glow comes from the outer shell where combustion is happening, and the middle, sitting inside that bright boundary, is the coolest part of the whole structure. In both cases the attention-grabbing part and the hottest point sit in different places, a mismatch that only becomes obvious once somebody goes looking with something other than a glance.

Flames without a cool core

This layered structure belongs to a diffusion flame, of the kind a candle produces, where fuel and air start apart and meet only at a boundary. A thoroughly premixed flame, with fuel and air blended before ignition, has no unburned core, since no region inside it holds fuel without the oxygen to react it. It behaves more like a single thin sheet of reaction, having solved before ignition the problem a candle flame keeps solving as it burns, which is part of why a clean blue gas flame looks so different from a candle's.

A strong draft disturbs the layering directly, thinning or tearing the reacting shell in places and letting pockets of fuel vapour escape to the outside air without passing through it. A candle guttering in a draft looks uneven for this reason, some sections of its shell interrupted more than others.