A stove flame is blue and a candle is yellow

Premixed against diffusion flames, seen in two everyday objects.

A stove flame is blue and a candle is yellow because a stove mixes fuel and air together before the flame ever ignites, burning that ready-made mixture cleanly and completely, while a candle burns wax vapour that has to find its own oxygen from the surrounding air as it goes, a slower and less complete process that leaves glowing carbon particles behind.

A gas burner project I worked on began, unglamorously, with the discovery that the small blue cone sitting at the base of a kitchen hob's flame is a far more complicated object than it has any business looking.

Air mixed in before the flame, or found at its edge

A gas hob draws air in and mixes it with the gas before either reaches the point of ignition, so every molecule of fuel arriving at the flame already has enough oxygen beside it to burn completely and immediately. That premixed combustion is fast and well controlled and leaves almost no carbon behind, which is why the flame burns a clean, pale blue.

A candle has no premixing step. Heat from the flame melts the wax, draws it up the wick and vaporises it, and that vapour meets oxygen only at the surface of the flame, where hot fuel and surrounding air diffuse into each other. Combustion there is slower and less complete, leaving tiny unburned carbon particles that glow as they pass through the hottest part of the flame before burning up or drifting away as smoke, which is also why candles add soot to a room's air in a way a blue hob flame does not.

Reading the colour

Blue light from a flame comes from excited molecules releasing energy as they react, a signature of combustion fast and complete enough that almost nothing is left over. Yellow and orange light comes from solid carbon particles glowing simply because they are hot, the same process that makes a piece of metal glow orange in a forge. A candle's flame is yellow because it is full of carbon that has yet to finish burning, not because its chemistry differs fundamentally from a hob's.

Paint stirred in the tray, paint blended on the wall

Stirring two colours of paint together in a tray before rolling either onto a wall gives an even colour the instant the roller touches the surface, because the mixing has already happened everywhere. Dabbing one colour directly beside a still-wet patch of another gives a slower, patchier blend, the two mixing only at the boundary where they touch.

A hob's flame is the paint pre-stirred in the tray, fuel and air combined before ignition and burning uniformly the instant it lights. A candle's flame is the wet-on-wet brushwork, fuel and oxygen meeting along a boundary that keeps forming and reforming as the flame burns, finding each other slowly along its visible edge. Left long enough, the wet colours do blend at their shared edge, much as the upper reaches of a candle flame, fed by rising air, finish burning carbon that started off unreacted lower down, which is part of why the very tip often looks less intensely yellow than the region just beneath it.

From a candle to the rest of a burner

The same distinction, premixed against diffusion, decides how efficiently a burner uses its fuel, how much soot it produces, and how easily its flame can be controlled or put out. A blue, premixed flame is generally the one worth designing towards wherever cleanliness and efficiency matter, and understanding why it looks the way it does is the first step toward understanding why the rest of a burner is built as it is, starting with the small holes that let air into the gas stream long before either reaches the flame.