It’s a matter of how much oxygen is available to the burning fuel. Lots of oxygen makes blue flames, while a limited amount of oxygen makes yellow ones. Let’s look at the yellow flame first.
A candle is really a very complex flame-producing machine. First, some of the wax must melt, then the liquid wax must be carried up the wick, then it must be vaporized to a gas, and only then can it burn, react with the oxygen in the air to form carbon dioxide and water vapor. This is far from an efficient process.
If the burning were 100 percent efficient, the wax would be transformed completely into invisible carbon dioxide and water. But the flame can’t get all the oxygen it needs to do that just by taking it out of the air in its immediate vicinity. The air, with its flame-nourishing cargo of oxygen, just can’t flow in fast enough to take care of all the melted and vaporized paraffin that is ready to burn.
So, under the influence of the heat, some of the unburnable paraffin breaks down into tiny particles of carbon, among other things. These particles are heated by the flame and become luminous; they glow with a bright yellow light. And that’s what makes the flame yellow. By the time the glowing carbon particles reach the top of the flame, almost all of them have found enough oxygen to burn themselves out.
The same thing happens in kerosene lamps, paper fires, camp fires, forest fires, and house fires: yellow flames, all. Air just can’t flow in fast enough to make the fuels burn completely to carbon dioxide and water.
If you don’t believe that there are tiny particles of unburned carbon in a candle flame, just insert the blade of a table knife in the flame for a few seconds, to catch them before they burn out. The blade will acquire a deep, velvety black coating of carbon. This carbon black is just about the blackest substance known, and is used in inks.
Gas grills and gas ranges, on the other hand, start out with a gaseous fuel, no vaporizing required. That makes it easy for the fuel to mix with lots of air, so that the burning reaction can go full blast. Because the fuel is burning almost completely, we get a much hotter flame. And it’s a clear, transparent flame because no glowing carbon particles clutter it up.
Want hotter yet? Why not mix pure oxygen, instead of air, with the fuel gas? After all, air is only about 20 percent oxygen. Glassblowers use a torch that mixes oxygen with natural gas (methane), to produce a flame temperature of about 3000 degrees Fahrenheit (1600 degrees Celsius). A welder’s oxyacetylene (oxygen plus acetylene gas) torch can reach about 6000 degrees Fahrenheit (3300 degrees Celsius).
Clear, blue flames, all, except when the torch is improperly adjusted so that the gas doesn’t get enough oxygen to burn completely. Result? A yellow, sooty flame.