How hot can an open-air fire reach?

Answer 1

The answer depends, of course, on what you are burning in the open air...

Last year, I did a bit of research to see if diamonds could burn in house fires. I found that house fires rarely reach above 600 C (1100 F).

For hotter fires, you'd need special devices (e.g., compressed flammable gases, burning metals).

For example, you can set sufficiently fine steel wool on fire. You can start it with a match and it will burn even if wet. It burns very hot but I can't find the temperature.

Magnesium was also used for "flash" photography. Magnesium can be set on fire as low as 900 F (475 C). Once on fire, magnesium will burn in open air, at a temperature of 4000 F (2200 C) and will be very difficult to extinguish. At that temperature, the water molecule is disassociated before hitting the fire, thus providing new oxygen (and hydrogen) to the fire.

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By the way, diamonds burn at around 800 C (1500 F). Expensive diamonds can be coated with boric acid. However, the coating will cease to protect the diamond if it reaches its melting point (1200 C).

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I just found that a controlled coal fire (not open air) can reach 2200 C (4000 F), a little higher than what can be achieved with natural gas (2000 C).

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Many simple things (e.g., wood) can also have very hot flames in controlled environments. However, in open air, turbulence will keep it down. (still hot enough to burn your hand).

Answer 2

Enough to melt metals and even soften rock if the fire is big enough. A safety match can reach 800 centigrade if that puts things in perspective.