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According to an old Wikipedia revision, charcoal burns at temperatures as high as 2600 °C. Other sources which seem less reliable give a figure of around 1000 °C. The Wikipedia article has since been updated to state a temperature of 1100 °C.

I know that it depends on the airflow, but how high of a temperature can normal charcoal (bought for BBQs) burn? Can it be used to melt metals with a melting point such as iron or steel?

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    $\begingroup$ Melting of steel is possible (how do you think steel was invented in the first place?), but requires special setup. 2600 °C seems unrealistic. $\endgroup$ Commented Sep 28, 2016 at 11:32
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    $\begingroup$ The high value of 2600 °C probably corresponds to the adiabatic flame temperature of a certain charcoal in air (i.e. a theoretical temperature based on the heat of combustion of charcoal and the heat capacity of the involved gases, without any heat losses from the flame). $\endgroup$
    – user7951
    Commented Sep 28, 2016 at 11:55
  • $\begingroup$ In open air at atmospheric pressure (BBQ) expect about 1000 °C or lower. It's enough to reduce iron, but not enough to melt it. That's why steel was first invented as porous substance, which must undergo thorough casting to form compact metal. In oxygen and specially constructed furnace about 1500 °C is expected. $\endgroup$
    – sa7
    Commented Oct 3, 2016 at 15:53
  • $\begingroup$ Perhaps even 2000 in oxygen. But I doubt about any useful setup to obtain 2600 from burning charcoal. $\endgroup$
    – sa7
    Commented Oct 3, 2016 at 16:01
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    $\begingroup$ The problem with the question is that there is no unique temperature at which charcoal burns. There could easily be a 1,000 degree difference between the temperature in an open barbeque and a specially constructed furnace built with insulating walls and forced air flow. Iron can be melted but furnace construction is a skilled job and the limiting factor in the temperature reached. $\endgroup$
    – matt_black
    Commented Aug 18, 2021 at 14:24

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This question has been unanswered for a while, but there are some great comments which I will use as part of the answer, as well as some answers to other questions here and on the Physics site.

The temperature is dependent on the location

In well-controlled flames such as a candle or a Bunsen burner, you can measure the temperatures in different locations of the flame and get a wide range of temperatures. Here is the example of the candle, copied from this Physics StackExchange answer:

enter image description here

In the question, the OP asks:

[...] how high of a temperature can normal charcoal (bought for BBQs) burn? Can it be used to melt metals with a melting point such as iron or steel?

You would need the hottest part, not some average temperature. So if you want to melt iron or steel samples, you would have to have some part of your setup get sufficiently hot to melt the sample, and this space would have to have sufficient volume so that heat conduction through the sample does not lower the temperature below what you need.

The temperature depends on the design of the furnace

[OP] I know that it depends on the airflow [...]

It depends on the airflow, the temperature of the supplied oxidant, the amount of heat transferred to the product gases (carbon dioxide and perhaps others). It also depends on the design and dimensions of the setup:

[Matt Black in comment to question] The problem with the question is that there is no unique temperature at which charcoal burns. There could easily be a 1,000 degree difference between the temperature in an open barbeque and a specially constructed furnace built with insulating walls and forced air flow. Iron can be melted but furnace construction is a skilled job and the limiting factor in the temperature reached

So there is no typical temperature associated with a given fuel. However, you can ask something specific, like about the temperature on the surface of the charcoal when they have burnt for a while. However, there is the concept of adiabatic flame temperature:

[user7951 in comments] The high value of 2600 °C probably corresponds to the adiabatic flame temperature of a certain charcoal in air (i.e. a theoretical temperature based on the heat of combustion of charcoal and the heat capacity of the involved gases, without any heat losses from the flame).

Ways to measure the temperature

You can either measure the temperature by inserting a probe into the flame, or by observing the radiation that is emitted (and deduce the temperature based on theory and some calibration data). Some methods works better for sooty flames whereas other work better for soot-free flames (see introduction to this manuscript).

Color due to black body radiation

If there is a solid in the fire (e.g. the charcoal, glowing embers or smoke, i.e. small solid particles), they will emit black body radiation. The surface of the sun, at first approximation, is a black body that emits white light, corresponding to a surface temperature of about 5,600 degrees Celsius. The color of light bulbs is described by a temperature, even though modern light bulbs don't use black body radiation, whereas incandescent light bulbs did. Red-glowing char coal corresponds to a temperature of about 700 degrees Celsius.

If you want to measure temperatures of the gases, you can lower a wire into the flame and check how it changes color.

Not all colors you observe are from black body radiation. For example, gases can emit light from electronic transitions (e.g. sodium vapor lamp) and vibrational transitions (e.g. Swan bands).

In the example below, a metal inoculation loop (probably nichrome) is placed into a flame for sterilization. The orange color of the loop is due to black body radiation, the blue flame color is due to Swan bands, and the orange color of the gas above the loop is due to sodium electronic transitions (sodium is on the loop before sterilization):

enter image description here

Source: Wikimedia

Infrared spectrum

You can measure the temperature of a gas from the vibrational states the particles are in. The infrared spectrum of carbon dioxide, for example, depends on the temperature. Temperature "guns" use the infrared emission to estimate the temperature of a sample, but they work best with surfaces.

Thermocouple

You would not be able to use a conventional alcohol or mercury thermometer in a charcoal pit. However, there are thermocouples that have a wide temperature range. One technical difficulty of using them in a flame is the formation of soot on the thermocouple, making the measurement less accurate.

Safety considerations

If you have a very hot fire that can melt metal, you have to make sure your grill doesn't melt (or deforms and collapses). Furnaces are made from material with higher temperature stability than steel, and very hot flames (such as an acetylene burner) are designed in a way that the hottest part of the flame points away from the metal parts.

You can always make a fire hotter by making it bigger as long as the air supply is ensured (camping fire vs. bonfire vs. forest fire), but that makes it more difficult to keep under control.

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  • $\begingroup$ You are confounding temperature with heat evolution in your last paragraph Bigger is not necessarily hotter; There is more heat evolved. $\endgroup$
    – jimchmst
    Commented Dec 29, 2023 at 4:12
  • $\begingroup$ @jimchmst The ratio of surface to volume gets smaller, so less heat escapes. $\endgroup$
    – Karsten
    Commented Dec 29, 2023 at 4:23

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