system
Given the following post, respond as if you're commenting on Reddit:
human
Fires burn at different temperatures, what's the 'coolest' fire that we know of? How is it made?
gpt
What you're referring to is called a "cool flame" which is classified as any flame cooler than 400 °C (752 °F). 

The short answer is: It depends entirely on the fuel being used. The lowest possible CFT (cool flame temperature) is only limited by our ability to craft a fuel with the right ratios. Generally speaking the lowest CFT is defined as "the lowest  temperature the human eye can detect light being radiated from a flame in a completely dark environment". It will usually appear as a pale blue. 

In conventional flames, the molecules are broken down into small particles which combine with Oxygen to produce CO^2. This process of burning gives off lots of heat and light. In "cool flames" these molecules do not have the chance to break down into smaller particles as much, so they are more likely to recombine and continue the cycle. Since the larger molecules are not able to combine with Oxygen at the same rate as a conventional flame does, there is far less light, heat and carbon dioxide being released. 

There are a few terms to consider. 

* Flashpoint is the lowest temperature necessary to ignite vapors of a fuel source, given an ignition source.

* Firepoint is the lowest temperature necessary to keep the flame lit, having removed the ignition source. 

* Auto-ignition temperature (AIT) is the lowest temperature necessary for the fuel source to create it's own ignition. 

The CFT can fall anywhere between the Firepoint and the AIT. Again, it depends purely on the fuel source. 

Butyl acetate for example has a very low CFT of 225 C, an AIT of 370 to 425 C, and a Flashpoint of 22 C. These temperatures may vary depending on the experiment conditions.