Fire is certainly high up on the top ten list of mankinds most useful inventions. It allows for a more efficient utilisation of nutriants, is the basis of chemistry and the material sciences, allowed us to settle the Northern regions, was the main way of storing energy for most of our history and gave us an edge over other competing species. One can hardly contemplate the rise of a technological civilisation without fire, thus making the range of atmospheric oxygen content where fire is useful important for my worldbuilding.

I'm already aware of a number of limits for oxygen levels. If the oxygen content rises above 35% fires won't stop burning and runaway wildfires will keep the oxygen levels at or below this value additionally the human breathing range lies between 0.16 and 0.5 atm.

Especially with the first constraint in mind I'm interested in the range of oxygen content the atmospere of the homeworld of an aspiring technological civilisation can have while fire remains useful to them. Useful means that the use of fire gives them huge advantages in their primitive environment like heating, lighting and cooking without demanding extensive caring (read if one person needs to supply air regularly just to keep the grill going its too much) or being extremely dangerous (if lighting a fire is like opening a gate to a dimension filled with ill tempered fire demons and one spark will annihilate the camp in a fierce cataclysm it is too much).

What is the range of atmospheric oxygen content I'm looking for? Does partial pressure or fraction of the atmospere matter?

Use earthlike conditions as your answers baseline, only taking away or adding nitrogen to adjust for the varying oxygen content. Should changing composition or pressure lead to interesting effects feel free to mention them. Thanks in advance guys.

  • $\begingroup$ virtual.vtt.fi/virtual/innofirewood/stateoftheart/database/… $\endgroup$ – Ville Niemi May 13 '19 at 12:56
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    $\begingroup$ Related: Fire on different oxygen density $\endgroup$ – a CVn May 13 '19 at 12:58
  • $\begingroup$ An answer for this cannot be given unless you also specify the fuel that you want to burn. Some fuels need oxygen rich environments to burn at all, others can "burn" without any oxygen. $\endgroup$ – XRF May 13 '19 at 20:51
  • $\begingroup$ @XRF Are there any naturally occurring self-oxidising fuels? Because the question says "primitive environment" and "aspiring" rather than established "technological civilisation" and all the self-oxidising or self-starting fuels I can think of take a chem lab to make in volume. $\endgroup$ – Ash May 14 '19 at 12:22

Given that the only tool using species we have examples of are forms of Earth fauna that require similar levels of oxygen to humans and tool use is the basis of the only advanced technological civilisation we know of the single-point-of-reference evidence we have would suggest that our current earthly levels are prerequisite for the type of civilisation you want to form.

To answer the question of what levels of oxygen are required to make fire a useful tool, the settlement of La Rinconada in Peru is instructive, La Rinconada has no fire department because fires cannot get out of control because the effective 11% oxygen means that fires need constant tending and/or specialised fire places/blowers to keep them burning and there is a distinct limit on the temperatures that can be reached; Gold, Silver, and even Copper can be smelted, with specialised equipment and techniques, but Iron cannot. Your upper limit of 35% is broadly correct for the carbon based flora and fauna we know. So for general cooking but also to have iron smelting etc... you're looking at something like 0.15-0.3atm partial pressure of oxygen. That 15% lower limit is a bit rough, may be a little higher that it needs to be, but it's an estimate based on the work of J.E. Rehder in The Mastery and Uses of Fire in Antiquity and correlates well with the highest altitude Roman Iron smelting sites found in the Alps at 2000-2200m which used standard designs that work equally well at sea-level.

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  • $\begingroup$ That's fascinating dude I had no idea there was a place on Earth where fire is simply not a danger! $\endgroup$ – nagamani May 19 '19 at 6:24

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