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In "Artemis" Andrew Weir says that the hot flakes produced by striking flint and steel together do not ignite acetylene in vacuum. He says that the reason steel flakes get white-hot on Earth is that they have very high surface to volume ratio and oxidize in air so fast that they basically burn. No air -> no oxidation -> no white-hot flakes. In air, the process is as follows:

1) Flint striking steel scrapes off a flake, exposing fresh iron at a certain temperature $X$ before oxidation.

2) The iron oxidizes. This is an exothermic chemical reaction, which raises the temperature of the flake from $X$ to well above 1000 °C. This high temperature is what causes the flake to glow white-hot.

3) Acetylene's autoignition temperature is 300 °C, so the white-hot flake is more than enough to ignite/oxidize the acetylene.

What is the initial temperature $X$ of flakes just after the strike, assuming no oxidation (e.g. we are striking in vacuum)?

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    $\begingroup$ "at least some sparks should be able to ignite acetylene" doesn't matter how hot the little bits of metal are, if there is no oxygen then acetylene will not burn $\endgroup$ – pentane Jan 5 '18 at 4:16
  • $\begingroup$ @pentane, bad wording of mine, in Artemis they use acetylene/oxygen mixture. It does not burn till oxygen proportion is increased. $\endgroup$ – Vashu Jan 5 '18 at 9:12
  • $\begingroup$ By definition before they start to burn the particles are just below the autoignition temperature... once they reach that temperature they start to burn $\endgroup$ – pentane Jan 5 '18 at 23:32
  • $\begingroup$ hey Vashu what do you think of my statement directly ^above? Maybe we can make your question a bit clearer if we work together $\endgroup$ – pentane Jan 16 '18 at 4:55
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    $\begingroup$ Can you provide the full direct quote? You can't have an oxyacetylene mixture "in vacuum"-- if you release gas into vacuum it's not a vacuum anymore (at least not locally where the gas is being released). $\endgroup$ – pentane Jan 20 '18 at 10:20
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I doubt that you can see sparks only when the spark material burns in air. I even remember that as a child hitting two stones together produced sparks that were visible in the dark. If the sparks are hot enough the spark material should glow and emit light. Stone age humans used flint (which is quartz) for producing sparks to initiate fire. Flint cannot oxidize in air.

Extension following comments by @pentane. He has kindly pointed out in his comments (see below), that there exist scientific investigations regarding the pre-historic fire-making with the help of flint stones. The scientific consensus seems to be that in spite of visible sparks produced by hitting flint on flint, these do not produce sufficient heat to ignite a fire. Therefore in the stone ages, for producing fire, flint was used together with stones of other chemical composition like pyrite, which oxidize in air and therefore produce hot sparks.

The references provided by pentane are:

Oakley 1955: https://www.cambridge.org/core/journals/proceedings-of-the-prehistoric-society/article/fire-as-palaeolithic-tool-and-weapon/81DEEE06BED454DA72F0A9FE2602EE85

Stapert & Johansen 1999: https://link.springer.com/article/10.1023/A:1003802432463

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    $\begingroup$ Flint is used in combination with iron (like pyrite or steel) for sparking, not by itself. $\endgroup$ – BowlOfRed Jan 5 '18 at 18:39
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    $\begingroup$ @BowlOfRed -You are right! But steel which has been used recently was not available in the stone age. And you get sparks with two flint stones. The question is whether this suffices to produce a fire. Here (simple.wikipedia.org/wiki/Flint) they claim "When two flintstones are hit together, they can make a spark. For many centuries, flint was one of the main ways for people in many countries to make fire." $\endgroup$ – freecharly Jan 6 '18 at 2:26
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    $\begingroup$ "The sparks produced by striking stones together must have been very evident to men of the early stone age working in dim light. The flash produced by flint on flint, or quartz on quartz (triboluminescence), has no incendiary properties ; but the use of a nodule of iron sulphide, or pyrites, as a hammerstone in flaking flint would readily lead to the discovery of the percussion method of making fire, for the sparks produced in this way are hot incendiary particles, easily igniting any sufficiently dry tinder." - Oakley 1955 doi.org/10.1017/S0079497X00017382 $\endgroup$ – pentane Mar 13 '18 at 17:44
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    $\begingroup$ "According to Oakley (1955), however, it is not possible to make fire by striking quartz against quartz; this procedure would result in cold sparks unsuitable for fire production (triboluminescence). This is also the opinion of G.J. Boekschoten (pers. comm., 1996). On the basis of experiments, Collin et al. (1991) concluded that making fire by striking two flints together is ‘probably a legend’ (see also Collina-Girard 1998)." - link.springer.com/article/10.1023/A:1003802432463 so it seems the answer to whether the flint-on-flint sparks suffice to make fire is a "no." $\endgroup$ – pentane Mar 13 '18 at 17:47
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    $\begingroup$ @pentane - Thanks for the links to the interesting publications! So scientists seem to have already investigated the question, how the fire was made using fire stones. It seems pretty well established that the oxidation is necessary for the sparks to make fire. $\endgroup$ – freecharly Mar 13 '18 at 19:12

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