Temperature when we strike a spark in vacuum 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)?
 A: 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
