I’ve heard that when you strike flint with steel, the flint shaves off small pieces of the steel and that’s what makes a spark, but even if that is true I don’t understand how shaved steel makes sparks.
In: Physics
You may be aware that fine steel wool burns quite easily.
Because most metals are good heat conductors, combustion isn’t self sustaining on massive, monolithic objects. Heat from oxidation with the air is just conducted into the interior of the part.
Combustion requires three things, fuel, oxygen, and heat. Remove the heat quickly and the combustion stops. Red hot steel will quickly oxidize, forming a layer of grey magnetite (Fe3O4) that gradually flakes off. This can be seen in forging operations. This isn’t self-sustaining though.
However, fine steel shavings combust much more easily because the have a high surface area but very low mass.
Shaving fine bits of steel at high speed with a much harder material like flint, can heat the shavings to such an extent that they ignite and burn just like steel wool.
You can see this process [in a more controlled fashion on a metalworking lathe in this video.](https://youtu.be/BU14tfjUfQU)
(Note, a specialized cutting tool is being used here that can tolerate the extreme force without chipping the edge off. )
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