Why Sparks from Steel and Flint?

steel and flint - flint chips
Pieces of Flint – Image: Adapted from Photo by Connecticut Department of Transportation
As a youngster, I recall a neighbor had several rocks that were white with large areas of darker stone I now know as flint. Were the white portions limestone? I suppose even by then I was aware if you strike flint with steel, you get sparks. The reader may find online more than one explanation of why striking steel against flint produces sparks. Frankly, not all of the explanations provided are completely accurate. The best explanation is also the one that is most logical, most reasonable.

Chip Off the Old Block

Freshly broken flint, while somewhat smooth to the eye, if examined microscopically, has a rough surface. Now flint, would you believe it, is actually harder than steel. So what is the mechanism whereby one get sparks from steel and flint?

Steel and Flint

When a piece of steel is dragged quickly across a fresh flint surface, very tiny pieces of metal are heated by the friction to a temperature above the ignition point for iron. Iron is the main component of steel. In the presence of oxygen, the steel particle burns at white heat to produce a brilliant spark and a burned out cinder!


steel and flint - produce cinders
Cinders from striking flint and steel – Robert Hooke, 1780.
Does this not seem reasonable to you? Then consider this. Still smaller pieces of iron do not even need friction to ignite them in oxygen. Such iron is said to be pyrophoric. The video embedded below, affords visual proof that the above explanation is not only reasonable, but true as well.

Note: You might also enjoy Differences Between Fluorescence, Phosphorescence, Incandescence

References: ← Back to Classic Science
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One thought on “Why Sparks from Steel and Flint?

  • Scott

    Friction can create heat, but steel shards burn because of the rapid oxidation of the iron. [Agreed. – Author]

    Friction is chemical bonds between two objects, not tiny grooves that run together to make heat. Rough surfaces do create more friction, but it’s because the edges make for better bonds, not because the edges are being pulled apart like puzzle pieces. [As I understand it, the two elements discussed here both contribute. -Author.]

    Iron, the main ingredient in steel, is a reactive metal. This means that iron oxidizes (reacts) when exposed to oxygen… All reactive metals are generally considered [to possess a] stable… coating of oxidation on the outside. This coating keeps the rest of the metal from being exposed to air. The coating is only a few molecules deep. This is so thin that [it isn’t readily apparent]. [Yes. -Author.]

    So, how flint and steel really works: the harder flint breaks off [extremely finely divided particles] of the steel. The iron… rapidly oxidizes… or… light[s] itself on fire. [The two processes jointly enable high-speed oxidation–sparks. -Author.]

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