As a youngster, I recall a neighbor had several rocks that were white with large areas of darker stone I now know as flint or chert. The white portions were limestone. From youth, watching black-and-white TV programs, I was aware if you strike a sharp edge of chert against steel, you get sparks.
The reader will find online many ‘explanations’ why striking steel against flint produces sparks. Frankly, not all 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 true mechanism whereby one get sparks from steel and chert?
Steel and Flint
When a piece of steel is dragged quickly across a fresh chert 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!
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. The name applied to a material that burns in this way is pyrophoric. The following brief video explains how you can obtain a suitable source of iron and rock to produce sparks and start a fire!
Note: You might also enjoy Differences Between Fluorescence, Phosphorescence, Incandescence
- Northern Illinois University: Flint and Steel Firebuilding
- Ragweed Forge: Firemaking with Flint and Steel
- Microscopic Observations, by Robert Hooke (1780)