Many a time, a metalworker will receive a piece of steel that he does not know the carbon content of. To make a good knife out of it, he needs to know if it will be hard enough, or maybe soft enough. Also, the ability to anneal, harden, and temper it is reduced if he does not know the percentage of the carbon content. He could send the piece to some university with equipment fro scanning the atomic content, but that would be VERY expensive, and if he’s anything like me, there is no way he would want to wait long enough for the steel to arrive at the university, scanned, and sent back. So, what does he do? He grinds it.
When a piece of steel is put against a grinding wheel, the wheel grinds away little bits of the metal at high speed. As they leave the main piece of the metal, the friction created by the high speed grinding wheel heats up those little bits, and, as there is no other steel to help disperse the heat, they burn as they are flung away, releasing energy in the form of light.
Different steels will burn in different ways; higher carbon normally means shorter but thicker sparks, going out a little then seemingly exploding, or many sparks separating from the main one. Lower carbon would typically mean really long sparks, fairly bright, but not “exploding” very much. Aluminum can be detected easily as it does not burn, and held against a grinder no sparks come. Titanium burns much hotter than steel, and when it is held against a grinder, the sparks are super long and blindingly white.
Here’s a little vid I found that demonstrates the spark test: