Why do steel sparks do this thing: they fly straight and seem to explode at the end of their trajectory.
The spark makes a line, then there’s a little star-shaped explosion at the end and it goes dark.
My suspicion is that the steel is oxidizing and it heats up more as it goes through the air, finally getting hot and breaking into pieces. If you look at the two sparks center-left you’ll see what I mean – there’s a line and then it looks like it’s breaking into 4 or 5 fragments. I’m guessing that, at that point, they no longer have enough thermal mass to stay at radiant temperature and go dark.
Anyone know? It’s not as if this is important, but it’s something I’ve wondered about at a low level on and off since I first saw someone grinding steel. It doesn’t work for all metals, e.g.: copper or brass (I’ve ground my share of brass) so it’s got to have something to do with oxidation?