it pays to have a good vocabulary

LOL

its the iron particles oxidizing in mid air after being sanded off. the iron oxidizes and burns until it is completely oxidized. like how steel wool turns black once you ignite it. aluminum when its sanded doesnt make sparks usually because when the aluminum particle heats up and starts oxidizing in the air aluminum creates a different oxidation layer thats clear and very hard and prevents oxygen from getting to the aluminum to cause it to further more burn up like a typical iron spark would

but tensile strength supposedly also plays in, in that aluminum is so soft that the particles never really "snap" while sanding like steel with a higher tensile strength does. i guess thats why soft steel sparks less than hardened steel? could be the carbon content also but i have used a grinder on a piece of soft high carbon steel and it makes a little less sparks than a hardened piece

Hahaha

😏

my brain went full nerd mode here and was about to google aqua thermal treatment and then i realized the joke. thats clever