wow .. thank you all !!

unfortunately I have no images
is there a mathematical formula how these cleaning (pepper) works ?
( I will have a look at Miller TIG Handbook )

Hi there, i am complete new to this topic - welding. A friend if mine was welding aluminum and around the arc, the workpiece was cleaned. He told me "spots on the surface" cleand it, I tried to google for spots but did not find anything only resistance welding. Hope someone knew anything a...