I worked for just about a year in an old General Motors foundry. Slag for us was the scum of crude melted glass on top of the puddle in the iron melt furnace. The slag rake for removing it was an iron plate on the end of a heavy pipe, mounted on a fork lift to reach into the door of the furnace; just picture a great big safety razor.
I got a slip for a two-week layoff one night, and later that night tripped over a slag rake and broke an arm.
My Dad did his own welding on the farm, so I know welding slag too.