Gerald,
most excellent and most professional - as always!
"There are many others, but slag is in fact possible to find on the surface.
A slag inclusion can show up anywhere in the weld, Root, Crown, or in the Fill." I have tried to find a picture on the www, showing a linear slag inclusion open to the surface, and found it on a German Car Mechanics' website(!).
I would like to embed this picture just as a humble addendum to your excellent response.
Stephan