why is smith called 'smudger'?

Alan Smith at Arsenil was called Smudger too. In the services if your surname is Smith it is pretty inevitable you'll end up being called Smudger. Why is shrouded in mystery.
 
Harry3534 in the Newbies reckons it derives from the habit of smithies smudging their faces with furnace dust. Seems a likely explanation.
 
there's a whole lost of old public school nikcnames the only other one i can remember is that if your surname is White, your nickname is Wally