EMPLOYEE WAS SLAGGING THE IMPELLOR INSIDE THE QUENCH TANK; HE WAS USING A JACK HAMMER AND STANDING IN MUD. WHEN THE SLAG CAME FREE, THE HAMMER WENT FORWARD & THE EMPLOYEE'S FEET WERE STUCK IN THE MUD. HIS HAND SLIPPED FORWARD ON THE HAMMER, CUTTING HIS RIGHT MIDDLE FINGER ON THE AIR COUPLING.