A WELDER AND A LONGWALL MECHANIC, WERE IN PROCESS OF ELECTRIC AIR CUTTIN ON THE CUTTER HEAD PLOW OF THE FACE SHEARER AT THE NO 20 SHIELD WHEN A SPARK IGNITED AN UNKNOWN BLEEDER, AT THE TOP OF THE BLUE CREEK SEAM. THE FACE WAS CRACKED. MSHA NOTIFIED, INVESTIGATION CONDUCTED.