Matting timbers were used to support shovel side frame caught fire which lasted more than 30 min. Presumed to have been caused by slag from welding/cutting torch the previous afternoon. Work was completed at 3:00 pm 11/23, final walk around at 5:45 pm. At 4:30 am 11/24, fire noticed by night shift personnel. MSHA notified at 5:02.