Rake over the red mud with a stiff brush to remove as much of the solid material as possible.
Fill a container with 1 cup water and 1 tbsp. borax. Stir until the borax dissolves completely.
Moisten a cleaning cloth with the solution and saturate the stained area thoroughly. Allow it to remain in place for 15 minutes.
Rinse the affected area of the comforter with cold water.
Fill a bucket 3/4 of the way with cold water if any of the stain remains visible. Pour in 1 cup household ammonia and submerge the stained section of the comforter in the bucket. This is best done in the bathtub to prevent a mess.
Let the comforter soak in the solution overnight.
Wring the excess moisture out and examine the comforter for stains. If any are still visible, continue to soak the comforter in the ammonia and water mixture until the stain is removed.
Launder the comforter as usual after the red mud stain is gone. Remember to consult the care label and follow the directions carefully.