1 cup ammonia
2 cups boiling water
1/2 cup baking soda
1 cup white vinegar
Preheat oven to 200 degrees F. Leave oven on for 15 minutes; turn off and leave the door closed.
Fill a shallow baking pan with ammonia and place on the top shelf of the oven. Fill another pan with 2 cups boiling water and place on the bottom shelf. Close oven and leave pans in for at least 2 hours or overnight. Remove ammonia and make a paste of ammonia, baking soda and vinegar. Spread paste on oven surfaces, leave on for 15 minutes, and then scrub off with a sponge or steel wool.