Without anything to rub or effort, Put the oven rack in a bag

Leave the bag in a safe, ventilated area for 8–12 hours (overnight is best).

Remove and rinse
After soaking, open the bag (carefully — avoid breathing in fumes), remove the rack, and rinse it with water. Most grime will wipe off easily or rinse away.

Important Notes:
Do not mix ammonia with other cleaners — especially bleach (toxic gas).

Use gloves and avoid inhaling ammonia directly.

This method works by letting the ammonia fumes loosen baked-on grime, so scrubbing is either very minimal or not needed at all.