My sister ruined her self cleaning oven many years ago, using oven cleaner on
You can wash off the bottom of the oven using mildly abrasive pads - just to
give it a head start. Then turn on the oven for the longest recommended time.
I recently cleaned mine and also had two badly crusted iron skillets in there.
Came clean with just 2 1/2 hours instead of the usual 3 1/2.
Be prepared to brush out the ash as soon as it is cool enough. Dry cloth works
best so you don't smear it around.