This is genius. A neighbor gave me this tip. Above are my cleaned grates and trust me those oldies are so clean now.
Here they were before. I used to have to try and scrub with a steel wool sponge and never get it all clean.
Here is what you need:
Large plastic zip lock bag
sunshine on a warm day
Okay head outside. Ammonia is strong stuff so hold your breath while you are working with it. Put the graters two at a time in the bag on top of the cookies sheet. Pour half a cup of ammonia into the bag and seal completely. Put the cookie sheet in the sun. I did it on a sort of cloudy day so had to leave out two days. That is why I did two at time so I could still use my stove. Hot sun is best.