I have found if the engine has turned dark when its removed from the antifreeze , to high of a temp was used, Low heat is all thats necessary,and unless there really bad 2-4 hrs as a rule is long enough to get one looking like new again.

And by all means never reuse a crockpot for any food products once its had antifreeze in it, also when useing a pot to clean with make sure its used in a well ventalated area, The fumes can be toxic also.

Theres several ways to clean up a engine, Heres a few methods I have used over the years
The crockpot IMO cleans the best.
1. Crockpot & antifreeze
2.Chevron engine cleaner
3.Dawn Dishwashing soap
4. Brake or carb cleaner