I remember my father building a fire to “cold pack”. Jars were done the same, but put into a large covered pot with water. He would keep the fire hot for several hours. It worked, but he only did that occasionally as we also had two pressure canners.
That’s interesting, I’d never heard of anyone doing that. I’ve canned butter before. One persons website I read says they’ve been doing it for years and sometimes the butter would be several years old before they used it. Then freaked out when I read about a dangerous bacteria you can get from canned butter, or lard. I can’t remember what the bacteria was called but, it wasn’t a good thing. It’s something that hides in fat and is very hard to kill. Can survive without oxygen, and I don’t think it was salmonella, botulism or listeria.