You are misunderstanding the purpose of the different things you are describing. People aren’t using the Crisco candles contained in sand to generate heat. Those are extended use light sources. The use of the terracotta pots placed over a candle or other flame heat source is to capture and re-radiate the heat using the terracotta pot as a larger surface for heat to radiate off of. Larger surface area = more USEFUL heat being radiated towards the people in the room. You are mixing up the ideas here. Which could work if you actually had a useful arrangement for the pot over the Crisco candle.