I did a surprise visit on a cousin yesterday. She had a large clay flower pot with a cast iron lid in the living room. It's her light bulb heater. It has a 3 way bulb as the control. She turned it on 50 and replaced the lid, 30 minutes later that lid was hot, 45 minutes later the living room was hot. She has one in the kitchen, bedroom and master bath. She said the other day with the cold spell we had her heat came on very little. They have cut her heating bill to almost nothing since the bulb doesn't use as much electricity as other heaters.
I have 4 large Terracotta pots in the shed. Some single base fixtures and cords, a good supply of 60, 75 and 100 watt bulbs to try before buying 3 way fixtures and bulbs.