The vessel they fitted over does not have to have been ceramic. Whats the possibility of them fitting over a metal vessel or even wood? There might be advantages to heavy lids and I have seen stone "pot lids" which were Victorian and used for ceramic vessels. But again the question of what they started out as still raises its head.
The dimples increase the surface area if you were trying to control evaporation of a liquid, just a thought. Might work for melting something over a boiling liquid.
The dimples increase the surface area if you were trying to control evaporation of a liquid, just a thought. Might work for melting something over a boiling liquid.