I am a bonsai artist. Recently, I discovered a real treasure at an estate sale--a set of three bonsai pots. One of them had been filled with concrete and large pebbles to make it a heavy base for artificial trees. I want to remove the concrete. The pot is of fired clay with fired glaze. The inside, where the concrete sits, is not glazed. I'd rather not take a hammer and chisel to it. I read where muriatic acid could work if the concrete was not more than 28 days old; however this is definitely old concrete. Help, please! The photo is an example of the kind of pot involved.