Guido - allow me to take a moment to pay repsctes. I have seen some VERY insightful posts from you on a vast range of topics. You are the audiophile's audiophile!
In addition pumice would display no, or negligible, electric conductivity. The disadvantage is that--being essentially a glass foam, it is relatively fragile, and will withstand torque or shearing stresses somewhat poorly. In that a metallic foam would be superior. . . but do we want the entire speaker to conduct electricity?
Maybe if you infused the pumice with various metals it would increase the structural rigidity of the cabinet allowing it to withstand the torque that is applied. I suggest various metals as I think that light show produced by the cabinets conductivity could be quite interesting and maybe even have multi-colored sparks...what could be better? It would be like the 4th of July every night in your listening room.
You would probably want to listen from a safe distance though.
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