Question:Rava proves from tzamid pasil that there can be no tumas medras on an earthenware vessel.
My question is: The tzamid pasil can certainly become tamei through a sheretz. For example, while it was not attached as a tzamid pasil a sheretz may have fallen into its airspace. Still, we dont say that this proves that an earthenware vessel can't become tamei at all. SInce the tzamid pasil may be tamei, there are situations when it will not stop the tumas meis from entering. We must say the pasuk means than any earthenware vessel that has a tahor tzamis pasil on it will be tahor in the ohel ha-meis. So perhaps likewise, if an earthenware vessel which has a tzamid pasil is set aside as a couch, the tzamid pasil will stop the tumah from entering only if it is tahor. So perhaps an earthenware vessel could become tamei medras, but the pasuk of tzamid pasil is not referring to a tamei tzamid pasil.
The Tzamid Pasil does not have to be an earthenware vessel; it does not have to be a Kli at all. Even a plug of cement serves as a Tzamid Pasil.
However, you are correct in assessing that even if he placed the top of a barrel, which is a Kli by itself, onto the top of the barrel and sealed it in place, it protects the barrel from becoming Tum'ah because it serves as a Tzamid Pesil. The reason why your question didn't bother the Gemara is because it is only with regard to the Kli itself which has a Tzamid Pasil, that the Pasuk states, "And all open utensils...." (i.e., even one which is reserved for sitting upon). The Pasuk does not state that all Tzamid Pasils can save a Kli, since, as you pointed out, it cannot save the Kli if the Tzamid Pasil is a Kli Tamei.
Thank you for your thought-provoking questions, and l'Hitra'ot1
Y. MONTROSE