Why is it that a Klei Cheres has different laws than other Kelim? How do you understand the Torah's ruling that it becomes Tamei only from the inside? And why should it be that metal utensils become Tamei even if they do not have a receptacle, as opposed to Cheres, wood, cloth, bone etc.?
Mordechai from Raananna
The SEFAS EMES offers a beautiful answer to your first question (Parashas Shelach 5633) in the name of his grandfather, the Chidushei ha'Rim. He explains that Cheres (earthenware), as opposed to the other materials you mentioned, does not have inherent value - since it is simply baked earth. What gives it value is the fact that it is useable, and can be used as a container. Therefore only its inside, the container (the Tzurah), can bring it Tum'ah, not the outside, which is simply clay (Chomer). Perhaps that is also why it has no Taharah in a Mikveh. Washing cannot cleanse a Kli Cheres since it only relates to the material; the Tzurah must be changed (i.e. it must be broken and lose its container).
A similar point can perhaps be made with regard to metal. We are told that metal is unique since it honors it owner and it is especially expensive (Chulin 25b). With other Kelim, although the material itself can receive and transmit Tum'ah, that is only when it has the additional value of a Tzurah (the container). But metal does not need a container in order to become Tamei, since it is inherently valuable. (It must be a Kli though, for otherwise it is like a piece of stone and does not serve a person in any regular way.)
Best wishes,
Mordecai Kornfeld
Kollel Iyun Hadaf