Makhshirin, Chapter Four, Mishnah Eight
1) If a pot full of water was placed in a mikveh, and a person who was unclean by a father of impurity put his hand into the pot, it becomes unclean.
2) But if [he was unclean] by a “touch of defilement,” the pot remains clean.
3) But any of the other liquids [contained in the pot] becomes unclean, for water cannot purify the other liquids.
Section one: The pot in this mishnah is made of earthenware, which does not become pure by having been put in a mikveh. If a person who is a “father of impurity,” meaning he has a high level of impurity (for instance, contact with a dead body or with a sheretz), puts his hand into the pot, he defiles the pot.
Section two: However, if the person putting his hand in the pot has a lower degree of impurity, meaning he touched someone who is a “father of impurity,” then the pot remains clean for such a person does not defile vessels. Only a “father of impurity” defiles vessels. The water remains clean as well, for the water is in the mikveh.
Section three: However, any other liquid that might be in the pot (oil, for instance) does become impure and then it will defile the pot. The water of the mikveh purifies only water, not other liquids (see Mikvaot 10:6). See also Kelim 8:4, where the irony of this situation was discussed.