Mishnayos Machshirin Perek 4 Mishnah 8

ופשט אב הטומאה את ידיו לתוכה טמאה – for an earthenware vessel receives ritual defilement from a primary source of ritual impurity (i.e., a corpse) that touches him, his ritual immersion did not count, for even though he was in the ritual bath/Mikveh, because the waters that are in it (i.e., the dish) interpose between him and waters of the ritual bath/Mikveh.

מגע טומאות טהורה – If it was not a primary source of ritual impurity that touched it, but rather a first-degree of ritual impurity, and he is called unclean [only by reason of] contact with unclean things, that is to say, he comes in contact with primary sources of ritual defilement, then the dish is ritually pure, for an earthenware vessel does not receive ritual defilement other than from a primary source of ritual defilement, and it is found that the dish was not defiled other than on account of impure liquids that were within it that had been defiled, and they go back and defile it, and because of this it does not defile, for the waters that were within it were dipped in the waters of a Mikveh/ritual bath and purified. But the rest of the liquids, as for example, wine, [olive] oil and milk, the ritual bath/Mikveh does not purify them, for they do not consider it the dipping of a vessel, filled with an unclean liquid, so as to make its surface level with the surface of the water into which it is dipped/contact between liquids (i.e., a ceremony of Levitical purification), therefore they return and defile the dish, for the Rabbis decreed upon liquids that they would defile, as a decree because of the liquid of a person afflicted with gonorrhea and/or a woman with a flux.