Mishnayos Keilim Perek 8 Mishnah 6

בית שאור המוקף צמיד פתיל (a clay leaven pot with an airtight lid) – we ae dealing with an earthenware vessel, that saves in the airspace of the oven with an airtight lid in the manner that it saves in the tent of a corpse (as only the space within an earthenware vessel can contract ritual impurity – and everything within it remains ritually pure), as it is written (Leviticus 11:34): “As to any food that may be eaten, [it shall become impure if it came in contact with water; as to any liquid that may be drunk, it shall become impure if it was inside any vessel,” but not all of the food, excluding that which is surrounded with an airtight lid on an earthenware vessel and placed inside the oven that does not become defiled from the airspace of the oven. But a vessel that requires only rinsing to be restored to Levitical cleanness (see Tractate Hullin 25a; Tractate Zevakhim 3b) does not save with an airtight lid on the foods that are within it, as this is taught in the Halakhic Midrash (Sifra) to the Book of Leviticus, [All] the food, including that which is surrounded by an airtight lid with a vessel that requires only rinsing to be restored to Levitical cleanness and placed within the oven, that it does not save it from the hand of an earthenware vessel [and makes it impure].

השאור והשרץ בתוכו והקרץ בינתים – as for example, that this vessel has two receptacles, and the partition that divides between receptacle and its fellow is called a קרץ/[squeezed-in] partition; it is the language of (Job 33:6): "מחמר קרצתי גם-אני"/[You and I are the same before God;] I too was nipped from clay,” and the partition reaches until its rim, and the leaven from one side and the unclean reptile/insect from the other and the partition is the divider that separates between them, but if the entire vessel is surrounded with an airtight lid, that it is found that the leaven and the unclean insect are surrounded by an airtight lid.

התנור טמא – on account of the unclean insect/reptile that is within the airtight lid. For the airtight lid saves/protects those pure things that are within from becoming susceptible to receiving ritual impurity, but it doesn’t save those unclean things found within from defiling other things.

והשאור טהור – for it is surrounded by an airtight lid by itself.

ואם היה שם פותח טפח – that the partition that was between the leaven and an olive’s bulk from a corpse was beached with an opening of a square handbreadth [in length, breadth and height].

הכל טמא – that a handbreadth by a handbreadth that is open brings in the defilement.