Keilim, Chapter 9, Mishnah 5. Our chapter deals with an earthenware oven, a tanur, and with tumah that reaches its airspace. This mishnah adds a further set of cases: objects that have soaked up tamei liquids inside themselves. As long as the liquid stays locked inside the object, it has never really entered the oven's airspace; only heat draws it out. So the mishnah teaches us that the oven's status depends on whether it was fired up or not.
A shard that absorbed tamei liquid
The mishnah begins: "Charasin shenishtamshu bahen mashkin temei'in venafelu la'avir hatanur, husak hatanur, tamei." Fragments of earthenware served as vessels for liquids that carry tumah, say a bodily secretion that is a source of tumah, and that liquid was drawn into the clay. Wipe the surface and nothing impure can be seen; the liquid, though, is lodged inside the material itself. One of these fragments now drops into the hollow space of a tanur that until now was tahor. Should the oven be fired at that point, it contracts tumah.
Why? The mishnah gives the reason: "shesof mashkeh latzet," the liquid will ultimately emerge. Heat drives the absorbed liquid out of the shard, and once it leaves, the tamei liquid is present in the oven's airspace and conveys tumah to the oven. But if the oven was never heated, the liquid remains sealed inside the earthenware. Nothing tamei ever entered the airspace, and the oven stays tahor.
Fresh olive pulp
"Vechein begefet chadashah." An identical ruling governs gefet, the mash of olives that remains once they have gone through the press. Imagine the pressing being done by someone in a state of tumah. The oil has been extracted, so to the eye the mash appears dry, but some oil is unquestionably still held inside it. Should such newly pressed mash drop into an oven that is being fired, tumah is contracted, for the heat draws a further trace of oil out of the mash, and that trace reaches the oven's inner space. And here too, an oven with no fire under it remains tahor.
Old pulp, and pulp that still gives off liquid
"Aval biyeshanah, tahor." Everything said until now applies only to freshly pressed pulp. Pulp that is old, more than a year, that falls into a heated oven leaves the oven tahor, since we assume that pulp of this sort has dried out completely; even under heat no liquid will come out of it, so there is nothing to convey tumah.
The mishnah ends by carving out an exception: "Ve'im yadu'a sheyotzei meihen mashkin," if it is established that liquid does in fact emerge from such mash, then "afilu le'achar shalosh shanim, nitma." Even mash that has sat for three years, the figure standing simply for a long span of time, no longer enjoys the presumption of dryness once we have evidence to the contrary. When the fire visibly draws real liquid out of it, that liquid enters the tanur's airspace and renders the oven tamei.
The thread running through the whole mishnah is a single principle: absorbed liquid is not yet liquid in the oven. Tumah is conveyed only at the moment the liquid actually emerges, and therefore everything turns on two questions: was the oven heated, and is there still liquid left inside to come out.