Parah, Chapter 10, Mishnah 3. This chapter is built around one governing principle: things that are perfectly acceptable in the world of kodesh can still be disqualifying in the world of chatas. The waters of the parah adumah and the ashes themselves are guarded at a level of taharah stricter than anything else, and our Mishnah applies that stringency to a vessel of ashes that comes into contact with something tamei.
The Mishnah opens: "Kalal shel chatas shenaga b'sheretz, tahor." A kalal of chatas that touched a sheretz is tahor. The kalal is the vessel in which the ashes of the parah adumah are kept, either an earthenware vessel or a vessel of stone.
Why should touching a dead sheretz, which carries the status of an av hatumah, achieve nothing here? The answer lies in the material of the kalal. Earthenware never absorbs tumah through its outside; a keli cheres takes in tumah only by way of its inner airspace. And when the kalal is made of stone, the substance itself simply lies outside the categories of things that receive tumah. Either way, the sheretz that grazed the outside of the vessel has left no mark on it, and the ashes stored within stay fit for use.
Resting the Vessel Upon the Sheretz
The Mishnah's next clause reads "Nesano al gabav", meaning that he rested the kalal down upon the sheretz. Here the Tannaim part ways: Rabbi Eliezer holds that the vessel remains tahor, while the Chachamim hold that it becomes tamei.
Rabbi Eliezer reasons straightforwardly: nothing has changed. The vessel itself cannot contract tumah from its exterior, whether it merely touched the sheretz or is now sitting upon it, so the ashes stay tahor.
The Chachamim look at a different verse. The Torah says of the ashes that a tahor man shall gather them and place them outside the camp b'makom tahor, in a pure place. A vessel of ashes propped up on a dead sheretz is not standing in a pure place, and that alone disqualifies the ashes, even though no tumah passed into the vessel. The halachah does not follow Rabbi Eliezer here; it follows the Chachamim, and the ashes are tamei.
Food, Drink and Kisvei Hakodesh
The Mishnah goes on: "Naga b'ochalin uv'mashkin uv'chisvei hakodesh, tahor." Should the kalal come in contact with foodstuffs, with beverages, or with sacred writings, its status is unaffected and it stays tahor. Here too, no tumah ever entered the vessel, and so the ashes it holds are untouched.
"Nesano al gabeihen, Rabbi Yosi metaher, vachachamim metam'in." If he set the kalal down upon them, Rabbi Yosi rules it tahor and the Chachamim rule it tamei.
Note carefully what Rabbi Yosi is and is not saying. He accepts the ruling of the Chachamim in the first case: a kalal resting on a sheretz does render the ashes tamei, because a sheretz is an av hatumah by Torah law, and a vessel perched on such a thing is not standing in a pure place. But foods, liquids and kisvei hakodesh convey tumah only by rabbinic decree. In Rabbi Yosi's view that is not enough to call the spot an impure place, so even when the kalal is laid directly upon them the ashes remain tahor.
The Chachamim disagree and declare the ashes tamei even here, and the halachah follows the Chachamim. The lesson of the Mishnah is that with chatas we are not measuring only whether tumah physically entered the vessel; we are measuring whether the ashes are being kept in a place worthy of them.