Mikvaos, Chapter 9, Mishnah 5. This chapter is devoted to the subject of chatzitzah, a barrier that comes between the water of the mikveh and the thing being immersed. Here the Mishnah turns to the immersion of vessels and other articles, and asks which substances clinging to them are considered a chatzitzah and invalidate the tevilah.
The Mishnah opens: "Eilu chotzitzin bakeilim," these are the things that form a barrier on vessels. "Hazefes vehamor," tar and a type of gum or sap that oozes from a tree, when they are stuck "biklei zechuchis," onto a vessel made of glass. And the Mishnah adds that it makes no difference where the substance is found: "bein mibachutz bein mibifnim," whether it is on the outside surface of the vessel or on the inside, either way it is a chatzitzah.
Why does the Mishnah choose specifically a glass vessel as its example? The Rishonim offer two approaches. Rashi explains that it is a matter of texture. Glass is so smooth that almost nothing adheres to it; only substances as sticky and gummy as tar and tree sap will cling to a glass surface, and therefore these are the only cases the Mishnah needs to mention. The Beis Yosef, citing the Semag (and the same explanation is found in Maseches Shabbos), takes a different route. Tar and gum are materials people commonly use to patch up and seal other kinds of vessels, so on such vessels they are not out of place and their owner is not bothered by them. Glass, however, is not ordinarily repaired with these substances, so when they are found on a glass vessel the owner minds their presence, and that is precisely why they constitute a chatzitzah.
From here the Mishnah moves to a series of cases that all turn on one principle: whether or not the owner is particular about the dirt. "Al hashulchan, al hatavla, al hadargash," if such specks of tar or gum are found on a table, on a board, or on a dargash, which is a small bed, and these articles are "haneki'im," the kind that are normally kept clean, then "chotztzin," the substance is a chatzitzah, because people generally are makpid and do not tolerate such marks on them. But "al habelusin," on worn out and grimy pieces of furniture, "einan chotztzin." Nobody cares about a bit of dirt on an item that is already shabby, and where there is no hakpadah there is no chatzitzah.
The same distinction is drawn between people rather than objects. "Al mita shel ba'al habayis, chotzetz." Dirt on the bed of a man of means is a barrier, since a wealthy householder does not want his bed soiled. "Al shel ani, eino chotzetz." On the bed of a poor man it is not a barrier, because he is not particular about it.
So too with riding equipment. "Al ukef shel ba'al habayis, chotzetz." On the saddle of a wealthy man the substance forms a barrier. "Ve'al shel zekakin, eino chotzetz." On the saddle of the zekakin, dealers who carry and sell leather pouches, it is not a barrier, since men in that trade are not fussy about such things. And "al hamarda'as, chotzetz," on the saddle cushion of an animal it is a chatzitzah, because in that case a person generally is particular.
A dissenting opinion closes the Mishnah. "Rabban Shimon ben Gamliel omer": in his view, tar or gum found on a donkey's marda'as does not spoil the immersion unless it has built up to a certain amount. He sets that shiur as "ad ke'issar ha'Italki," the dimensions of an Italian issar, roughly what we would picture as a penny. A smaller smudge than that goes unnoticed by the owner, and the tevilah remains valid.
The thread running through the entire Mishnah is clear: the halachah of chatzitzah on vessels and articles is not measured only by the substance itself, but by whether its owner minds having it there.