Kelim, Chapter Fifteen, Mishnah Three
1) The container of the flour-dealers sifter is susceptible to impurity, but the one of a householder is clean.
2) Rabbi Judah says: also one that is used by a hairdresser is susceptible to impurity as a seat, since girls sit in it when their hair is dressed.
Section one: Albeck explains that they would attach a container to either the bottom or the top of the flour-dealer’s sifter to catch loose flour when sifting. The type of container used by a professional is large enough to be considered an independent vessel and therefore it can become impure on its own. But the one used by a householder is small and insignificant and is therefore clean.
Section two: A similar type of container is used by women when dressing other women’s hair. This container is sometimes used as a seat, and therefore it is susceptible to “sitting impurity.” This means that if an impure person sits on it, the chair is impure even if the person doesn’t touch it.