Keilim 22:4

If a bridal chair lost its coverings, Beis Shammai say that it remains susceptible to ritual impurity while Beis Hillel rule it insusceptible. Shammai says that even the frame of a chair is susceptible to impurity. If one attached a chair to a kneading-trough, Beis Shammai rule it susceptible to impurity and Beis Hillel rule it insusceptible. Shammai says that even a chair that was constructed from the trough is susceptible to impurity.

Keilim 22:5

If the seat slats of a chair don’t protrude and they’re removed, the chair remains susceptible to ritual impurity. This is because it’s common to rotate it onto its side and sit on it.