Kelim, Chapter Twenty-Four, Mishnah Three
There are three different types of baking-troughs:
1) If a baking-trough of a capacity from two log to nine kav was split it is susceptible to midras uncleanness;
2) If it was whole it is susceptible to corpse uncleanness;
3) And if it holds the prescribed measure it is free from all uncleanness.
Section one: This section was explained above in 20:2. There I wrote: A trough that is large enough to hold between two logs of liquid (1/2 of a kav) and nine kav and then is split, is subsequently susceptible to midras impurity because it can be used for sitting. Note that when it was used to hold liquids it would not have been used as a seat and therefore it would not have been susceptible to midras impurity.
Section two: As stated above, when whole the trough could not have been used as a seat and therefore it was just a normal vessel, susceptible to corpse impurity.
Section three: If the trough can hold 40 seahs, it is too big to be transported and therefore it is not susceptible. This halakhah was learned in 15:1.