And every open vessel that has no cover fastened around it is unclean.
Torah Temimah: Shabbos 84b — From where do we know that midras of an earthenware vessel is pure? Rava said: As it is written “and every open vessel that has no cover fastened around it is unclean” — implying that if it has a cover fastened around it, it is pure. And are we not dealing even with a case where he designated it for his niddah wife? Yet the Torah says it is pure.
English: From where do we know that midras of an earthenware vessel is pure? Rava said: As it is written “and every open vessel that has no cover fastened around it is unclean” — implying that if it has a cover fastened around it, it is pure. And are we not dealing even with a case where he designated it for his niddah wife? Yet the Torah says it is pure.
Inference from the positive and negative implications of the verse.
An earthenware vessel cannot become impure through midras (pressure/sitting impurity), even when designated for use by a niddah.