Kelim, Chapter Nineteen, Mishnah Five
1) If around a bed that had contracted midras uncleanness one wrapped a mattress, the whole becomes subject to midras uncleanness.
a) If it was removed, the bed remains subject to midras uncleanness but the mattress is unclean only from contact with midras.
2) If around a bed that had contracted seven-day uncleanness one wrapped a mattress, the whole becomes subject to seven-day uncleanness.
a) If it was removed, the bed remains subject to seven-day uncleanness but the mattress is unclean until the evening.
3) If the bed was subject to evening uncleanness and around it he wrapped a mattress, the whole becomes subject to evening uncleanness;
a) If it was removed, the bed remains subject to evening uncleanness but the mattress becomes clean.
Section one: When he wraps the mattress around the bed, the mattress becomes part of the bed and takes on the impurity status of the bed. And when he removes the mattress from the bed, it loses the impurity status that the bed had. But since it had contact with something that had “midras” impurity, the mattress remains unclean, albeit with a lower type of impurity (first degree impurity).
Section two: The same halakhah is true with regard to seven day uncleanness, the type of uncleanness contracted from contact with a dead body. When the mattress is attached to the bed it has the same impurity as does the bed. When it is removed, it has the lower status of “evening uncleanness,” meaning it will become pure in the evening. This is because the bed was a “father of impurity” and it caused the mattress to have first degree impurity, but first degree impurity lasts only until evening.
Section three: However, if the bed was itself unclean only with “evening uncleanness” it does not defile at all other vessels that have contact with it. When the mattress is attached, it has “evening uncleanness” but when it is removed, it is completely pure.