Kelim, Chapter Twenty-Seven, Mishnah Eight
1) If a sheet that had contracted midras uncleanness was made into a curtain, it is pure from midras uncleanness but is still unclean from contact with midras uncleanness.
2) Rabbi Yose said: but what midras uncleanness has this touched!
a) Only if a zav had touched it is it unclean from contact with a zav.
Section one: Once he turns the sheet into a curtain it loses its midras impurity because it is no longer something that is sat or lain upon. However, the curtain is considered to have had “contact with itself,” and therefore it is still unclean by virtue of contact with midras uncleanness.
Section two: Rabbi Yose holds that having “contact with itself” does not cause something to be impure. In other words, if the sheet/curtain is no longer impure with midras, then it is not impure by virtue of contact with midras.
However, this does not necessarily mean that the curtain (former sheet) is pure. If a zav (the one who conveys midras impurity) touched it when it was a sheet, he gave it both midras impurity because he sat on it and contact impurity by touching it. When it becomes a curtain, it loses its midras impurity because it is no longer sat or lain upon, but it retains its impurity by virtue of contact with the zav.
Note that if the zav had sat or lain upon the sheet without touching it, it would have only had midras impurity and when it was turned into a sheet it would be completely pure.