Tosefta Yadayim 2:5 (quoted by Rashba on Mishnah Yadayim) says:
תיק הספרים ומטפחות ותיבה של ספר בזמן שהן טהורות מטמאות את הידים
What does "pure" mean here? What could happen to the wrappings or the ark that they might still be in ritual use (so identifiable as the box for the scroll) but could no longer take on that secondary scriptural status? Or has something happened to remove them from ritual use?