TT #177 · Parshas Shemini · Aliyah 5 · Passuk 32
Chabad.org — Vayikra Chapter 11 with Rashi
English: And anything upon which one of them falls when dead shall be impure, including any vessel of wood or cloth or skin or sack; it shall be put into water, and it shall remain impure until evening, then it shall be clean.
Sifra (Toras Kohanim).
Wikisource — Sifra, Shemini, Chapter 7
Category enumeration — The listed materials define the classes of vessels subject to this impurity.
The wording distinguishes excluded simple wooden objects and included vessels.