What are the implications of the word "Ach"?
Rashi #1 and Seforno: It implies that even though Moshe taught them the Dinim of Tum'ah, there still remained the Dinim of Hag'alah (with regard to metal vessels - Seforno). 1
Rashi #2 (citing the Yalkut Shimoni) and Targum Yonasan: it comes to preclude utemsils that contain rust, which must be removed before the vessel is Kashered 2 (and utensils that are not unfinished (Gulmei Keilim) - Targum Yonasan).
What is the significance of the metals that the Torah lists?
Rashi (in Rosh Hashanah, 19b): The Torah is confining Tum'ah to vessels made of these six kinds of metals.
In what form is metal subject to Tum'ah?
Targum Yonasan and Sifri: Only when it has been manufactured into a vessel is it subject to Tum'ah 1 (which we learn via a Binyan Av from Tum'as Meis, where the Torah writes in Chukas, 19:18 "ve'Hizah al ... ha'Keilim" - Sifri).
Targum Yonasan: Not in its raw state and not even whe it is formed but not finished. Se Na'ar Yonasan.
