Parashas Metzora · Passuk 14:41
Chabad.org — English translation + Rashi
Torah Temima points to Toras Kohanim.
English: “The dust …” — if [only] “which they scraped,” one might think [even] pebbles; therefore it says “dust.” If [only] “dust,” one might think even that which falls of itself; therefore it says “which they scraped.”
Definition of the removed material
Chazal require the actual scraped plaster/dust, neither loose pebbles nor material that fell spontaneously.