Sforno on Exodus 27:5/ספורנו על שמות כ״ז:ה׳

1כרכב המזבח, ledges which are common to all vessels made of wood, as our sages confirm in Chulin 25 “these are wooden vessels in their original unfinished state that are not susceptible to ritual impurity; any such vessel or instrument that is not fit for intended for scraping the scales off fish, etc.”