Mishnà su Tamid 3:10

The metal cover of a basket of householders: Rabban Gamaliel says: it is susceptible to impurity, The sages say that it is clean. But that of physicians is susceptible to impurity. The door of a cupboard of householders is clean but that of physicians is susceptible to impurity. Tongs are susceptible to impurity but stove-tongs are clean. The scorpion [-shaped] hook in an olive-press is susceptible to impurity but the hooks for the walls are clean.