משנה על בבא מציעא 167:20

The clean parts of a wagon are the following: the yoke that is only plated [with metal], side-pieces made for ornamentation, tubes that give out a noise, the lead at the side of the necks of the cattle, the rim of the wheel, the plates and mountings, and all other nails, all of these are clean. Metal shoes of cattle are susceptible to impurity but those made of cork are clean. When does a sword become susceptible to impurity? When it has been polished. And a knife? When it has been sharpened.