Mishnah Zevahim 11-6

Mishnah says that would not invalidate it from going through the proper procedure to purge and rinse out this copper vessel. So we say, you can go back in. You have to bring it back into the courtyard, and then it could be purged and rinsed in a, in the holy place of the courtyard of the Beit HaMikdash (Holy Temple). But if it became Tameh (impure) when a person was outside of the courtyard, he can't now bring it back into the courtyard while it's in that state of Tum'ah (impurity). We say he should make a large gash in the side of this copper vessel in order to make it Tahor (pure), even though a metal vessel has the ability to become purified in a mikveh (ritual bath), by dipping it in a mikveh, unlike the vessel we were talking about in the previous Mishnah, which was earthenware and cannot. Here, technically, you can dip it in a mikveh when it's outside, but we say that can't be done outside of the courtyard because some of the residue of the korban (offering) would become washed away. Part of the process of purification here is that it has to be purged and rinsed only in a makom kadosh (holy place), so therefore, he couldn't dip it in a mikveh in order to purify it outside of the courtyard. What he has to do is make a gash in it and lose its purpose as a keli (vessel) now, and he makes it Tahor. It can't be a small hole like he could make in an earthenware vessel, but here, he has to make a large gash, a large hole in it. And then he could bring it into the courtyard, and it's purged and rinsed in a makom kadosh, in the holy place.