Zevachim Chapter 11, Mishnah 6 - a direct continuation of the previous Mishnah, to the point that it is almost unclear why it was taught as a separate Mishnah in its own right. Here we will deal with copper vessels, and metal vessels in general. We learned that a metal vessel that absorbed the leftovers of holy sacrifices requires, at the end of the set time, cleaning and koshering anew in the Azarah. This presents the exact same problem: if the vessel contracted tumah outside the Azarah, it is impossible to return it inside - so how can the requirement to kosher it be fulfilled?
The difference between an earthenware vessel and a metal vessel:
With an earthenware vessel, we said that a small hole is sufficient, since through this the vessel loses its original status. This is not the case with a metal vessel. Metal was very expensive, and they did not usually produce vessels from it for minor needs, but only for specific uses. Take a frying pan as an example: it is impossible to make it from another material, and in the time of the Mishnah, it was expensive and difficult to produce. If a small hole is punctured in it, it does not matter at all - it continues to be used for frying in its usual manner.
Therefore, a small hole does not work for a metal vessel. For the vessel to lose its status as a vessel and not be susceptible to tumah, a large hole must be made in it, damaging it significantly. However, this creates the opposite difficulty: since it is no longer a vessel, there is no benefit in returning it inside, as it is impossible to purify it from what it absorbed - there is no vessel here.
It turns out, then, that the process is done in two stages:
Breaking the vessel: A large hole is made in it that disables it from its function, and through this its tumah is removed.
Restoring it to working order: It is struck with a hammer until it can function again as a frying pan, even in a flimsy manner, and once it has returned to being a vessel, it can be purged with boiling water and cleaned of the fat and absorptions.
The text of the Mishnah:
"Klei nechoshes sheyatze'u chutz latachum - nichnas umoreko veshotfo bemakom kadosh" - a metal vessel that contained leftover sacrifices or absorptions and went outside the Azarah: we return it to the Azarah, and here we scour it (the meaning of scouring will be explained in the next Mishnah) and rinse it in a holy place, in the Azarah.
"Nitma chutz latachum - pok'o, venichnas umoreko veshotfo bemakom kadosh" - if the vessel contracted tumah while outside the boundary, such as if a dead mouse touched it, "pok'o" - he breaks it and removes its status as a vessel, not with a small hole but with significant destruction. Since in this state it will not be able to function as a vessel at all, he strikes it with a hammer and restores it to working order to the necessary extent, and then brings it back into the Azarah: now it is pure, but the leftover absorptions are still inside it - and therefore he scours and rinses it in the holy place, in the Azarah, as commanded by the verse.
In summary: This Mishnah deals with a metal vessel that absorbed holy sacrifices. Since a small hole does not nullify the use of a metal vessel, significant destruction is required to nullify the tumah, followed by restoring it to working order with a hammer - so that it will be a vessel suitable for scouring and rinsing in the Azarah. In the next Mishnah, we will examine the meaning of the scouring itself.