Keilim, Chapter 5, Mishnah 4. The opening mishnah of this chapter established that a clay oven is not yet a finished vessel, and therefore cannot yet receive tumah, until it has been fired to a certain heat. That firing is its gmar melacha, the completion of its manufacture. Only once that has happened does the oven count as a finished utensil that is susceptible to tumah. Our mishnah now asks about heatings that came about in irregular ways: the fire was in the wrong place, or nobody intended it, or the owner had not yet taken possession of the oven. Does such a heating still serve as the gmar melacha?
Three irregular heatings
The Mishnah presents them one after the other. "Tanur she'husak me'achorav": an oven that was fired from behind, meaning the fire was set alongside the oven on its outside rather than in the normal manner, on the inside, and from there the oven reached the necessary heat.
The second situation is introduced with the words "o she'husak shelo leda'ato." Here the fire burned in its proper place, inside the oven, and yet the owner had no idea it was happening. No one carried out the act of firing as a deliberate step toward finishing the vessel.
The third situation the Mishnah raises is "o she'husak beveit ha'uman." The oven reached its heat while it was still sitting in the shop of the potter who made it, before he had declared his work done and before any owner had ever acquired it.
In all three of these cases the ruling is "tamei": the oven is now capable of contracting tumah. Once the oven has in fact been brought to the required temperature, it is a completed vessel, and the circumstances that produced that heat, the place of the fire, the absence of intention, the identity of the one who heated it, make no difference at all.
The fire in Kfar Signa
The Mishnah now brings a case that actually occurred: "Ma'aseh shenafela deleka," a fire once broke out, "betanurei Kfar Signa," among the ovens of the village of Signa. They were still standing in the shop of the artisan who had formed them, and the flames raised them to precisely the temperature their manufacture calls for.
"Uva ma'aseh leYavneh": the matter was brought for a ruling to the sages sitting in Yavneh. "Vetimei'am Rabban Gamliel": Rabban Gamliel declared those ovens to be vessels that can now contract tumah.
Notice that this single episode contained all three of the irregularities the Mishnah listed. The fire burned outside the ovens rather than within them; it happened with nobody's consent or intention; and the ovens were still unfinished, sitting in the craftsman's workshop, when it happened. Even so, they were held to be completed vessels, fit to receive tumah.
The principle
The lesson of the mishnah is that the gmar melacha of an oven is measured purely by the result. If the oven has been heated to the temperature that its manufacture requires, it is a finished vessel and susceptible to tumah, whatever the circumstances that brought that heat about.