הלכות טומאת מת

What is the source that teaches that a sealed covering saves the contents of a container from contracting ritual impurity in a shelter in which a corpse is located? Numbers 19:15 states: "Any open container that does not have a sealed covering on top of it is impure." One can derive from this that if there is a sealed covering on it, it is pure.
According to the Oral Tradition, it was taught that the verse is speaking only about an earthenware container, for it is a container that contracts impurity only through its opening. Therefore, if its opening is closed with a sealed covering, all of its contents are protected.
From this, we can infer that the contents of any of the containers which are not susceptible to ritual impurity are protected when the container is closed with a sealed covering. Such containers include: containers made from cow turds, stone containers, containers made from earth, containers made from the bones or skin of a fish or the bones of a fowl, oversized wooden containers, wooden boards that are flat and are not containers, metal keilim which have not been completely fashioned. The contents of all of these are protected by a sealed covering.
Now if the contents of a utensil closed with a sealed covering are protected, we can infer that this also applies to keilim that are swallowed or under an ohel. What is the difference between ohalim and containers that protect because of a sealed covering? That the covering of the containers must be sealed close, while for an ohel, any covering is sufficient.

All containers that protect their contents when sealed closed also protect anything that is under them to the very depths if they are turned upside down and stood on the earth, when their inner space is a handbreadth by a handbreadth by a handbreadth, even though one did not spread clay around the sides to seal them. The rationale is, in that position, they are like an ohel and an ohel protects from impurity. The only exception is an earthenware container, for an ohel formed by an earthenware container does not protect from impurity.
What is implied? When an earthenware jug is turned upside down, even if one smeared clay on its sides everything under it is impure, for the prooftext speaks of "a sealed covering on top of it," and not a closed covering on its back. If one attached its opening to the wall and smeared clay on its sides, it protects everything inside of it and everything opposite its opening in the wall. If one did not smear clay on its sides, it does not protect its contents, because an earthenware container does not protect as an ohel, as we explained.
All of the other containers that protect their contents when sealed close protect their contents when their openings are attached to the walls of a house even when they do not have a sealed covering, because they protect as an ohel. Therefore it is necessary that the walls of the utensil be at least a handbreadth high, for containers do not protect their contents with their walls as an ohel unless the wall of the container is at least a handbreadth.
If the wall of the container was half a handbreadth, there was a border of half a handbreadth protruding from the wall, and they were attached to each other, it is not considered as an ohel and does not protect its contents even though there is a handbreadth of empty space. It is necessary that the handbreadth come from one entity.

Just as such containers protect their contents from impurity when they are inside an ohel and attached to its walls, so too, do they protect their contents when they are outside the ohel if they are attached to the ohel, for the ohel is considered as a covering in all instances.
What is implied? A samovar that has walls that are a cubit high was placed on its side on staves outside an ohel and its opening was placed immediately next to the wall of the tent. If there is impurity under it, the keilim inside of it are pure. If it was placed next to the wall of a courtyard or the wall of a garden, it does not protect its contents, because these are not the walls of a tent. Therefore any keilim in the container are impure, for they were held above the impurity.

If there is a beam that is a handbreadth wide running from wall to wall, there is impurity below it, a pot was hanging from the beam and the beam was touching the entire opening of the pot and covering it, the keilim in the pot are pure. The rationale is that they were saved by the ohel covering them. If the opening of the pot was not covered by the beam, but instead there was some empty space between them, everything in the pot is impure and the pot itself is impure.

If there is a beam that is a handbreadth wide running from wall to wall, there is impurity below it, a pot was hanging from the beam and the beam was touching the entire opening of the pot and covering it, the keilim in the pot are pure. The rationale is that they were saved by the ohel covering them. If the opening of the pot was not covered by the beam, but instead there was some empty space between them, everything in the pot is impure and the pot itself is impure.

The following laws apply when there is a cistern in a building, there is impurity in the building, and there are keilim in the cistern. If the cistern was covered with a flat board or a container that can protect its contents from impurity because it has a wall that is a handbreadth high, everything that is in the cistern is pure. If the cistern had a border built around its opening that was a handbreadth above the ground, whether he covered it with a container that can protect from impurity because it has a wall or whether the container did not have a wall, the container protects the contents of the cistern from impurity, because there is a wall of a handbreadth from another source.

The following rules apply when a cistern is built inside a building and there is a lamp in it with its flower protruding and covering the opening of the cistern. One placed a container that can protect from impurity in an ohel where a corpse is located over the opening to the cistern and it is resting on the flower of the lamp. We see if the container that can protect from impurity would remain in its position if the lamp was removed. When this is the case, it protects everything that is in the cistern from impurity. The keilim that are between the edge of the container that serves as a cover and the edge of the cistern are pure until the very depths. Even the lamp is pure despite the fact that the edge of the flower is visible between the covering and the cistern. If the container would not remain in position, everything is impure.

The following laws apply when a cistern is built inside a house and a container that could protect its contents from ritual impurity was placed over its opening. If there was impurity between the edge of the container and the edge of the cistern or within the cistern, the house is impure. The rationale is that an ohel inside a building does not prevent the spread of impurity, as we explained.
If there was impurity in the house and there is a handbreadth by a handbreadth by a handbreadth of empty space in the cistern, the keilim stored in the walls of the cistern are pure. If not, they are impure.
If the width of the walls of the cistern extends outside the house, they are nevertheless pure. The rationale is that they are not the walls of the house and just as the inside of the cistern is protected from impurity, so too, its walls protect.
We have already explained that an old oven is like all other keilim that convey impurity and is not considered as an ohal. For this reason, it does not protect its contents from ritual impurity unless it is sealed close like other containers that protect their contents. We have already explained that an oven is not considered as a k'li in this context and does serve as an ohel. Therefore it protects its contents from impurity merely by covering them without the cover being sealed close like other ohalim. The covering of an oven is called a serida.