Keilim, Chapter 10, Mishnah 5. The subject of this chapter is the vessel that saves whatever it holds from becoming tamei. For that rescue to work, the Torah stipulates that the vessel be closed with a covering fastened tightly upon it, and a good part of the chapter is devoted to determining exactly where that closure has to be positioned. In our mishnah we meet two containers whose sealing substance is perfectly whole, but which is attached on the interior instead of stretching across the mouth from above.
The first case: "chavit she'nitkalfa ve'hazefet shelah omedet", an earthenware barrel whose clay wall chipped and flaked off, while the tar lining coated on its inner wall stayed firm. Visualize it: at the place where the pottery broke away there is now what looks like a hole, and yet nothing can leak or enter through it, since the tar that encircles the barrel from within has remained solid and keeps the whole thing shut tight. The only deficiency is that this tar never reaches up onto the edge of that gap; it closes it from the inside alone.
A second, similar case follows: "ve'chen kevutim shel muryas she'gifatan im ha'safa", and so too jars of muryas (a brine made from fish) whose closure of gefes was laid along the inner face of the lip. Jars of this sort have a widened, flaring mouth, and that shape makes it easy to press the sealing substance against the inside of the flare. The gefes, a mineral used for stopping vessels up, was packed against the inner surface of that lip, working much the way a cork works when it is wedged into a bottle. Once again the vessel is genuinely shut, and once again the material lies below the edge rather than spanning it.
"Rebbi Yehuda omer: einan matzilin." Rebbi Yehuda holds that neither container saves what is stored in it from tum'ah. The Torah calls for a covering bound onto the vessel, and he takes those words at face value: the closure has to rest on the mouth of the vessel, coming down on it from overhead. In these two instances the sealing substance never crosses the opening; it adheres below it. For Rebbi Yehuda, then, such vessels grant no protection at all.
"Vehachamim omrim: matzilin." The Chachamim hold that these vessels do save their contents. In their reading, the Torah asks only that the opening be blocked and shut; whether the substance doing the blocking is laid over the mouth or fitted inside it makes no difference whatsoever. Since the barrel's breach is held closed by its tar, and the jar's mouth is plugged by its gefes, the vessel is considered to have a fastened covering, and everything within it stays tahor.