The gemara in trying to figure out the source for there not being tumas midras by earthenware vessels quotes Rava citing a pasuk that implies that an earthenware vessel that is sealed would always remain tahor. The implication is that even if a Nida would sit on it, it would remain tahor. The seal only prevents tumas ohel, but wouldn't prevent tumah from a nida sitting on it. From this we conclude that there is no tumas midras by earthenware.
Rashi understands that the implication of the pasuk is that there would be no way at all for this sealed earthenware vessel to become tamei through a Nidah. Why wouldn't it become tamei by the nidah moving it, - טומאת היסט? Rashi explains that only something that is susceptible to tumah through touch can be tamei by moving it, therefore a sealed vessel which one cannot touch on the inside, wouldn't become tamei from moving it. Tosafos disagrees with Rashi and says that the צמיד פתיל - seal, doesn't prevent טומאת היסט and if a Nida would move it, it would become tamei.
The gemara on 21b says that they found a jar of oil that was sealed by the seal of the kohein gadol indicating that it was still tahor. Tosafos asks how can they be sure it was tahor? Although a goy would not have touched it, there is a chance that a goy moved it rendering it tamei through טומאת היסט. Even though on a Torah level goyim aren't tamei, chazal were gozer to treat them like zavin for all purposes (83a). Why are we not concerned that a goy moved it? This question is really Tosafos lishitasam that צמיד פתיל doesn't prevent tumas heset, but according to rashi there is no concern for tumas heset since it is sealed.
Tosafos answers that it was situated in the ground in a way that it is clear that it hadn't been moved at all. The Ran says that since they were looking for precious items, had they found it, they would have opened it. The fact that it was still sealed indicates that they never found it, therefore there is no concern that they moved it.