Mishnà su Numeri 19:15

They can make the mixture in all kinds of vessels, even in vessels made of cattle dung, of stone or of earth. The mixture may also be prepared in a boat. It may not be prepared in the walls of vessels, or in the sides of a large jug, or in the stopper of a jar, or in one's cupped hands, for one does not fill up, or mix in, or sprinkle the hatat with anything but a vessel. Only on a vessel does tightly fitting cover afford protection, for only in vessels is protection afforded against uncleanness within an earthen vessel.

A flask that one has left uncovered and on returning found it to be covered, is invalid. If one left it covered and on returning found it to be uncovered, it is invalid if a weasel could have drunk from it or, according to the words of Rabban Gamaliel, a snake, or if it was possible for dew to fall into it in the night. The hatat waters are not protected by a tightly fitting cover; But water that had not yet been mixed with the ashes is protected by a tightly fitting cover.