Parah 5:5

The ashes may be mixed with water in all kinds of vessels, even in those made of cattle dung, stone or clay; one may even do so in a boat. It may not be mixed on the walls of vessels, on the sides of a pitcher, in the stopper of a jug or in a person’s cupped hands. This is because we only fill, mix and sprinkle the sanctification water with a vessel. Only vessels are protected from impurity by a tight-fitting lid and only vessels protect against impurity in earthenware vessels.

Parah 5:6

A potter’s “egg” is a fit vessel for mixing the water, though Rabbi Yosi rules it unfit. Rabbi Meir and Rabbi Yehuda rule a chicken’s eggshell fit as a vessel but the Sages declare it unfit.