Leviticus 15:12 NASB
However, an earthenware vessel which the man with the discharge touches shall be broken, and every wooden vessel shall be rinsed in water.
It is said according to the book of Leviticus earthen vessels contaminated by unclean persons which culminated in them being destroyed. But according to the Gemara, earthen vessels were immune to contamination.
GEMARA (Yoma 2a):
The halakha of sequestering the High Priest prior to his performance of the Temple service on Yom Kippur is comparable to the sequestering of the priest designated to burn the red heifer. Therefore, the Gemara cites that which we learned in a mishna there, in tractate Para: Seven days prior to the burning of the red heifer, the Sages would remove the priest who burns the heifer from his house to the chamber that was before the bira at the northeast corner of the courtyard on the Temple Mount. And that chamber was called the Chamber of the Stone House. The Gemara explains: And why was it called the Chamber of the Stone House? It is because all the actions associated with the red heifer were performed in dung vessels, stone vessels, and earth vessels, which are vessels that cannot become ritually impure
Why were the earthen vessels broken?