Jamieson, Fausset and Brown on Leviticus 15:12

12. the vessel of earth, that he toucheth which hath the issue, shall be broken--It is thought that the pottery of the Israelites, like the earthenware jars in which the Egyptians kept their water, was unglazed and consequently porous, and that it was its porousness which, rendering it extremely liable to imbibe small particles of impure matter, was the reason why the vessel touched by an unclean person was ordered to be broken.