Mishnà su Salmi 40:78

The matted hair on the heart and on the beard and on a woman's hidden parts; pus outside the eye, hardened pus outside a wound and the plaster over it, dried-up juice, clots of excrement on the body, dough under the finger nails, sweat-crumbs, miry clay, potter's clay, and road-clay. What is meant by 'miry clay’? This means the clay in pits, for it is written: "He lifted me out of the miry pit, the slimy clay" (Psalms 40:3). "Potter's clay" according to its literal sense. Rabbi Yose declares potter's clay clean, but clay for putty unclean. And "road-clay." These become like road-side pegs in these [kinds of clay] one may not immerse oneself nor immerse [other things] with them; But in all other clay one may immerse when it is wet. One may not immerse oneself with dust [still] on one's feet. One may not immerse a kettle with soot [on it] unless he scraped it.