Mishnah Mikvaos 9:1: The following are [items which are considered] an interposition on a person [immersing in a mikveh, and thereby prevent one’s immersion from being valid]: wool threads, and flax threads, and ribbons [worn] on the heads of girls. Rabbi Yehuda says: those of wool and of hair do not interpose, because water can enter them.

אֵלּוּ חוֹצְצִין בָּאָדָם, חוּטֵי צֶמֶר וְחוּטֵי פִשְׁתָּן וְהָרְצוּעוֹת שֶׁבְּרָאשֵׁי הַבָּנוֹת. רַבִּי יְהוּדָה אוֹמֵר, שֶׁל צֶמֶר וְשֶׁל שֵׂעָר אֵינָם חוֹצְצִין, מִפְּנֵי שֶׁהַמַּיִם בָּאִין בָּהֶם:

Mishnah Mikvaos 9:2: [The following are also considered interpositions on a person attempting to immerse:] tangles [of hair] on the chest, or on the beard, or in the hidden areas of a woman, a secretion outside the eye, or a crusting on the outside of a wound, or a bandage that is upon it, or dried sap, or hardened feces on one’s flesh, or dough underneath a fingernail, or dirt particles, or Greek clay, or potters’ clay, or Greek mud. What is Greek clay? This is the clay used for pits, as the verse states, (Psalms 40) "And he raised me out from the pit of tumult, from the Greek clay." The potters’ clay is as it sounds. Rabbi Yose would consider that of potters to be pure [i.e. and not to be an interposition preventing something from being purified by immersion], and he would consider impure [i.e. and therefore an interposition] that [earth] which was used to polish and mend vessels. And regarding Greek mud, this is [white earth used to make] road pegs, in which one may not immerse, nor may one immerse them [i.e. one may not immerse while they are on his body]. And regarding all other clay, one can immerse in it when it is moist. And one should not immerse with the dust on his feet. One should not immerse a kettle with soot [on it], unless he rubbed [it off].

קִלְקֵי הַלֵּב וְהַזָּקָן, וּבֵית הַסְּתָרִים בָּאִשָּׁה, לִפְלוּף שֶׁחוּץ לָעַיִן, וְגֶלֶד שֶׁחוּץ לַמַּכָּה, וְהָרְטִיָּה שֶׁעָלֶיהָ, וּשְׂרָף הַיָּבֵשׁ, וְגִלְדֵי צוֹאָה שֶׁעַל בְּשָׂרוֹ, וּבָצֵק שֶׁתַּחַת הַצִּפֹּרֶן, וְהַמִּלְמוּלִין, וְטִיט הַיָּוֵן, וְטִיט הַיּוֹצְרִים, וְגֵץ יְוֵנִי. אֵיזֶהוּ טִיט הַיָּוֵן, זֶה טִיט הַבּוֹרוֹת, שֶׁנֶּאֱמַר (תהלים מ), וַיַּעֲלֵנִי מִבּוֹר שָׁאוֹן מִטִּיט הַיָּוֵן. טִיט הַיּוֹצְרִין, כְּמַשְׁמָעוֹ. רַבִּי יוֹסֵי מְטַהֵר בְּשֶׁל יוֹצְרִין וּמְטַמֵּא בְּשֶׁל מָרֵקָה. וְגֵץ יְוֵנִי, אֵלּוּ יִתְדוֹת הַדְּרָכִים, שֶׁאֵין טוֹבְלִין בָּהֶן וְלֹא מַטְבִּילִין אוֹתָן. וּשְׁאָר כָּל הַטִּיט, מַטְבִּילִין בּוֹ כְּשֶׁהוּא לַח. וְלֹא יִטְבֹּל בָּאָבָק שֶׁעַל רַגְלָיו. לֹא יִטְבֹּל אֶת הַקֻּמְקְמוּס בַּפֶּחָמִין, אֶלָּא אִם כֵּן שִׁפְשֵׁף: