If earthenware absorbs ĥametz hot, even via a kli sheni, hagala is not effective. Although libun would effectively burn the taste absorbed in the utensil, the Sages forbid koshering earthenware utensils through libun because there is concern that they will crack (see above 10:7). If used cold, however, one may kosher them by washing them thoroughly. If a ĥametz beverage was allowed to sit in such a vessel for twenty-four hours, one may kosher it by soaking it in water for three consecutive days (ibid. 14).
China, clay, and ceramic utensils are considered earthenware.
Regarding porcelain which, though made like earthenware, has a smooth surface like glass, most poskim maintain that it has the status of earthenware, and there is no way to kosher it. This is the halakha (MB 451:163; Kaf Ha-ĥayim ibid. 305). However, some poskim believe that since the surface of porcelain is smooth like glass, it does not absorb at all. When there are additional doubts, their lenient opinion is taken into account.
. Shiyarei Knesset Ha-gedola OĤ 451, Hagahot Beit Yosef 30 states that the universal custom is to use porcelain on Pesaĥ even if it was used for ĥametz throughout the year, since porcelain is like glass which does not absorb. He was stringent for himself, as per the opinion of Radbaz, but did not rule stringently for others. She’elat Yaavetz 1:67 also permits it. Pri Ĥadash states that one may only be lenient if he has authentic porcelain, but nowadays, since there is much counterfeit porcelain, all agree that one must be stringent. This is also the opinion of Maĥzik Berakha 451:10, as quoted in Kaf Ha-ĥayim 451:305, and Kol Mevasser 1:80. This is the accepted custom. However, one may be lenient if there are other doubts, as explained by the Aĥaronim cited in Hagalat Kelim 13:368 and Yabi’a Omer YD 1:6 and 7:10.
When porcelain breaks, one can see that its inner texture is coarse, like earthenware, and its outer texture is smooth. There are also porcelain utensils that are smooth on the inside and coarse on the outside. It seems that if a utensil like this absorbed in its coarse area, everyone would agree that it has the status of earthenware. Nowadays, most dishes are made of tempered glass, which is sand-based and has a hard texture like glass, even though it is not always smooth as glass. I am often asked whether such dishes can be koshered via hagala if they were used for non-kosher or for milk and meat. In my humble opinion, these dishes should be considered like glass, since when they break one can see that they are made of dense material – like glass, not like porcelain. They thus have the status of quality glass utensils that can withstand the hagala process, as will be explained in the next section. Even the Ashkenazic custom is to be lenient when necessary to kosher such a utensil from forbidden taste it absorbed. To dispel all doubt, one should kosher the utensil with hagala three times (it is not necessary to use three separate refills of water), since according to Itur this would even work for earthenware. It seems that even according to the custom of SA glassware does not need to be koshered, since today’s dishes are not so smooth that they should undergo triple hagala. (I have not mentioned any particular brand names since companies constantly change the composition of their products; I have referred specifically to utensils whose hard texture is similar to that of glass.)