If an earthenware kli absorbs ḥametz hot, even if it was in a kli sheni, hagala is not effective. It is also impossible to kasher it by means of libun (above, 10:9). If used cold, however, one may kasher it by means of hadaḥa, that is, by cleaning and rinsing it. If a ḥametz beverage sat in it for 24 hours, one may kasher it by means of hagala or by soaking it three times, each time for 24 hours (SA 451:21; above, 10:13). Clay and ceramic kelim are considered earthenware.
The poskim are divided about the status of porcelain (china). Porcelain is made as an earthenware kli, but it is coated with glass by brushing it with or immersing it in the substance from which glass is made. In practice: le-khatḥila, porcelain kelim should not be kashered, but in times of need, as long as there are no cracks, they can be kashered by means of hagala.
It seems that the primary reasoning of those who adopt the stringent view is concern that the glass coating is imperfect. It emerges from Pri Ḥadash and Ḥida (Maḥazik Berakha) that if the porcelain is real, there are grounds for leniency, but it is often counterfeited. Indeed, in the past, many would coat the earthenware with a glasslike substance with a paintbrush, and sometimes the coating was imperfect and had cracks. However, when the earthenware is immersed in liquid glass, the coating is uniform, smooth, and strong. Therefore, although le-khatḥila one should show concern for the stringent view, when necessary, one may be lenient, since nowadays the manufacture of porcelain is high quality, i.e., the glass coating is strong and the kli does not absorb. However, when the kli has cracks, its status reverts to being like earthenware.
Today there are high-quality plates made (by Arcopal and others) from sand. Their texture is hard like glass, and, as can be seen when they break, the interior material is hard and impervious, like glass. Although their surfaces are made rough for aesthetic reasons, they have the status of glassware and are kashered by means of hagala, as will be explained in the next section.