Enamel kelim are made of metal and coated with a thin layer of enamel for aesthetic reasons. The inside of the pot is usually painted white, while the outside is decorated with different colors. Enamel is made of sand like glassware, but it is manufactured differently. In practice, one may perform hagala on enamel kelim like all other metal kelim.
The same applies to all types of metals, leather, wood, and bone. Only earthenware cannot be kashered, because, due to the unique composition of earthenware, it absorbs a lot and does not release all that it absorbs.
Plastic kelim that have absorbed the taste of boiling hot food are kashered via hagala with boiling water, like all other kelim. However, sometimes plastic kelim get scratched, making it hard to clean them well. Therefore, the accepted recommendation is to replace plastic baby bottles and pacifiers before Pesaḥ, as it is hard to clean them to the point that their original shine is restored. Nevertheless, in times of need, they can be kashered by means of a good cleaning and hagala or pouring boiling water from a kli rishon.
. Igrot Moshe (OḤ 1:92) states that one should not perform hagala on kelim made of plastic or other synthetic materials not mentioned by the Rishonim, since such materials might be like earthenware, which does not release what it has absorbed. This is also the view of R. Yaakov Shaul Elyashar and Lehorot Natan 6:69. Nonetheless, most poskim agree that plastic kelim can be kashered by means of hagala. These include: Responsa Ḥelkat Yaakov 2:163; Seridei Esh 2:160; R. Levi Yitzḥak Halperin; R. Mordechai Eliyahu, Hilkhot Ḥagim 5:86; Ḥazon Ovadia, Pesaḥ, p. 151; and Or Le-Tziyon 3:10:13.