The American Society for Testing and Materials ( ASTM ) defines a ceramic article as â an article having a glazed or unglazed body of crystalline or partly crystalline structure , or of glass , which body is produced from essentially inorganic , non-metallic substances and either is formed from a molten mass which solidifies on cooling , or is formed and simultaneously or subsequently matured by the action of the heat . A ceramic is an inorganic , non-metallic solid prepared by the action of heat and subsequent cooling . Ceramic materials may have a crystalline or partly crystalline structure , or may be amorphous ( e.g. , a glass ) .