(Also see Ceramic )
Porcelain tile is made from high-quality clay and fired at an extremely high temperature. It can be glazed or unglazed. With unglazed porcelain tile, the color of the tile is the same from top to bottom – this is called through-body porcelain tile. Unglazed porcelain tile is sometimes polished like marble to achieve a shiny, reflective finish. With glazed porcelain tile, a glaze is applied to the top surface and then the tile is fired. The glaze creates the color and pattern of the tile. The durability of the glaze on porcelain tile can vary greatly. Check the PEI rating of the tile for durability. Sometimes porcelain tile is cut a second time after it is fired in order to achieve a perfect square edge. This type of porcelain is called rectified-edge porcelain.
Porcelain tile is generally considered to be impervious (non-porous) and therefore is highly recommended in exterior freeze/thaw zones.