Hi Fimac,
We offer a competitve super white porcelain that comes in 60x30 and 60x60 sizes. With these being a polished porcelain they do need to be sealed. This is to prevent any liquids seeping through the polish and staining the tile. When the polish is layered onto the tiles they are filed and buffed down to create an even surface. This leaves micro cavities where air bubbles have been trapt, and so sealing the product ensures that all the microscopic air holes are sealed. All this entails is using a polished porcelain sealer, we use an L.T.P. product called 'MPG sealer', in which a small amount of liquid is applied to the porcelain, buffing after 20 minutes (to remove any wipe marks).
To test if any natural stone or polished porcelain tile needs sealing you can perform an easy watermark test; leave a spot of water on the tiles and after 20 minutes if there is still a visible water mark within the tile this indicates that liquids are being absorbed into the product. Simply clean and reseal!
Hope this is of some help!