Broadly, this term is used when the manufacturer of the tiles apply sealer coating to the tiles at the end of the manufacturing process.
Why not factory pre-seal?
Pre-sealing tiles has presented problems in the field as the sealers do not seem to have consistent characteristics. They tend to produce what the experts call optical hazing.
Optical hazing is the surface effect found on some polished porcelain. It is generally only visible in certain types of low angle natural light and may not present on all tiles in the same batch. It must be noted that although tiles that exhibit hazing effect is not defective (according to test method set by ISO 10545 part 2 section 7), it is considered undesirable by many consumers and it therefore leads to complaints.
What’s the better alternative?
We recommend using quality porcelain tiles to start with and then perform the sealing once the installation has been carried out. This will not only reduce the entry of staining materials to the tiles surface but also the grout used in the installation.
Generally two types of sealers are used; topical surface coatings and penetrating sealers. The purpose of the sealer and the characteristics of the tile should be considered prior to sealants being used to ascertain the performance requirements.
Surface coatings seal the surface pores and generally reduce the roughness of the finish, thereby making the tile easier to clean, however it may alter the slip resistance.
With polished porcelain, the process of polishing opens up the tiny clay pockets of the tile which will accept stains and therefore polished porcelain need to be treated with a special sealer.
The sealer impregnates and lines the small cavities, prohibiting the penetration of stains. Penetrating sealers do not generally alter the appearance of polished porcelain products since they are designed to only fill the microscopic pores and not coat the tile, nor should they affect the slip resistance of the tile.
Tile Today magazine, issue 60 Sept. 2008.
AS 3958.1-2007 Ceramic tiles - Guide to the installation of ceramic tiles.