Natural stone is porous and that's a fact! This basically means it will absorb liquid if that liquid is left on it for any period of time. This can cause staining. Limestone tiles are particularly susceptible to moisture absorption. Granite tiles on the other hand are practically impervious to water but we still recommend that you seal them just in case. Its really easy to do and once sealed properly your natural stone tiles should be prevented from absorbing liquids. Marble tiles aren't as porous as limestone tiles and travertine tiles are about the same.
Sealers are basically an "impregnator" which means that they are a liquid that soaks into natural stone tiles to create a kind of barrier. Tiles should be sealed to what is known as "Saturation Point" this usually takes two coats but can take more than two on limestone tiles. Saturation point is where the tile will accept no more sealer into them ie. no more will soak in, it just sits on the surface.
Typically your installer will do this for you in the first instance. If they are any good, they will know exactly how to seal your natural stone tiles properly. Once this has been done, you need to use the right cleaning products as general household cleaners can take the seal out of the tiles over a period of time making them more susceptible to moisture and dirt absorption. We recommend using the Stonefix Maintenance liquid for cleaning as this will not affect the seal in your tiles. Its inexpensive and a bottle will last for about a year!
The procedure for sealing tiles is this. Once your limestone tiles, travertine tiles or marble tiles have been installed and before they have been grouted, clean the tiles thoroughly with a residue remover (obtainable practically anywhere). This will ensure the tiles are perfectly clean so that they can absorb the sealer properly. Apply a thin coat of the appropriate sealer (there are different ones for polished natural stone tiles and for honed (matt) natural stone tiles) using a lint free cloth. Make sure the coat is even and all tiles are covered. After about an hour, if there is any residue left on the surface of the tiles, wipe off with a dry cloth like an old tea towel. Leave for a few hours then grout the tiles. It is important to seal before grouting as limestone tiles are especially susceptible to "bleed" or "pulling in". This is where the color of the grout soaks into the edge other the tile through the joint and creates a shadow on the edge of each tile. Sealing the tiles first before grouting prevents this and also stops the grout from drying out too quickly as with limestone tiles that aren't sealed, the moisture from the grout on the surface of the tiles when they are being grouted is drawn out of the grout and the grout drys out sufficiently to stick to the surface of the tile before you have had chance to wipe it off!!!
Once the tiles have been grouted and properly cleaned, repeat the procedure as above. This should be enough for travertine tiles and marble tiles but you may have to give limestone tiles one more coat. Remember, any residue still left after an hour should be wiped off with a dry cloth!