Sealing Natural Stone

The sealing of stone is quite different to sealing other floor types. When sealing wooden floors for example a protective coat is added to the outside. When sealing stone the pores in the rock , or the spaces between the crystals that make up the rock are filled with the sealant with no trace of the sealant on the outer surface.
The purpose of this process is to stop liquids seeping into the stone and staining it. A sealant does not protect the stone from physical damage as a sealant on another surface would..
So what is the best sealant to use?
Natural stone must be allowed to breathe. What does this mean, because stone does not breathe like we do. Stone is made up of a variety of crystals. Each crystal is made up of different minerals. The minerals that make up the crystallised structure of the stone determine whether it is marble, or limestone or granite. To maintain the integrity of these crystals they must interact with the air and the component parts of the air in particular the oxygen. Hence they must be allowed to breathe. It is also essential that any water that gets into the stone is able to evaporate by moving through these spaces until it reaches the surface and can evaporate.
Most importantly and a little considered aspect of stone structure is the interaction between the crystals of the stone and the many millions of bacteria that live in these pores. Very slowly research is demonstrating that bacteria would seem to play a vital role in maintaining the integrity of the stone in ways that are as yet little understood. In order for these bacteria to survive and prosper the sealant used should be one that does not damage them but possibly promotes their activity.
Acrylic type sealants or impregnators should be avoided at all costs because they fill up the pores killing the bacteria and blocking completely the movement of water through the stone. This can ultimately lead to the stones cracking if the underside becomes wet for any reason. They are also very difficult to remove if excess sealant is applied and not properly removed whilst still wet. What you end up with is a covering of sealant on the surface of the stone which discolours and dulls it.
The best sealers to use are those based on natural products and made up of fatty acids such as soap. These can be applied in the normal cleaning process to top up the seal and clean at the same time. Any excess is washed off with pure water. Most importantly these fatty acids fill the spaces but not in a rock hard manner. Because fat is hydrophobic it repels water stopping it penetrating during any spillage so preventing absorption into the stone and subsequent staining. Being semi hard only it does not stop the natural movement of water through it so it can still escape by evaporation. Most importantly it encourages the bacterial population by providing another source of nutrient and a comfortable environment in which to live and reproduce.