Stone sealing is the application of a surface treatment to products constructed of natural stone to retard staining and corrosion. All bulk natural stone is riddled with interconnected capillary channels that permit penetration by liquids and gases. This is true for igneous rock types such as granite and basalt, metamorphic rocks such as marble and slate, and sedimentary rocks such as limestone, travertine, and sandstone. These porous channels act like a sponge, and capillary action draws in liquids over time, along with any dissolved salts and other solutes. Very porous stone, such as sandstone absorb liquids relatively quickly, while denser igneous stones such as granite are significantly less porous; they absorb smaller volumes, and more slowly, especially when absorbing viscous liquids.
Why Seal? Natural stone is used in kitchens, floors, walls, bathrooms, dining rooms, around swimming pools, building foyers, public areas and facades. Since ancient times, stone has been popular for building and decorative purposes. It has been valued for its strength, durability, and insulation properties. It can be cut, cleft, or sculpted to shape as required, and the variety of natural stone types, textures, and colors provide an exceptionally versatile range of building materials. The porosity and makeup of most stone does, however, leave it prone to certain types of damage if unsealed. Staining is the most common form of damage. It is the result of oils or other liquids penetrate deeply into the capillary channels and deposit material that is effectively impossible to remove without destroying the stone.
Efflorescence is the formation of a gritty deposit, commonly white, on the surface. Efflorescence is usually the result of mineral solutions in the capillary channels being drawn to the surface. If the water evaporates, the minerals remain as the so-called efflorescence. It also can be the result of chemical reaction; if badly prepared cement-based mortar is applied to maintain the stone in position, free calcium hydroxide may leach out. In the open air the lime reacts with carbon dioxide to form water-insoluble calcium carbonate that might take the form of powdery efflorescence or dripstone-like crusting.
Acid Attack. Acid-soluble stone materials such as the calcite in marble, limestone and travertine, as well as the internal cement that binds the resistant grains in sandstone, react with acidic solutions on contact, or on absorbing acid-forming gases in polluted air, such as oxides of sulfur or nitrogen. Acid erodes the stone, leaving dull marks on polished surfaces. In time it may cause deep pitting, eventually totally obliterating the forms of statues, memorials and other sculptures. Even mild household acids, including cola, wine, vinegar, lemon juice and milk, can damage vulnerable types of stone. The milder the acid, the longer it takes to etch calcite-based stone; stronger acids can cause irreparable damage in seconds.
Picture Framing occurs when water or grout moves into the edges of the stone to create an unsightly darkening or "frame" affect. Such harm is usually irreversible.
Freeze-thaw Spalling results when water freezes in the surface pores. The general term is Frost weathering. The water expands on freezing, causing the stone to spall, crumble, or even to crack through. Source: Wikipedia