Sealing is the application of a surface treatment to retard staining [1]. All natural stone is porous since it is made up of countless interconnected capillary pores through which liquids and gases can move. Natural stone includes; granite, basalt, marble, limestone, travertine, sandstone and slate. These porous materials act like a "hard sponge" and actually suck in liquids over time, along with any dissolved salts and other minerals. Very porous stone, such as sandstone will absorb liquids relatively quickly, while denser igneous stones such as granite are significantly less porous and may take an hour or more to absorb oils and water-based liquids.
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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 as a building and decorating material. Its strength, durability, excellent insulation properties, ability to be shaped, and the variety of stone types and colours make it an exceptionally versatile building material. The porosity and makeup of most stone does, however, leave it prone to certain types of damage if unsealed.
Staining is the most commonly occurring damage, which occurs when oils and other liquids are sucked deeply into the material through its system of capillaries, where it can be almost impossible to remove.
Salt Attack occurs when salts dissolved in water are carried into the stone. The two most common types of salt attack are efflorescence and spalling. Salts drying i.e. crystallizing on the surface can cause surface spalling. For example, the salt magnesium sulfate found in sea water when it dries will expand to three times its volume by taking on seven water molecules.
Efflorescence is the formation of a white deposit on the surface. It is usually the result of water containing water-bourne salts reaching the surface and the water evaporating. In the case of cement based mortar being used to fix the stone in place free calcium hydroxide is leached out onto the surface. Here it reacts with carbon dioxide in the air to form water insoluble calcium carbonate.
Acid Attack. calcite-based stone such as marble, limestone and travertine react with acidic substances on contact, breaking down the surface and leaving dull marks or even deep pits over time. This is known as acid etching. Even mild household acids, including cola, wine, vinegar, lemon juice and milk, can damage these types of stone. The milder the acid, the longer it takes to etch calcite-based stone; stronger acids can damage the stone in seconds.
Picture Framing occurs when water or grout moves into the edges of the stone to create an unsightly darkening or "frame" affect. The result is usually irreversible.
Freeze-thaw Spalling results when water is able to freeze in the surface pores. The water expands by 9% of its volume causing the surface to rupture or spall.
The longevity and usefulness of stone can be extended if it is effectively sealed against the ingress of damaging liquids and minerals such as salts. The ancient Romans often used olive oil to seal their stone, which provides some protection against the ingress of water and against general weathering by the elements, but stains the stone permanently.
During the renaissance Europeans experimented with the use of topical varnishes and sealers made from ingredients such as egg white, naturally occurring resins and silica, which were clear, could be applied wet and harden to form a protective skin.
Modern stone sealers are divided into 3 broad types: