Sealing is the application of a surface treatment to retard staining . 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

Marble

Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasons use the term more broadly to encompass unmetamorphosed limestone.Marble is commonly used for...

, 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.

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 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

Efflorescence

In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...

 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

Efflorescence

In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...

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.

Protecting stone

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

Renaissance

The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...

 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 sealers

Modern stone sealers are divided into 3 broad types:

Topical sealers

Generally made from polyurethanes or acrylics. These sealers may be effective at stopping stains but, being exposed on the surface of the material, they tend to wear out relatively quickly, especially on high-traffic areas of flooring. This type of sealer will significantly change the look and slip resistance of the surface, especially when it is wet. These sealers are not breathable i.e. do not allow the escape of water vapour and other gases, and are not effective against salt attack, such as efflorescence

Efflorescence

In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...

 and spalling.

Penetrating sealers

The most penetrating sealers use siliconates, fluoro-polymers and siloxanes, which repel liquids. These sealers penetrate the surface of the stone enough to anchor the material to the surface. They are generally longer lasting than topical sealers and often do not substantially alter the look of the stone, but still can change the slip characteristics of the surface and do wear relatively quickly. Penetrating sealers often require the use of special cleaners which both clean and top up the repellent ingredient left on the stone surface. These sealers are often breathable to a certain degree, but do not penetrate deeply enough (generally less than 1mm) to be effective against salt attack, such as efflorescence

Efflorescence

In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...

 and spalling.

Impregnating sealers

Uses silane

Silane

Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...

s or modified silanes. These are a type of penetrating sealer, which penetrate deeply into the material, impregnating it with molecules which bond to the cappillary pores and repels water and / or oils from within the material. Some modified silane sealers impregnate deeply enough to protect against salt attack, such as efflorescence

Efflorescence

In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...

, spalling, picture framing and freeze-thaw spalling. A good depth of penetration is also essential for protection from weathering and traffic.