Sealing in the Beauty of Natural Stone: Granite, Marble, Quartzite & More

Knowing how to properly care for and protect natural stone is very important for preserving the beauty of the natural surface.  It all starts with understanding the science of stone.

There are many different types of natural stone, just as there are many different types of wood. Each type has characteristics that determine its hardness, durability, chemical resistance, type of finish and even translucency (the ability of light to shine through). By understanding these characteristics, we can make better decisions in the use, maintenance and care of stone.

Why is there so much variation even in one specific type of stone? Natural stone is not manufactured according to strict laboratory standards. Stone is created in the earth as a product of natural forces. Its origin and composition depends on constantly changing factors such as time and place, heat, pressure and chemistry. This is what makes each stone a unique work of art.

Consumers often assume that natural stone is “stain-proof;” however, all stone is permeable to some degree. In fact, DuPont lab testing shows that all natural stone, including granite, is permeable, and that the lighter and more uniquely patterned natural stones — such as Kashmir White Granite — stain easily. A simple accident of spilling wine or oil on the surface can stain an unsealed countertop in minutes, sending the dissatisfied customer complaining to their supplier.

The best way to prevent staining on natural stone is to treat the surface with a protective sealer. This creates a protective barrier that repels spills on the surface, allowing time to wipe them away. Otherwise, the liquid may damage the stone and leave behind unsightly stains.

All untreated natural stone used in construction is permeable to some degree and should be sealed. It is important, however, to understand that the majority of impregnating sealers used to seal natural stone do not prevent etching of calcite or calcium carbonate based stone by acidic liquids. These stones include limestone, travertine, marble and onyx.

The hardness of a stone is no indication of its permeability to liquids. The factors in determining a stone’s permeability have much more to do with the stone’s composition (silicate or calcium carbonate) and its origin (igneous, metamorphic or sedimentary).

Stones such as granite, gneiss, sandstone, slate, quartzite and schist are made up of silicate minerals. Silicates will have similar chemical resistance characteristics as glass. They are generally nor acid sensitive and are relatively hard and/or durable. Limestone, travertine, onyx and marble are all composed of calcium carbonate minerals. They are all acid sensitive and are generally softer than the silicates.

Igneous means “born of fire” and would include any rock formed by the cooling and crystallization of magma (molten rock) or by the consolidation of volcanic ash. Granite is an example of igneous origin. Metamorphic means “changed in structure” and would include any rock that has been altered by temperature, pressure and/or the chemical activities of fluids. Slate, quartzite and marble are metamorphic in origin. Sedimentary means “the cementing of grains,” and would include any rock that is composed of an aggregate of solids derived from preexisting rocks, the precipitation of mineral matter from solution or extracted from solution by organisms. Rocks exposed to weathering or degradation is broken down into smaller particles and may be transported by wind, water or ice and eventually deposited as sediment. Sandstone, limestone, travertine and onyx would be examples of sedimentary origin.

Many of the highly patterned or veined stones we refer to as granite are extremely permeable because of the tiny crystals that make up the stone. Between the crystals are microscopic spaces that form a capillary structure, which allows deep penetration of liquids. The smaller the crystals, the more permeable the stone and the more deeply a stain may penetrate the stone. Homogeneous granites with larger crystals have fewer spaces and are less permeable to liquids. Calcite- and calcium carbonate-based stones may stain easily but generally stain widely rather than deeply.

Before choosing a sealer, all stone professionals should know that the sealant selected should be based on the permeability and type of stone or masonry to be sealed. This knowledge is essential to delivering the best possible results.

Proper testing of sealers on the actual surfaces to be installed will show which sealer gives the best performance. Testing will also help determine the number of sealer coats required to produce the best performance. Some dense granite may require only one coat while a tumbled limestone paver may need three or more to achieve an efficient seal. Testing also helps determine the method and ease of sealer application.

That’s why it’s important to use a sealer that bonds with the stone, preserving its beauty and protecting the substantial investment that homeowners make when they choose natural stone. Using a high-quality impregnating sealer is relatively inexpensive insurance that customers will nor be faced with a potentially permanent discoloration of a relatively expensive and unique work of nature.

Once the natural stone is sealed and installed, it’s important to inform consumers how to clean it properly. Most consumers are unaware of the science of the natural stone they have purchased and do not know how to properly care for the surface. Especially significant is that most do not realize using an acid- or alkaline- based cleaner could damage the surface and negatively impact the performance of the sealer.

Specially formulated cleaners that are pH neutral have been developed to remove spills and other messes from natural stone. These provide gentle yet effective cleaning.

Most general-purpose cleaners, acidic and alkaline solutions, abrasives, ammonia and bleach can break down sealers and damage stone surfaces. When cleaning up messes on natural stone, it’s important to use specially formulated cleaners.

That’s why you should recommend to your customers that they seal their natural stone and use cleaners that are scientifically formulated for gentle yet effective cleaning. That way, you can rest assured you’re offering your customers the best care for their investment and you’re building your reputation for quality, and long-lasting beauty in the surfaces you install.