Sealing Stones

Lore & Background

The source details how natural stone has been used since ancient times in kitchens, floors, walls, bathrooms, dining rooms, around swimming pools, building foyers, public areas, and facades. It has been valued for its strength, durability, and insulation properties. The porosity of most stone leaves it prone to damage if unsealed. Staining occurs when oils or other liquids penetrate deeply into capillary channels, depositing material that is effectively impossible to remove without destroying the stone. Salt attack happens when salts dissolved in water are carried into the stone, causing efflorescence (a gritty white deposit on the surface) or spalling (surface flaking due to expanding salt crystals). Acid attack erodes acid-soluble stone materials like calcite in marble, limestone, and travertine, as well as the internal cement binding grains in sandstone. Even mild household acids like cola, wine, vinegar, lemon juice, and milk can damage vulnerable stone types. Picture framing occurs when water or grout moves into the edges of the stone, creating an unsightly darkening that is usually irreversible. Freeze-thaw spalling results when water freezes in surface pores, causing the stone to spall, crumble, or crack.

In Their Own Story

The ancient Romans often used olive oil to seal their stone. Such treatment provided some protection by excluding water and other weathering agents, but it stained the stone permanently. During the Renaissance, Europeans experimented with topical varnishes and sealants made from ingredients such as egg white, natural resins, and silica, which were clear, could be applied wet, and hardened to form a protective skin. Most such measures did not last long, and some proved harmful in the long run.

Reader's Guide

Modern stone sealers are divided into three broad types: topical sealers, penetrating sealers, and impregnating sealers. Topical sealers are generally made from polyurethanes, acrylics, or natural wax. They may be effective at stopping stains but tend to wear out relatively quickly, especially on high-traffic areas of flooring. They significantly change the look and slip resistance of the surface, especially when wet. They are not breathable and are not effective against salt attack. Penetrating sealers use siliconates, fluoro-polymers, and siloxanes, which repel liquids. They penetrate the surface enough to anchor the material, are generally longer lasting than topical sealers, and often do not substantially alter the look of the stone. However, they still can change slip characteristics and wear relatively quickly. They are often breathable to a certain degree but do not penetrate deeply enough (generally less than 1mm) to be effective against salt attack. Impregnating sealers use silanes or modified silanes. These penetrate deeply into the material, impregnating it with molecules that bond to capillary pores and repel water and/or oils from within. Some modified silane sealers impregnate deeply enough to protect against salt attack, efflorescence, spalling, picture framing, and freeze-thaw spalling. Some silane stone sealers based on nanotechnology claim to be resistant to UV light and higher pH levels found in new masonry and pointing.

Did You Know?

• All bulk natural stone, including granite, basalt, marble, slate, limestone, travertine, and sandstone, contains interconnected capillary channels that allow liquids and gases to penetrate.
• The ancient Romans used olive oil to seal stone, but it stained the stone permanently.
• Even mild household acids like cola, wine, vinegar, lemon juice, and milk can damage vulnerable types of stone such as marble and limestone.
• Efflorescence is a gritty white deposit on the surface caused by mineral solutions in capillary channels being drawn to the surface and left behind when water evaporates.
• Some modern silane-based impregnating sealers claim to be resistant to UV light and higher pH levels found in new masonry and pointing.