The worst enemy of stone is water, which penetrates via tiny pores. Capillaries with diameters from 0.1 to 100 microns, much finer than a human hair, are particularly effective at transporting water into the stone interior. As the water evaporates, salts crystallize out, either subjecting the stone to internal pressure due to their high volume, or forming deposits on the surface. Alternatively, if the water in the stone freezes, the higher-volume ice may also generate pressure. Salt and ice formation results in microcracks which decrease the stone’s stability. Whole sections may chip off, or the stone may be softened and weakened to the extent that it can be rubbed away by abrasion. Air pollution, for example in the form of acidic gases such as sulfur dioxide, worsens the destructive effect of the water. The resulting salts may seal off the surface of the stone, preventing the moisture underneath from escaping. In addition, the effect of freeze-thaw cycles is intensified. Fungi and moss which grow on stone retain water on and in the stone, which may exacerbate its destructive effect.
The strengthening of stone starts at its chemical structure. Most stone resembles quartz sand, composed of silicon dioxide (SiO2). The silicon atom has four covalent bonds with oxygen atoms. Stone is based on a silicate structure composed of silicon-oxygen compounds. If the silicon atoms are not tightly bound by the four oxygen bonds, so-called lattice defects occur in the stone structure.
Such lattice defects accumulate due to the aging processes described above. The lattice defects may be repaired by introducing organic silicate compounds. WACKER Stone Strengthener OH 100 consists of a silicate containing ethyl groups. The liquid is applied directly to the cleaned stone, where it penetrates through capillaries into the stone in the same manner as atmospheric humidity. The stone strengthener and water react inside the stone. During the consolidation reaction alcohol splits off and evaporates. The silica units fill the lattice defects, thus restoring the stone’s original hardness. The stone becomes denser without losing its all-important breathability. This type of stone strengthening may be repeated without problems if signs of aging reappear. To delay the aging process, strengthened stone should also be impregnated with a water repellent to prevent water from penetrating and causing damage.
