Stone Needs to Breathe

“Your stone is breathing…”

Quite a statement, but in a way natural stone does breathe. Stone has an internal structure that is not totally solid. There are two physical properties found in stone: pores and capillary structures that are interconnected. The structure, size and orientation of these networks affect the degree which moisture can migrate by capillary action through the stone.
Moisture migrating through natural stone is called “moisture vapor transmission”
For example: Moisture present in the ground can be wicked or drawn up into the stone by capillary action. The porosity of the stone- that is the amount of voids in the stone and its permeability – a network of pores, move the moisture vapors like a sponge through the stone. The vapor is then released into the atmosphere.

Granite for example, during the formation process was under high pressure and temperature. This allows for very little open pore space the grain sizes of feldspar, quartz etc can increase porosity. During the cooling process of the stone after high temperature the quartz grains can contract more than half their size. This allows for extensive cracking around the quartz. So it’s more of a fracture than a pore throughout the stone.

Marble during the formation process takes place by an increase of temperature and pressure. The original minerals of the stone merge to form smaller crystals to larger crystals. These are then reformed into a new texture. It is during this process that micro-pores develop between the grains. Mineralogy and the degree of metamorphism of the stone cause the size and shape of pores in marble.

Sedimentary stone such as Limestone and Travertine consist mostly of fossilized material found in ancient sea, lake and river beds. Pores develop as a result of compaction of various minerals, organic material secondary older stone from the area and sediment. Due to the process an unlimited variety of pore sizes and shapes are prevalent in sedimentary stone. Granite and marble are generally low in porosity while limestone is considered highly porous.

When the pores and capillary structures are interconnected, the result is permeability.
Liquids can be absorbed into the interior of the stone or move from the substrate by this capillary action of pores and fractures. Permeability may be greater in one direction or another depending on pore size, shape and the distribution of fractures within the stone.

So what happens if a stone can’t breathe? If natural vapor transmission is not allowed to take place the moisture gets trapped and can cause chemical and mineralogical changes within the stone. This action of decay in natural stone may take the form of pitting, spalling, flaking and oxidation.

Using a coating or polymer on your natural stone will essentially cause the problems of blocking the necessary vapor transmission process your stone needs. So when it’s time to restore your natural stone surface, think twice about the quick fix alternatives of just coating the surface. A natural stone restoration expert will be able to assess and recommend the correct procedure to keep your natural stone healthy and happy.