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Porosity of Stone

What is stone porosity, permeability & absorption?

Stones internal structure is not totally solid and so all stone is absorbent to some degree. The more absorbent a stone is the easier it will stain. Therefore it is important to understand stone porosity and permeability in order to plan a new maintenance schedule or decide how a stone sealer should be applied.

Porosity

The porosity of a stone is its ratio of pores or 'micro-voids' to its total solid volume. Igneous stones generally have a much lower porosity than metamorphic stones and sedimentary stones.

Igneous Stone Porosity:

Due to its formation under high heat and pressure, igneous rocks such as granite and basalt develop very limited pore space. However, during the cooling process certain igneous stone grains can contract by over 50% causing cracking .For this reason regularly used igneous stones such as granite are characterised as having a fracture system rather than a pore system.

Granite has a porosity ratio of between 0.4% – 1.5%

Metamorphic Stone Porosity:

Micro-voids appear in metamorphic stone during the re-crystallisation process. This is because the natural crystals grow and re-arrange or 're-pack' causing pores to develop between the grains. Porosities of metamorphic stones can vary considerably depending on the 'protolith' and other factors such as levels of heat, pressure and friction present during the complete process.

Marble has a porosity ratio of between 0.5% – 2%
Slate has a porosity ratio of between 0.4% – 5%

Sedimentary Stone Porosity:

The porosity of sedimentary stones such as limestone and travertine is developed during 'lithification'. Pressure causes the cementation of sediments which results in voids appearing during formation. Some sedimentary stones can have a very high porosity which must be taken into consideration when deciding on a natural stone installation.

Limestone has a porosity ratio of between 0.6% – 31%

Permeability

A stone's permeability is closely linked to its porosity. It refers to its ability to transmit fluids. This is affected by the interconnectedness of the pores and capillary structures within it. A high level of fracturing in the stone or a presence of soft veining will also increase its permeability. It is possible for a stone to have a high level of permeability and also have a low porosity. This would happen if a stone develops a good network of pores while maintaining a low percentage of micro-voids.

Absorption

A stone's level of porosity and permeability will determine its absorbent it is. In the stone care industry this is probably the factor which is most important to us as it will give us an idea of how prone to staining a particular stone surface will be, how this can be minimised and how it can be prevented.

Honed stones will absorb liquids much more readily than polished varieties due to the increased presence of open pores or voids at the surface. Vitrification is a process which also closes off pores at the surface which results in decreased absorption. These are general rules however as some stones have such large capillary structures that even full diamond polishing will fail to lower absorption significantly.

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