It is very important to understand the internal composition of stones in order to be able to chalk out a successful maintenance schedule. By nature, stones are not absolutely solid. They are made of several crystal fluid materials which have consolidated and bonded together for centuries to form stones. As a result, almost all stones have three internal physical characteristics- Porosity, Permeability and Absorption.
Geologically, the term ‘POROSITY’ means open or void space. Often spaces remains between the particles that form the stones. Porosity is used in stones to measure the percentage (%) of pores they contain and it is measured in fraction. Different stone has different porosity ratio. The metamorphic stones have higher percentage of porosity while the igneous and sedimentary stones have comparatively low ratio of porosity.
The processes of compaction and recrystallization change the texture of stones during metamorphism. As a result some open and void spaces develop between the grains. The degree and intensity of porosity vary from stone to stone belonging to metamorphic group of stones. For example, ratio of pores of quartzite and slate are not the same though both are metamorphic stones.
Igneous stones typically have less porosity as their grains grow together during the formation period. However after the cooling process is over, some grains of some stones contract more than others- quartz grains contract more than half their original size.
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Due to overpressure for a long period of time during the formation process, the cementation of sedimentary stones get solid and this decreases the porosity level of such stones. The porosity loss due to cementation is seen to have a strong impact on the permeability, which leads to the formation of a pressure seal.
Percentage (%) of Porosity of some common stones:
Granite 0.4 – 1.5
Limestone 0.6 – 31
Marble 0.5 – 2
Sandstone 0.5 – 35
Quartzite 0.4 – 3.9
Permeability is a measure of the ease with which a fluid can move through a porous stone. Normally, the rate of permeability and ratio of porosity go hand-in-hand. Stones, which have high permeability also have high porosity and large grain size. But in some cases, if a particular type of stone has high fracture and softer veining materials, that stone might be highly permeable though it has low porosity ratio.
Permeability measures the extent to which pores and capillary structures of a stone are interconnected which lead us to understand the degree and depth to which moisture, vapors and liquids can be absorbed by that stone. Connections between pore spaces are stones like sandstone, and are narrower in fine-grained materials clay, mudstone). However, not all pore spaces may be connected while others may contain clay minerals that can expand in the presence of liquid substance to block passageways and reduce permeability.
All stones are more or less absorbant-some more than the other. The absorbency of stones is the result of their natural characteristics-porosity and permeability. The more porous and permeable a stone is, the more absorbent it is. Absorbaecy of the stones also depend on honing and finishing. Honed and textured stones tend to have more open pores than highly polished one because windows of pores get blocked during the polishing process. These type of stones have low absorbency rate. However, there are some stones which have high absorbency rate even if they are well polished.