POROSITY
PERMEABILITY ABSORPTIONStone has a internal physical structure that is not totally solid.
POROSITY
Porosity is the ratio of pores (micro-voids) in the stone, to its total solid volume.
Pores and the capillary structure develop differently in each of the three
stone groups.
PORES & CAPILLARY STRUCTURE
Igneous Stones : The formation process of these stones under high pressure and
temperature allows very little open pore space. However, the quartz
grains contract more than half their volume during the cooling process,
and extensive cracking develops across and around the quartz grains.
The grain size of granite and granite-type stones range from small to
large and a particular variety may be composed of two grain sizes,
this increases its porosity. It can be said that granite has more of a
fracture system than a pore system.
Metamorphic Stones : During the re-crystallization process of limestone to marble the
small grains of calcite are reorganized into a larger crystal structure
and are being repacked into a new texture. It is during this growth
and repacking that micro-spaces (pores) develop between the
grains. Mineralogy and degree of metamorphism causes sizes and
shapes of pores to differ in quartzite and slate.
Sedimentary Stones : Pores develop as a result of compaction and the cementing process.
A nearly unlimited variety of pore sizes and shapes are characteristic
of sedimentary stones.
The porosity ratio is lower
in igneous and metamorphic stones, generally higher in
sedimentary
types.
POROSITY RATIOS % |
|||
| Granite | 0.4 - 1.5 | Slate | 0.4 - 5 |
| Marble | 0.5 - 2 | Limestone | 0.6 - 31 |
| Quartzite | 0.4 - 3.9 | Sandstone | 0.5 - 35 |
PERMEABILITY
Associated with stones porosity is its permeability. This
is the extent to which the pores
and capillary structures are interconnected throughout the stone. These networks,
their
size, structure and orientation affect the degree and depth to which moisture, vapors and
liquids can be absorb into the interior of the stone or migrate from the substrate by
capillary
action through the stone. Permeability may be greater in some directions than others based
upon the pore size, shape and the distribution of the interconnectedness of the system.
Permeability is increased when a stone is
highly fractured or the veining material is soft or
grainy. A particular variety of stone may be highly permeable ( a well defined
interconnected
network of pores), although its porosity is low (a low percentage of voids).
The size and shapes of pores and
the capillary structure differs in stones and is an important
factor in relation to stone decay.
ABSORPTION
Absorbency is the result of these two
properties. Absorbency
is an important determining
factor in stones sensitivity to stains. The size of the pores, their orientation,
how well
they are networked and the type of finish the stone has are important contributing factors
to
a stones overall absorbency. In relation to cleanability this factor is
more important
than how porous a stone is.
Honed and textured surfaces are more susceptible to soiling
and staining due to the fact that there are more open pores at the surface than a highly
polished finish. The polishing process has a tendency to close off pores leaving fewer
ones
exposed, resulting in a low absorbent surface. However, some varieties of stone have large
pores and capillary structures and even when these stones are polished they still remain
very absorbent.
| Granite | 0.2 - 0.5 | Slate | 0.1 - 1.7 |
| Marble | 0.2 - 0.6 | Sandstone | 0.2 - 9 |
| Quartzite | 0.1 - 1.4 | Limestone | 0.2 - 12 |
NOTE : Most common oils can be easily absorbed into all types of stone.
