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Porosity
The porosity of an object depends on the degree of compaction. A material with high compaction significantly reduces porosity by shrinking the size of the pores or filling them with a finer sediment. Furthermore, the naturally occurring or human-induced sorting of a porous material greatly affects porosity. In sediment, for example, soil naturally sorts itself into layers; a well-sorted layer of sediment will contain grains of the same or similar size, which greatly increases porosity. Badly sorted soil contains grains of a wide range of size, greatly reducing porosity. Porosity is measured through a simple calculation that divides void volume by the material volume. A cubic meter of soil, for example, has a porosity of 0.3, where as a cubic meter of clay has a porosity of 0.5.
Permeability
Permeability refers to the amount of air, water or gas the porous material can absorb at a given time and how quickly it flows. A highly compacted and very well sorted porous material, like clay, will retain water extremely well, and it will not allow the water to flow very quickly; thus, the permeability is very low. In fact, according to the Michigan Technological University, it takes 200 years for water to pass through 1 meter of clay; whereas, it takes only two minutes for water to pass through a meter of gravel, a highly permeable porous material.
Natural Porous Materials
Soil and rock are naturally occurring porous materials. Pores in soil make space for roots and insects and retain water and nutrients to nourish life. Soils are highly permeable and well sorted because they need to allow water to flow well to protect plant life from drowning. In rocks, there are several types of occurring porosity due to fractures and chemical alterations caused by leeching minerals. Vuggy porosity, for example, is the dissolution of a rock's larger features, which creates large holes.
Manufactured Porous Materials
The most common purpose porous materials are manufactured for is liquid filtration. Ceramic filters, for instance, have millions of micro pores that trap dirt, bacteria or living organisms too large to fit through, thus making the water safe to drink. Other porous material include manufacturing metals with an increasing number of pores to reduce weight, save materials, and increase heat retention. Sinterpaper, for example, is made out of powdered copper or stainless steel. The result is a lightweight, inexpensive, paperlike sheet of metal that can retain heat and filter liquid and gases.