Exploring of Asbestos
Asbestos (Greek a-, "not"; sbestos, "extinguishable") is a group of
fibrous metamorphic minerals. The name is derived for its historical
use in lamp wicks; the resistance of asbestos to fire has long been
exploited for a variety of purposes. It was used in fabrics such as
Egyptian burial cloths and Charlemagne's tablecloth, which, according
to legend, he threw in a fire to clean.
When asbestos is used for its resistance to fire or heat, the fibres
are typically mixed with cement or woven into fabric or mats. It was
used in brake shoes and gaskets for its heat resistance, on electric
oven and hotplate wiring for its electrical insulation at elevated
temperature, and in buildings for its flame-retardant and insulating
properties, its tensile strength, flexibility, and resistance to
chemicals. However, the inhalation of some kinds of asbestos fibres is
now thought to cause various illnesses, including cancer, and thus
most uses of asbestos are banned in many countries. Fibre glass has
been found to be a suitable substitute for thermal insulation and
woven ceramic fibre performs as well or better as an insulator of
high-temperature electrical conductors.
Most respirable asbestos fibres are invisible to the unaided human eye
because their size is about 3.0-20.0 µm in length and can be as thin
as 0.01 µm. Fibres ultimately form because when these minerals
originally cooled and crystallized, they formed by the polymeric
molecules lining up parallel with each other and forming oriented
crystal lattices. These crystals thus have three cleavage planes as
other minerals and gemstones have. But in their case, there are two
cleavage planes that are much weaker than the third direction. Thus
when sufficient force is applied they tend to break along their
weakest directions, resulting in a linear fragmentation pattern and
hence a fibrous form. This fracture process can keep occurring over
and over until they have been broken down to their smallest unit
dimensions. For this reason, one larger asbestos fibre can ultimately
become the source of hundreds of much thinner and smaller fibres in a
normal environment over the course of time. As they get smaller and
lighter, they become more mobile and more easily entrained (wafted)
into the air, where human respiratory exposures typically result.
Confusingly, the Modern Greek word asbestos means quicklime.
Types of asbestos.
Chrysotile, or white asbestos, is obtained from Canadian serpentine
rocks. It is less friable (and therefore less likely to be inhaled)
than the other types and is the type most often used industrially.
Chrysotile should not be confused with chrysolite, a synonym of