Asbestos - Classification And Properties, Occurrence And Mining, Uses, Health Considerations
Asbestos is the general name for a wide variety of silicate minerals, mostly silicates of calcium, magnesium, and iron. Their common characteristics are a fibrous structure and resistance to fire. The two most common families of asbestos minerals are called amphibole and serpentine. The mineral has been known and used by humans for centuries. The ancient Romans, for example, wove asbestos wicks for the lamps used by vestal virgins. The story is also told about Charlemagne's effort to impress barbarian visitors by throwing a table cloth woven of asbestos into the fire.
One of the first complete scientific descriptions of asbestos was provided by J. P. Tournefort in the early 1700s. He explained that the substance "softens in oil and thereby acquires suppleness enough to be spun into Threads; it makes Purses and Handkerchiefs, which not only resist the Fire, but are whiten'd and cleansed by it." Travelers to North America in the 1750s also told of widespread use of asbestos among both colonists and Native Americans.
After asbestos is removed from the earth, it is processed in order to divide it into groups according to fiber length. Longer fibers are separated out for weaving into a cloth-like material. Shorter fibers, known as shingles, are combined with each other and often with other materials to make composite products. Perhaps the best known of these composites is asbestos cement which was invented in the late 1800s. Asbestos cement contains about 12.5% asbestos with the remainder consisting of portland cement; this mixture is used for a variety of construction purposes.
The first step in making asbestos cement is to form a thick, pasty mixture of cement and asbestos in water. That mixture is then passed along a conveyor belt, where water is removed. At the end of the belt, the damp mixture of cement and asbestos is laid down on some type of base. Layers are allowed to build up until a material of the desired thickness if obtained. It is then dried.
- Asbestos - Classification And Properties
- Asbestos - Occurrence And Mining
- Asbestos - Uses
- Asbestos - Health Considerations
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