Pencil lead
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Pencil lead is the marking substance inside a pencil or mechanical pencil. It is not actual lead (which is poisonous), but rather a form of graphite. It is called lead either because graphite was once nicknamed "black lead", because lead and graphite leave similar marks, or because pencils once used lead.
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History
The Napoleonic Wars birthed the modern graphite and clay-based pencil out of necessity. France, under naval blockade imposed by Great Britain, was unable to import the pure graphite sticks from the British Seathwaite Fell mines - the world's only known source of solid graphite. France was also unable to import the inferior German graphite pencil substitute. In 1795 Nicholas Jacques Conté, a Army officer, discovered that firing the available powdered graphite with clay would work as a susbstitute pencil filler.
Hardness
Pencil lead is graded by hardness: a greater percentage of clay results in a harder lead. H (harder), F, HB and B (softer) are pencils of average hardness. Pencils ranging from 2B to 9B are softer still and are used for sketching; 2H to 9H are harder than average. A common American #2 pencil is equivalent to HB.
Mechanical pencils
In mechanical pencils, pencil lead is a long, thin rod of graphite. The most common thicknesses are 0.5mm and 0.7mm, and common lengths include 60mm, 75mm, and 120mm. The pencil lead is inserted at the back of the pencil.
External links
- Pencils & Pencil Lead (http://pubs.acs.org/cen/whatstuff/stuff/7942sci4.html)
- Pencil Lead and Health (http://health.yahoo.com/ency/adam/002817/0)
- How do they get lead in a wooden pencil? (http://science.howstuffworks.com/question465.htm)