Wednesday, 12 March 2014

The Lead in your Pencil

The 'lead' in your pencil is actually made from graphite combined with a clay binder.

When a vast deposit of graphite was discovered in the 16th century in Cumbria, England, it was mistaken for a form of lead ore.  Local farmers had been using the graphite to mark their sheep.

It was easy to break this pure and soft graphite into sticks, and a lead holder was eventually developed by hollowing out a stick of wood, cutting it lengthways, inserting the graphite, and then sealing the two sides back together.

The modern pencil is made in much the same way.  The lead part of the pencil is now made up from finely ground graphite and clay, mixed with water and pressed together at high temperatures into thin rods. Graphite and clay are mixed in differing ratios to create variations in the blackness and hardness of the lead and the different grades of the lead are identified by a letter, and or, number on the side of the pencil.

Although there is no specific industrial standard, the grade shown on the pencil can be used as an indication of the lead's blackness or hardness, and it will differ from manufacturer to manufacturer.  The letter 'B' stands for Black and the letter 'H' for Hard.  The lead used in a standard writing pencil is HB.  As the ratio of graphite is increased, the lead softens and the pencil mark becomes blacker - B, 2B, 3B etc.  As the ratio of clay is increased, the lead becomes harder and the pencil mark lighter - H, 2H, 3H etc.

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