|SCIENCE & TECHNOLOGY
WHAT'S THAT STUFF?
Volume 79, Number 42
CENEAR 79 42 p. 35-
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Most adults probably realize that there isn't any elemental lead in a pencil. But I worried about that when I was a kid after I had the point of a freshly sharpened No. 2 lodged in the palm of my right hand. It's still there, 30 years later.
The connection between graphite and lead stems from the days of the Roman Empire (and likely before that), when lead rods were used by scribes to write on papyrus. Both graphite and lead leave a gray mark on paper, although graphite is a bit darker. Graphite didn't come into widespread use for writing until after the 1564 discovery of a very pure graphite deposit in Borrowdale, England. At the time, graphite was thought to be a type of lead and consequently was called black lead or plumbago.
In 1779, Swedish chemist Carl W. Scheele determined that black lead was actually a form of carbon; in 1789, German geologist Abraham G. Werner reportedly gave it the name graphite, after the Greek graphein, meaning "to write." Inks were already widely available and were usually applied to paper with a brush called a peniculus, which is Latin for "little tail," hence the basis for the word "pencil." Charcoal probably was used in ancient times to mark on paper as well.
Graphite from Borrowdale originally was used in chunks called marking stones. Because graphite is softer and more brittle than lead, it requires a holder when carved into pencil-shaped sticks for writing. At first, sticks of Borrowdale graphite were wrapped with string, and the string was slowly unwound as needed as the writing core wore down. Later on, graphite was inserted between two slats tied together or into wooden sticks that were hollowed out by hand to create the first wood-cased pencils.
The Borrowdale deposit was pure enough to use without modification. But lower quality graphite needs help to keep it in a usable form. Various binders mixed with graphite powder have been tried, such as gum, resin, or glue. Sulfur also has been mixed with graphite, which results in writing cores most like pure graphite. In 1795, French chemist and Napoléon courtier Nicolas-Jacques Conté invented a process to mix graphite with clay and water, a process that is still used.
TODAY, GRAPHITE AND clay are crushed into a fine powder in a rotating drum containing large rocks. Water is added and the mixture is blended for up to three days. The water is pressed out of the mixture, leaving a gray sludge that is air-dried until it hardens.
The dried sludge is ground into a powder, water is added again, and the mixture is blended to form a soft paste. Carbon black may be added to increase the dark-ness of the lead. The paste is extruded through a metal tube to form thin rods that are cut into pencil-length pieces--called leads--that are then dried. The leads are heated in an oven to 1,800 °F (about 1,000 °C) or higher to make them smooth and hard. The ratio of graphite to clay can be adjusted to vary the hardness of the lead: the more clay, the harder the lead; the harder the lead, the less graphite comes off onto the paper, making a lighter line.
Pencils are made by cutting blocks of wood into slats that are machined to form a groove (two to nine per slat) to place the leads. A second slat is glued onto the first, sandwich fashion, then individual pencils are cut from the sandwich and sanded smooth. The pencils are next painted with five to eight coats of paint, and a recess is cut for the ferrule--the metal ring that holds the eraser. The ferrule and eraser are crimped into place, and a metal stamp is used to press a label onto the pencil.
Various types of wood have been used to make pencils over the years. Red cedar from Kenya and the U.S. was an early favorite, but today nearly all pencils worldwide are made from incense cedar, a species that grows in California's Sierra Nevada Mountains. The first pencils were unlabeled and unpainted to show the fine grain of the wood used. But by the 1890s, manufacturers started stamping their names on pencils and painting them.
The hardness of the lead is indicated by a number (1 to 4) stamped on the side of most pencils--the higher the number, the harder the lead. Sometimes there are other markings: H indicates hard, B indicates the blackness of the pencil's mark, and F indicates that the pencil can sharpen to a fine point. Sometimes combinations are used: HB, hard and black; HH, very hard.
Many of the names stamped on pencils are manufacturer's model names or company name, but these generally have historical significance. For example, the Ticonderoga, made by Dixon Ticonderoga Co., is named after the Revolutionary War fort in upstate New York, which is near one of the purest graphite deposits known, 99.9% pure carbon.
Here are some amazing pencil facts: An average pencil can write about 45,000 words, or a line about 35 miles (55 km) long. Laid end-to-end, the number of pencils made annually in the U.S. would encircle Earth about 15 times. In 1858, erasers were attached to the ends of pencils for the first time; most pencils in the U.S. have erasers, but those in Europe do not. A pencil lead or a line drawn by a pencil will conduct electricity. Colored pencils are made from chalk, clay, or wax mixed with binders and pigments.
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