No Lead Pencils

Pencils are made from a slat of wood with several grooves cut in it to hold the leads. A similar grooved slat is then placed on top and glued to form a sandwich from which individual pencils are cut, sanded smooth, and painted. Although many types of wood have been used over the years to make pencils, the current favorite is incense cedar from the Sierra Nevada Mountains of California. 

Pencils are very useful—especially for doing chemistry problems—because we can erase our mistakes. Most pencils used in the United States have erasers (first attached to pencils in 1858), although most European pencils do not. Laid end-to end, the number of pencils made in the United States each year would circle the earth about 15 times. Pencils illustrate how useful a simple substance like graphite can be.  

Ionic and covalent bonds are the We can also think of a bond in terms of forces. The simultaneous at- 

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Figure 5.15. Spectrum of graphite in No. 2 pencil lead obtained with Dilor microscope and 50 x objective. Five spectral segments with total acquisition time of 20 min 5 mW at sample, 100 pm slit, resolution of 4 cm .

The softness of natural graphite is exploited in lubricants and self-lubricating seals. The particularly pure graphite from Sri Lanka is preferred for such applications, since no solid abrasive impurities are present. For the same reason, graphite is used as the lead in lead pencils. 

Fun fact We commonly refer to the graphite in pencils as pencil lead, but there s no lead (Pb) in there. The Romans did use lead for writing, but the practice didn t meike it much farther in history than that. The paint on the outside of pencils did, however,... 

Freehand technical sketching can be done anywhere. All you need is a no. 2 lead pencil or softer, eraser, paper (grid or plain), and a hard smooth surface. This is one reason why designers and engineers often carry clipboards or engineer s notebooks with them (see Figure 4-15). 

The terminology of the elements suffuses our language, sometimes divorced from the questions of composition to which it once referred. Plumbing today is more likely to be made from plastic pipes than from the Romans (lead) the lead in pencils is no such thing. Cadmium Red paints often contain no cadmium at all. Tin cans have no more than the thinnest veneer of metallic tin it is too valuable for more. The American nickel contains relatively little of that metal. And when was the last time that a Frenchman s pocketful of jingling argent made of real silver ... 

Use only a pencil with soft, black lead (No. 2 or HB) to complete this answer sheet. Be sure to fill in completely the space that corresponds to your answer choice. Completely erase any errors or stray marks. 

Quarles [1988] used pencil lead breaks on wood blocks of various species to study the effects that grain angle and moisture content might have on AE event rates. The conclusion was that there were significant differences between species, but only minor differences in the moisture content effects within species. Both Quarles [1990] and Sakui [1990] confirmed that AE signal attenuation was relatively constant up to fiber saturation point (the moisture content corresponding to saturated cell walls, but no water in the cell cavity). At moisture contents above fiber saturation point much more significant attenuation was observed. 

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