226 APPLIED SCIENCE 



wood. This particular type of wood is selected because ifc 

 can be cut easily and smoothly with a penknife. The eas> 

 with which it can be cut is due to the closeness of its grain 

 and the softness and tenderness of its fiber. The graphite 

 used in lead pencils is of the highest grade. It is mined in 

 Ceylon and Mexico and conies from the earth in the form of 

 large, crude stones. 



This crude graphite is crushed to a powder in a large roll- 

 ing machine. A smooth clay, called a binding agent, is 

 added to the graphite to hold the particles together. The 

 ratio of the clay to the graphite determines the hardness of 

 the "lead" in the pencil; increasing the proportion of clay 

 makes the pencil harder. The mixture is washed to remove 

 all particles of grit and other impurities. 



To make the pencil rods, or " leads," the mixture of graphite 

 and clay is placed in the bottom of a steel cylinder which 

 contains dies of the proper gauge for the thickness of the 

 "lead." Under enormous pressure the mixture is forced 

 through the dies and emerges like a cylindrical shoe-string 

 at the rate of 170 ft. per minute. This cylindrical string is 

 straightened and dried, cut to pencil lengths, and placed in a 

 crucible to harden. The heat toughens and gives the proper 

 temper to the rods. 



Six pencils are made at one time. The red cedar wood, 

 already mentioned, is cut into slats. Each slat is slightly 

 longer than a pencil, slightly thicker than half a pencil, and 

 as wide as six pencils. The slat is well seasoned kiln-dried 

 and passed through a planing and cutting machine. This 

 machine planes the surface of the slat smooth and cuts in it 

 six lengthwise grooves. Into each of these grooves a piece 

 of lead is inserted by hand . Then another slat, similarly 

 grooved and planed, is fitted over the slat into which the 



