A lead pencil consists of a stick of graphite in the center of a cylindrical piece of red cedar wood. This particular type of wood is selected because it can be cut easily and smoothly with a penknife. The ease 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 comes from the earth in the form of large, crude stones.
This crude graphite is crushed to a powder in a large rolling 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 lead has been placed. This second slat is coated with glue before being fitted over the lead, so that the two slats hold fast after being brought together. After the glue has set thoroughly, the slats are fed lengthwise into another machine which separates their six parts into six pencils.
Since there is a demand for pencils of every grade, from the soft pencil of the news editor to the hard pencil of the draftsman, pencils are made in sixteen grades of hardness. These grades vary widely enough to meet every demand.