Fig. 1 shows the shape in which the cedar slats arrive at the factory.
These slats after grading are boiled in steam to remove what remaining
sap there may be in the wood. The slats are then dried in steam-drying
rooms. Then the next step is grooving and gives the results shown by
Fig. 2. Now the wood is ready to receive the “leads” (which you will
remember are a mixture of graphite and clay), which are placed between
two slats sandwich fashion, glued, put in forms that hold them over
night under a thousand pounds pressure. Fig. 3 shows the leads laid in
one of the grooved slats.]
How Are Lead-pencils Made?
The graphite, as it comes from the mines, is broken into small pieces,
the impure particles being separated by hand. It is then finely
divided in large pulverizers and placed in tubs of water, so that the
lighter particles of graphite float off from the heavier particles of
impurities. This separating, in the cheaper grades, is also done by
means of centrifugal machines, but the results are not as satisfactory.
After separation, the graphite is filtered through filter-presses.
What Makes Some Pencils Hard and Others Soft?
The clay, after having been subjected to a similar process, is placed
in mixers with the graphite, in proportions dependent upon the grade
of hardness that is desired. A greater proportion of clay produces a
greater degree of hardness; a lesser proportion increases the softness.
[Illustration: FIG. 4.