Pencil 'leads' contain no lead whatever. They are made of a mixture of graphite and clay, finely ground, thoroughly mixed, and fired in ovens to produce a strong fused stick, similar to chinaware.
Graphite is pure carbon and is found in two forms. Crystalline graphite, from Ceylon and Malagasy, comes in the form of tiny silvery flakes of such oily smoothness that it is frequently used as a lubricant. Amorphous graphite, mainly obtained from Mexico, is powdery, formless and extremely black, and it is this type that is largely used in the manufacture of pencils.
Graphite is a natural lubricant. It is therefore virtually impossible to grind it small enough by conventional methods. Some years ago Berol invented a special type of mil called the attrition mill. It is based on the idea that particles of graphite should grind particles of graphite. This is done by blowing two jets of highly compressed air, containing graphite particles, directly at each other. Thus the particles of graphite grind themselves by attrition. The particles, so small that they float naturally, are gradually drawn off and collected for future use.
The Clay, which comes mainly from Bavaria in Western Germany, is similar to the type used in the manufacture of the highest grade of porcelain and china, but produces a stronger fired product. It is mixed with water and refined to remove all grit and heavier elements, leaving only the finest microscopic particles which will be mixed with the graphite.
The graphite and the clay are now mixed together in the exact proportion which determines the hardness (or degree) of the pencil. Conventionally the centre point for grading is HB; this stands for Hard and Black, and has become the most popular degree.
The more graphite that is introduced into the mixture the softer and blacker the pencil will become: this group ranges from B up to 6B; the latter degree has a far greater proportion of graphite to the clay and is much favoured by artists.
At the other extreme the more clay the harder the pencil; these are manufactured from H up to 9H. The 9H pencil contains a very large proportion of clay and very little graphite and is used mainly by stonemasons and steelworkers.
The final mixture, still suspended in water, is poured into revolving pebble mills and further mixed and ground for days until both graphite and clay are reduced to still smaller particles which are perfectly dispersed.
The mixture is then pumped into filter presses where the water is squeezed out, leaving a stiff solid, which is compressed and extruded to compact it still further.
Finally, it is extruded from a cylinder, in the bottom of which there is a diamond lapped sapphire die the diameter of the finished 'stick'. Under many tons of pressure, the mixture is forced through the die, cut into appropriate lengths and further dried ready for the final process.
When completely dry, the sticks are packed in crucibles and fired for many hours at white heat (1200oC) in electrically controlled gas or oil-fired furnaces, where they are tempered like fine steel. They come out of the furnace almost ready for use. There is one thing to be added - wax; to give an even smoother writing point. The wax is a petroleum derived product; spermaceti is no longer used. However, the stick has to be glued into the wood in such a way that it will not slip out. It is a well known fact that glue will not stick to wax unless it is specially treated.
Some years ago Berol invented a process which overcame this difficulty, with the result that the lead was in fact 'super-bonded' into the wood. The trade mark 'chemi-Sealed' assures the user that Berol pencils have been super-bonded.
Super-bonding means that the stick will not shatter inside the wood if the pencil is dropped. It means that the pencil can be sharpened without bits breaking loose and clogging the sharpener. It also means that a firm writing pressure can be exerted without the stick slipping out of the top of the pencil, frustrating the user as it did in the old days.