We usually become acquainted with lead pencils in early childhood when we experience some of our earliest graphic pleasures in making lines with them. As we continue to use pencils we learn that the marks made with soft lead, although they cover paper well, smear and come off unless we are very careful. We learn, too, that hard pencils yield sharp, clear marks that hardly rub away at all. Why? We do not ask, really, because we have become so accustomed to these common tools for jotting down notes and making sketches that we hardly so much as think about the technical processes behind them.
If we were to give thought to these questions, the very term "lead" pencil might occasion some bewilderment, since ordinary pencils no longer have a trace of metallic lead. Since the fourteenth century, metallic lead - which can be used for drawing if pure enough - has come to be replaced by a crystalline form of carbon called graphite. In its pure form graphite is too soft for efficient use - although originally it was used pure. for the last 175 years, clay (a common earth formed chiefly of a mixture of oxides of silica and aluminum) has been mixed with it in order to make it harder. Before then, efforts were made to develop graphite sticks and blocks in which powdered graphite was held together by glue-like binders; results were less than satisfactory.
Pencil "leads" contain between 30 and 70 per cent of clay, according to grade of hardness. The clay is ground fine, mixed with the graphite, and formed under pressure into cylindrical rods. Such rods are extremely fragile and, therefore, are clinched between two pieces of wood glued together; in this way they can be held in the hand comfortably and sharpened easily. Cedar, a dark, smooth, aromatic wood, is used in the best quality pencils. There are 15 degrees of drawing pencils. The harder spectrum ranges from 10H, the hardest, to H, the least hard; F and HB are, respectively, hard-medium and softmedium; and the softer spectrum ranges from B, the least soft, to 7B, the softest. Artists rarely use pencils harder than 6H or softer than 6B. HB is the most popular degree for ordinary use. Soft leads are thicker and more fragile than hard leads. A knife and piece of sandpaper are more satisfactory than a mechanical pencil sharpener for pointing them.
I recommend the increasingly favored practice of using refill instead of woodclinched pencils, buying the different degrees of leads in boxes of a half dozen.
Colored leads and chalks may be bought the same way.
So-called "chalks" have even thicker leads than soft graphite pencils, about one-eighth inch in diameter; they include all colors and carbon. The word "chalk" is misleading here, for the leads are not natural products, like charcoal, but materials artificially compressed like pastels, only more finely ground and purified. The leads of real chalks of natural derivation are, unfortunately, made only in black, from amorphous carbon, white, from a mixture of clay and baryta white, and red, or sanguine, from a clay reddened naturally by iron oxide.
There are three grades of hardness: hard, medium, and soft, each differentiated by the admixture to the very soft raw material of corresponding quantities of binder instead of clay. If the chalks are thick or tough enough they can be used without a wooden or refill case; this is true particularly with the comparatively resistant Conte crayon and compressed charcoal.
You will remember from your school
days how fragile chalk is. Binder must be used very sparingly or the color will not come off. White blackboard chalk
consists of real chalk, or carbonate of lime, deposited as a sediment in the Cretaceous geological period by myriads of minute sea creatures. The chalk powder is obtained by grinding and then washing the chalk in water so that the particles which are still too large for use settle on the bottom. Prehistoric cave arists used chalk, presumably in its natural lump state. for black they used charcoal, a carbonized, not burnt, wood, the same used today for charcoal drawing.
Charcoal sticks are made from thin, peeled twigs of the lime tree, which are made to glow without free access of air until they are completely carbonized. Lime wood, or willow, is best, as it is very soft and free of resin and gives a very even color. Charcoal made of hardwood is more uneven. Resinous pines and firs produce vegetable tar when carbonized, which would smear in drawing.