Black Lead

Synonyms:

Black Lead, Plumbago, Graphite, Graphit

The material of which so-called black-lead pencils are made is essentially nothing but one of the three forms in which the element carbon occurs; diamond is another; lamp-black may be taken as a good representative of the third form. Graphite is the most appropriate name for the mineral commonly known as black lead, but which in reality contains no lead, and is in no way related to that metal. Graphite was found of good quality at Borrowdale in Cumberland, a locality which has been stated, probably erroneously, to be now exhausted; but excellent supplies of this mineral come from the Albert Mine, Siberia, Mount Alibert in the Ural, Ceylon, and many other places. Graphite, however, is never found in a state of perfect purity, always leaving, when burnt, some incombustible matter or ash.

In order to prepare graphite for use as a pigment, the purest pieces should be selected; these should be at first broken into small fragments in order that obviously contaminated bits may be rejected before the material is ground. Inferior qualities of graphite may be purified by being coarsely ground, and then heated with strong hydrochloric acid to remove iron and alumina. After washing, the thus-far purified material should be placed in a leaden or platinum vessel and gently warmed with a solution of hydrofluoric acid to remove silica. The graphite, after this treatment, is to be washed with abundance of water, and ground to a very fine powder, which is then ready for admixture with gum water or, after drying, with oil. The addition of a little china clay during the grinding of graphite in oil tends to produce a more manageable paint.

Graphite has a very dark-grey colour, and forms pure tints of grey with white pigments. It is equally well adapted for use in water-colour, oil, fresco, and tempera painting. It is absolutely permanent, and without action on other pigments.

Graphite is sometimes adulterated with charcoal or lamp-black. When a very thin layer of the suspected pigment - freed from oil or other vehicle - is spread on a piece of platinum foil and strongly heated over a lamp the lamp-black will burn away, leaving the true graphite unaltered. If a change of hue or tint occurs, this is a sign of the presence of the above-named adulterant.