When the evolution of fumes ceases, the mass is allowed to cool, and is carefully washed with a large volume of water, the residue being then dried and heated to redness. During this calcination the graphite undergoes a peculiar change, increasing considerably in bulk and forming an exceedingly soft powder which, after another washing, consists almost entirely of chemically pure carbon. Graphite purified in this way can be used for any purpose for which this material is employed, and may be made into the finest lead pencils. However, as already mentioned, this process is usually too expensive for general application. The use of graphite for writing is more ancient than is usually supposed, having been tentatively employed between 1540 and 1560. It was during this period that the graphite mines in Cumberland were discovered; and the extremely pure graphite found there soon began to be used as a writing material. Up to the close of the eighteenth century, lead pencils were made by selecting pure lumps of graphite and sawing them into thin rods, which were then encased in wooden sticks. Apart from their high price, these pencils exhibited various defects, one of the chief being that a stick of such pencil was seldom of uniform hardness throughout its length, most of them being so soft in parts as to make a deep black, smeary mark, whilst other parts would hardly give any mark at all. The defects inherent in native graphite are completely removed by the method now generally employed in making lead pencils; and on this account the old process of sawing the lumps has been abandoned. Graphite with a fine earthy texture alone is suitable for lead pencils, scaly varieties being useless for this purpose, unless specially prepared, since they will not give a solid black streak. By means of the Brodie process, however, even the most highly crystalline kinds can be rendered suitable for this purpose. Siberian graphite is distinguished by extremely high covering power, and is specially preferred for the manufacture of pencils. Excellent varieties for this purpose are also found in many parts of Europe; and indeed, a large proportion of all the lead pencils used throughout the world are made from Bohemian, Styrian and Bavarian graphite. At present, all pencils are made from ground graphite, the extremely finely ground and levigated material being kneaded into a paste with clay. This operation fulfils a twofold purpose, the plasticity of the clay increasing the cohesion of the individual particles of graphite, whilst the amount of clay used determines the hardness of the pencil. The larger the proportion of clay, the harder the pencil when baked, and therefore the paler the mark the pencil will make on paper. In the pencil factories, the clay is incorporated in special machines; and the operation requires extreme care, since only a perfectly uniform mixture will give a composition of regular character in all cases.