To complete the tale of the applications of graphite, its employment as a lubricating agent for machinery, especially for reducing friction in machines made of wood, may be mentioned. Latterly also, the finest levigated graphite has come into use, in admixture with solid fats or mineral oils, for lubricating large engines, for which purpose it is excellently adapted.
Black Chalk
Black chalk, slate black, Spanish chalk, crayon, etc., is not a chalk at all, in the mineralogical sense, but consists of clay shale of varying colour. Some kinds of this shale are pure black, almost velvet black, and these are considered the best. Others have a more greyish or bluish tinge and are of low value as pigments.
The purer the black, the finer the grain of the material, and therefore the greater its value to the colour-maker. The variety obtained from Spain is generally admitted to be the best, and for this reason the name of Spanish chalk has been applied to all similar minerals.
In all cases the black colour of Spanish chalk is due to carbon; but the particular modification of carbon present has not yet been accurately identified. According to some, it is chiefly graphite, whereas others ascribe the colour to amorphous carbon. Apparently, the material found in different deposits contains either one or the other of these modifications of carbon.
Deposits of black chalk are fairly plentiful, but in many of them the material is so contaminated with extraneous minerals that a somewhat troublesome method of preparation is needed to fit them for the purpose of the draughtsman. With this object, the native product must be ground extremely fine, and the powder levigated; and owing to the expense of these processes, they are now seldom used, it being possible to obtain a good black chalk far more cheaply than by levigating the natural material.
This artificial black chalk is prepared by mixing ordinary white chalk, or white clay, with a black colouring matter, shaping the mass into prisms, and sawing these into suitable pieces when dry. The white pigment may either be mixed with some very deep black substance, such as lampblack, or stained with an organic dyestuff, which is, in reality, not black, but either very dark blue or green.
The usual colouring matter used with white chalk is lampblack, mixed to a uniform paste with thin glue, a suitable amount of clay or chalk being incorporated with the mass. The production of a perfectly homogeneous mixture entails subjecting the paste to a somewhat protracted mechanical treatment. When the mass has become perfectly uniform throughout, it is shaped into prisms, which are exposed to the air to dry and are then cut up with a saw. Instead of prisms, the mass can be shaped into thin sticks, which dry more quickly.
A very handsome black chalk can be made, with comparatively little trouble, by treating chalk with a suitable quantity of logwood decoction previously mixed with sufficient green vitriol solution to render the liquid a deep black. This liquid is added to the dry chalk, intimately mixed therewith, and the pasty mass shaped into sticks. The colouring agent may be replaced by a solution of logwood extract blackened by the addition of a small quantity of chromate of potash; or black dyestuffs may be used.