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.
CHAPTER XI
THE COMMERCIAL NOMENCLATURE OF THE EARTH COLOURS
Mention has already been made of the great confusion prevailing in the nomenclature of pigments, and that many of these are put on the market under a variety of names taken from different languages.
Although the number of the earth colours is far smaller than that of the artificial colouring matters, the nomenclature is in a no less confused condition.
Most frequently, earth colours are named after the localities where they are either discovered or prepared, in combination with the word indicating the colour of the product—for example: Cologne white, Vienna white—or the term “earth” (Verona earth, Veronese green, etc.). Whilst these names give, to some extent, an indication of the nature of the pigment, others have no reference to it at all; such as colcothar, bole, umber, etc. Finally, a number of other names in use are calculated to produce the impression that the earth colours in question are of an entirely different nature to their real one. As an example, we may cite the name “French chalk,” which is not a chalk at all, but consists of the mineral talc. Black chalk, again, is not chalk (calcium carbonate), but a black shale; and graphite is often termed blacklead, although it contains no lead at all, and the name is merely a survival from the time when pencils of metallic lead were used for drawing.
In order to bring some kind of order into the various names which are applied to the earth colours, a list of those in current use is appended. Many of these names, it may be stated, have been selected in a purely arbitrary manner, some manufacturers, for instance, selling ordinary chalk under a variety of foreign names, for the purpose of thereby obtaining higher prices. These borrowed names would seem to be superfluous, to say the least. Pure and properly levigated chalk is the same article everywhere, whether prepared from English, French or German limestone; and in all cases the simple name, “chalk,” with an explanatory “single,” “double,” or “triple” levigated, should be quite sufficient.
In the case of earth colours that are really obtained of special quality in certain localities, such as terra di Siena, green earth from Verona, or the like, the corresponding name might be retained, even if the pigment did not originate from the locality in question, as a generic term for a pigment possessing certain properties and of a certain composition.
In the following classification, the names of the earth colours are given in accordance with their colour and chemical composition.