BLACK EARTH COLOURS
Only two minerals are known that can be used as black earth colours,
namely black chalk or shale black, and blacklead or graphite. Whereas
the former of these is of merely subordinate importance, most of the
black chalks being prepared artificially, graphite is all the more so
because it is employed, not only as the sole material for lead pencils,
but also for making graphite crucibles, as blacklead stove polish, as
a lubricant, etc. One of its numerous applications is in connection
with the electro deposition of metals, its high electrical conductivity
causing it to be used for coating the interior of the moulds in which
this deposition is effected.
GRAPHITE
Graphite, also known as plumbago or blacklead, consists of carbon. It
is usually spoken of as pure carbon, but from a very large number of
carefully conducted analyses, it would appear that native graphite is
never quite pure, even the finest grades of the mineral containing
96·8% of carbon at the most. The accompanying substances--which in
some cases form nearly 50% of the whole--are of divergent composition
and consist of iron, silica, lime, magnesia and alkalis. Even the
combustible constituent of graphite is not pure carbon, but always
contains a certain--though small--proportion of volatile substances.
These slight traces of volatile matter are of considerable importance
in connection with the hypothesis on the origin of the mineral.
Contrary to the old idea, it is now almost universally considered that,
instead of being of volcanic origin, graphite consists of the remains
of long-dead organisms, and in this respect is closely related to
coal. This hypothesis, however, fails to explain one point, namely the
crystalline nature of graphite; for even anthracites, which form the
oldest coals known to have had their origin in the decomposition of
organic substances, do not reveal the faintest traces of crystalline
structure. The upholders of the theory that graphite was formed by
the action of plutonic forces adduce, in support, the fact that
graphite can actually be produced, in certain chemical processes, at
high temperature. Molten cast-iron in cooling causes the separation
of carbon in the form of graphite; and the same substance is also
formed, in large quantities, in gas retorts, through the decomposition
of certain carbonaceous compounds when brought into contact with
the glowing walls of the retorts. Recent investigations, however,
have shown that the temperature necessary for the transformation of
non-crystalline carbon into crystalline graphite is by no means so
high as was formerly supposed; and it is now known that the change
takes place at as low as red heat. Possibly the two theories could be
reconciled by the assumption of a very old coal--such as is found, for
instance, as anthracite in many parts of the world--being so strongly
heated, by plutonic action, as to change into graphite.
Native graphite crystallises in the form of hexagons, mostly tabular;
but really well-developed crystals are of extremely rare occurrence,
and by far the greatest quantities of this mineral are found in the
condition of dense lumps, in which only the crystalline structure, and
not any decided crystals, can be discerned. The hardness of the mineral
fluctuates within fairly wide limits, ranging from 0·5 to 1·0. The
sp. gr. averages 1·8018–1·844, but, in the case of impure lumps may
increase to 1·9–2·2.
The following analyses will give some idea of the considerable
divergence existing between graphites from different deposits:--
SIBERIAN GRAPHITE
1 2
Carbon 94·28 40·55
Ash 5·72 56·56
Water -- 2·80
PORTUGUESE GRAPHITE
Carbon 42·69
Water (chemically combined) 3·96
Ash 53·35
BOHEMIAN GRAPHITE
1 2
Carbon 61·01 69·04
Alumina 7·80 6·86
Silica 17·34 14·18
Magnesia 1·03 0·53
Lime 2·56 0·80
Ferric oxide 5·54 4·00
Potash 0·87 0·91
Water and volatiles 3·24 2·89
Sulphur 0·51 0·62
GRAPHITES FROM UPPER STYRIA
1 2 3
Carbon 85·00 87·16 82·21
Ash 14·89 12·66 17·92
4 5 6
Carbon 82·40 81·10 55·50
Silica 12·38 11·61 21·00
Alumina 3·90 5·60 14·56
Ferric oxide 0·53
Manganese protoperoxide 0·62 2·00 4·84
Lime 0·02 2·00 4·84
Alkalis Trace Trace 0·62
Sulphur -- -- 0·30
Loss on incineration -- -- 2·43
Of these Styrian specimens, Nos. 1–4 are crude kinds, of sp. gr.
2·1443; No. 5 was levigated in the laboratory, and No. 6 was levigated
from an inferior quality at the mine.
According to the character of the crystalline structure, the colour of
graphite varies, but is mostly deep black. Very pure specimens, such
as the beautiful graphite blocks (from the renowned Alibert graphite
mines in Siberia) which, as a rule, are only to be seen in exhibitions
and mineralogical collections, have the appearance of unpolished steel
or white pig iron (spiegeleisen). The most important property of native
graphite is its low hardness and cohesion, in consequence of which it
leaves a streak when drawn over the surface of paper.
Graphite seems to be of frequent occurrence all over the world, though
only few deposits are known which yield a product that is suitable for
all the purposes to which graphite is applied.
In European countries, Austria is particularly rich in graphite; and
very large deposits of this mineral are found in Bohemia. Considerable
deposits also occur in Bavaria, where they have long been worked.
English graphite is celebrated for its excellent quality. All these
European deposits, however, are surpassed, both in extent and in the
quality of their products, by those discovered in Siberia, the largest
being that producing the aforesaid Alibert graphite and situated, near
the Chinese frontier, in eastern Siberia. At one time, America imported
all her blacklead pencils from Europe, having, at that period, no known
graphite deposits furnishing a suitable product. At present, however,
deposits of this kind have been found in California, and there can be
little doubt but that many others of this valuable mineral remain to be
discovered in that enormous continent, the geological investigation of
which is still far from being complete.
The graphite of some deposits is so highly contaminated by extraneous
minerals that it cannot be utilised, since the cost of purification
would exceed the value of the product. On the other hand, the purer
kinds, when suitably refined, yield a graphite that is fully adapted to
all requirements.
The refining process may be either chemical or mechanical, the choice
of methods depending entirely on the character of the associated
minerals. If these mainly consist of coarse, stony fragments,
preference should be given to mechanical treatment; but if they are of
such a character that they cannot be eliminated in this way, chemical
methods must be employed. Sometimes the two systems are combined, by
first subjecting the graphite to a rough mechanical purification, and
then completing the operation with chemical reagents.
The mechanical treatment consists in first removing as many of the
impurities as possible by hand-picking, and grinding the remainder in
edge-runner mills, along with water. The turbid liquid, containing the
powdered graphite and extraneous minerals in suspension, is led through
long launders, the sides of which are notched at intervals to allow
the water to overflow into large pits. The graphite settling in the
first of these pits contains numerous particles of the heavy associated
minerals; but that remaining suspended in the water and carried on to
the further pits constitutes the bulk. The water is left to clarify
completely in the pits, and is then drawn off, the pasty residue being
shaped into prisms, which are compressed under heavy pressure, to
increase their density, when partially dry.
Although levigation will remove most of the accompanying extraneous
minerals, it cannot eliminate the ash constituents of the graphite.
Experiments made in this direction have demonstrated that the ash
content of the levigated graphite is exactly the same as that of the
crude material. Whilst these ash constituents do not affect the quality
of graphite for certain of its uses, they nevertheless impair its
beautiful black colour to a considerable degree. The chemical treatment
necessary to eliminate these constituents is attended with many
difficulties, the chief of which resides in the fact that the ferric
oxide present is in a form that is not readily accessible to the action
of chemicals. For this reason, attempts to purify graphite with crude
hydrochloric acid are hardly likely to prove successful, since both
the ferric oxide and the accompanying silicates obstinately resist the
action of this acid.
In order to obtain graphite of a high state of purity, the attempt must
be made to bring this ferric oxide and the silicates into a soluble
condition. This can be accomplished in various ways, and the choice
of the method will depend on the purpose for which the graphite
is intended. For example, the operations may either be confined to
purification, or else include the attainment of a maximum condition
of subdivision. When foliaceous graphite has to be treated--and this
kind of graphite cannot, in its original condition, be used for making
lead pencils--it is preferable to employ a method which will produce
both the above results. The purification may consist in crushing the
graphite to powder, and fusing this with a mixture of sulphur and
carbonate of soda, whereby the silicates present are converted into
soluble compounds, and the ferric oxide into ferric sulphide. On
extracting the melt with water, a portion of the contained salts pass
into solution and is carried off. The residue is then treated with
dilute hydrochloric acid, which dissolves out the ferric sulphide, with
liberation of sulphuretted hydrogen, and leaves the graphite in a very
pure condition after washing.
In order to render foliaceous graphite suitable for lead pencils, a
different method is pursued, but should only be employed in special
circumstances, on account of the expense entailed.
According to the process recommended by Brodie, the graphite, ground to
coarse powder, is mixed with about one-fourteenth of its own weight of
chlorate of potash, this mixture being heated, with two parts by weight
of sulphuric to each part of graphite, in a water bath so long as fumes
of hypochlorous acid continue to be disengaged. The heating must be
performed in stoneware or porcelain vessels, those made of any other
materials being strongly corroded by the chlorine compounds formed.
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.
The intimately mixed material is formed into thin rods, which are
dried and then baked, the heat driving out the water in the clay and
transforming it into a solid mass.
An addition to this main application of graphite, the mineral is also
used for making crucibles, chiefly for melting the noble metals.
Crucibles of this kind are largely manufactured near Passau, Bavaria,
and similar crucibles are made in England from Ceylon graphite.
Another important use for graphite is as a coating for iron articles to
protect them from rust. For this purpose, however, only the inferior
kinds are employed; and these can also be made up into excellent
cements capable, in particular, of offering considerable resistance to
the action of heat and chemicals.
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.