GENESIS OF THE DIAMOND
Speculations as to the probable origin of the diamond have been
greatly forwarded by patient research, and particularly by improved
means of obtaining high temperatures, an advance we owe principally
to the researches of the late Professor Moissan.
Until recent years carbon was considered absolutely non-volatile
and infusible; but the enormous temperatures placed at the disposal
of experimentalists by the introduction of electricity show
that, instead of breaking rules, carbon obeys the same laws that
govern other bodies. It volatilises at the ordinary pressure at a
temperature of about 3600° C., and passes from the solid to the
gaseous state without liquefying. It has been found that other
bodies, such as arsenic, which volatilise without liquefying at
the ordinary pressure, will easily liquefy if pressure is added to
temperature. It naturally follows that if along with the requisite
temperature sufficient pressure is applied, liquefaction of carbon
will take place, when on cooling it will crystallise. But carbon at
high temperatures is a most energetic chemical agent, and if it can
get hold of oxygen from the atmosphere or any compound containing
it, it will oxidise and fly off in the form of carbonic acid. Heat
and pressure therefore are of no avail unless the carbon can be
kept inert.
It has long been known that iron, when melted, dissolves carbon,
and on cooling liberates it in the form of graphite. Moissan
discovered that several other metals, especially silver, have
similar properties; but iron is the best solvent for carbon. The
quantity of carbon entering into solution increases with the
temperature.
[Illustration: FIG. 20. MOISSAN’S ELECTRIC FURNACE.
To face p. 116.]
For the artificial manufacture of diamond the first necessity is
to select pure iron--free from sulphur, silicon, phosphorus,
etc.--and to pack it in a carbon crucible with pure charcoal from
sugar. The crucible is then put into the body of the electric
furnace and a powerful arc formed close above it between carbon
poles, utilising a current of 700 ampères at 40 volts pressure
(Fig. 20). The iron rapidly melts and saturates itself with carbon.
After a few minutes’ heating to a temperature above 4000° C.--a
temperature at which the iron melts like wax and volatilises in
clouds--the current is stopped and the dazzling fiery crucible is
plunged beneath the surface of cold water, where it is held till
it sinks below a red heat. As is well known, iron increases in
volume at the moment of passing from the liquid to the solid state.
The sudden cooling solidifies the outer layer of iron and holds
the inner molten mass in a tight grip. The expansion of the inner
liquid on solidifying produces an enormous pressure, and under the
stress of this pressure the dissolved carbon separates out in
transparent forms--minutely microscopic, it is true--all the same
veritable diamonds, with crystalline form and appearance, colour,
hardness, and action on light, the same as the natural gem.
Now commences the tedious part of the process. The metallic ingot
is attacked with hot nitro-hydrochloric acid until no more iron
is dissolved. The bulky residue consists chiefly of graphite,
together with translucent chestnut-coloured flakes of carbon,
black opaque carbon of a density of from 3·0 to 3·5 and hard as
diamonds--black diamonds or carbonado, in fact--and a small portion
of transparent, colourless diamonds showing crystalline structure.
Besides these there may be carbide of silicon and corundum, arising
from impurities in the materials employed.
The residue is first heated for some hours with strong sulphuric
acid at the boiling-point, with the cautious addition of powdered
nitre. It is then well washed and for two days allowed to soak in
strong hydrofluoric acid in cold, then in boiling acid. After this
treatment the soft graphite disappears, and most, if not all, the
silicon compounds have been destroyed. Hot sulphuric acid is again
applied to destroy the fluorides, and the residue, well washed, is
attacked with a mixture of the strongest nitric acid and powdered
potassium chlorate, kept warm--but not above 60° C., to avoid
explosions. This treatment must be repeated six or eight times,
when all the hard graphite will gradually be dissolved and little
else left but graphitic oxide, diamond, and the harder carbonado
and boart. The residue is fused for an hour in fluorhydrate or
fluoride of potassium, then boiled out in water and again heated
in sulphuric acid. The well-washed grains which resist this
energetic treatment are dried, carefully deposited on a slide, and
examined under the microscope. Along with numerous pieces of black
diamond are seen transparent, colourless pieces, some amorphous,
others with a crystalline appearance. Fig. 21 B shows one of these
crystalline fragments. Although many fragments of crystals occur,
it is remarkable I have never seen a complete crystal. All appear
shattered, as if on being liberated from the intense pressure
under which they were formed they burst asunder. I have singular
evidence of this phenomenon. A fine piece of artificial diamond,
carefully mounted by me on a microscopic slide, exploded during
the night and covered the slide with fragments. Moissan’s crystals
of artificial diamond sometimes broke a few weeks after their
preparation, and some of the diamonds which cracked weeks or even
months after their preparation showed fissures covered with minute
cubes. I have explained that this bursting paroxysm is not unknown
at the Kimberley mines. So far, all such artificial diamonds are
microscopic. The largest artificial diamond is less than one
millimetre across.
[Illustration: FIG. 21. ARTIFICIAL DIAMOND MADE BY THE AUTHOR FROM
MOLTEN IRON.]
[Illustration: FIG. 22. MOISSAN’S ARTIFICIAL DIAMONDS.
To face p. 120.]
These laboratory diamonds burn in the air before the blowpipe to
carbonic acid. In lustre, crystalline form, optical properties,
density, and hardness they are identical with the natural stone.
In several cases Moissan separated ten to fifteen microscopic
diamonds from a single ingot. The larger of these are about 0·75
mm. long, the octahedra being 0·2 mm.
The accompanying illustrations (Fig. 22) are copied from drawings
in Moissan’s book _Le Four Electrique_.
Along with carbon, molten iron dissolves other bodies which possess
tinctorial powers. We know of blue, green, pink, yellow, and orange
diamonds. One batch of iron might contain an impurity colouring the
stones blue, another lot would tend towards the formation of pink
stones, another of green, and so on. Cobalt, nickel, chromium, and
manganese, all metals present in the blue ground, would produce
these colours.
A NEW FORMATION OF DIAMOND
I have long speculated as to the possibility of obtaining
artificially such pressures and temperatures as would fulfil the
above conditions. In their researches on the gases from fired
gunpowder and cordite, Sir Frederick Abel and Sir Andrew Noble
obtained in closed steel cylinders pressures as great as 95 tons
to the square inch, and temperatures as high as 4000° C. According
to a paper recently communicated to the Royal Society, Sir Andrew
Noble, exploding cordite in closed vessels, has obtained a pressure
of 8000 atmospheres, or 50 tons per square inch, with a temperature
reaching in all probability 5400° Ab.
Here, then, we have conditions favourable for the liquefaction of
carbon, and were the time of explosion sufficient to allow the
reactions to take place, we should certainly expect to get the
liquid carbon to solidify in the crystalline state.[7]
By the kindness of Sir Andrew Noble I have been enabled to work
upon some of the residues obtained in closed vessels after
explosions, and I have submitted them to the same treatment that
the granulated iron had gone through. After weeks of patient toil
I removed the amorphous carbon, the graphite, the silica,[8] and
other constituents of the ash of cordite, and obtained a residue
among which, under the microscope, crystalline particles could be
distinguished. Some of these particles, from their crystalline
appearance and double refraction, were silicon carbide; others
were probably diamonds. The whole residue was dried and fused at a
good red heat in an excess of potassium bifluoride, to which was
added, during fusion, 5 per cent of nitre. (Previous experiments
had shown me that this mixture readily attacked and dissolved
silicon carbide; unfortunately it also attacks diamond to a slight
degree.) All the operations of washing and acid treatment were
performed in a large platinum crucible by decantation (except the
preliminary attack with nitric acid and potassium chlorate, when a
hard glass vessel was used); the final result was washed into a
shallow watch-glass and the selection made under the microscope.
The residue, after thorough washing and then heating in fuming
sulphuric acid, was washed, and the largest crystalline particles
picked out and mounted.
From the treatment the residual crystals had undergone, chemists
will agree with me that diamonds only could stand such an ordeal;
on submitting them to skilled crystallographic authorities my
opinion is confirmed. Speaking of the largest crystal, one eminent
authority calls it “a diamond showing octahedral planes with
dark boundaries due to high refracting index.” After careful
examination, another authority writes of the same crystal diamond,
“I think one may safely say that the position and angles of its
faces, and of its cleavages, the absence of birefringence, and the
high refractive index are all compatible with the properties of the
diamond crystallising in the form of an octahedron. Others of the
remaining crystals, which show a similar high refractive index,
appeared to me to present the same features.”
It would have been more conclusive had I been able to get further
evidence as to the density and hardness of the crystals; but from
what I have already said I think there is no doubt that in these
closed vessel explosions we have another method of producing the
diamond artificially.