chloride by means of potassium. The chlorine leaves the aluminium to
combine with the potassium, and thus the former is set free. Wöhler
effected some improvements in Œrsted’s process, and he first obtained
the metal in malleable globules. It is, however, to Deville that we are
indebted for the invention, in 1854, of a process which admitted of
application on a manufacturing scale. He obtains chloride of aluminium
by mixing alumina (the oxide of the metal) with powdered charcoal made
into a paste with oil, and heating the mixture in a tubular earthenware
retort, like those sometimes used in the manufacture of coal-gas, while
a current of dry chlorine is made to pass through the vessel. The
charcoal combines with the oxygen, forming carbonic oxide, a permanent
invisible gas; and the aluminium unites with the chlorine, giving rise
to aluminium chloride, which, being volatile, sublimes into a chamber
lined with glazed tiles, in which it condenses as a yellow translucent
mass. The metal is reduced from the chloride in the following manner: A
tube of hard glass, about an inch and a half in diameter, is placed over
a furnace, or chaffing-dish, as shown in Fig. 336, where D C is the
tube, and G G an iron pan for containing the red-hot charcoal. Into the
part of this tube marked E, about half a pound of dry aluminium chloride
has previously been introduced, and is kept in its place by plugs of
asbestos. A current of dry hydrogen gas, perfectly free from air, is
passed through the tube; the gas being generated in the vessel, A, and
in B passed over some substance which removes from it all moisture. The
aluminium chloride is then gently heated by placing red-hot charcoal
beneath it, so that any hydrochloric acid it may contain may be
expelled. A long narrow porcelain tray, or “boat,” containing pieces of
sodium, F, is then introduced into the tube; and, the current of
hydrogen being still maintained, heat is applied to the part of the tube
containing the sodium, and the aluminium chloride is made to distil over
by a regulated heat. As it passes over the sodium, it is reduced with a
vivid glow. The aluminium is set free, and collects in the tray with the
double chloride of sodium and aluminium which is produced by the
reaction. The tray is removed and more strongly heated in a porcelain
tube through which a current of hydrogen is passing, and the metal is
thus obtained in globules.
[Illustration:
FIG. 336.
]
Messrs. Bell, of Newcastle, undertook the manufacture of aluminium by a
system founded on this process. The first step is the preparation of
pure alumina, which may be obtained by igniting ammonia alum, or by
precipitating from a solution of alum free from iron, or from sodium
aluminate made from the mineral called _bauxite_. The precipitate of
hydrated alumina, mixed with charcoal and common salt, is made into
balls and dried. These balls, which are about as large as an orange, are
placed in upright earthenware retorts, which are heated to redness,
while a current of dry chlorine is passed through them. The volatile
double chloride of aluminium and sodium distils over, and is condensed
in chambers lined with earthenware. This substance is mixed with
powdered fluor-spar, or with _cryolite_ (itself a compound of
aluminium), which serves as a flux; and small pieces of sodium are
interspersed throughout the mixture. The proportions are ten parts of
the double chloride, five of fluor-spar, and two of sodium. This mixture
is thrown upon the hearth of a reverberatory furnace, and the doors are
shut to exclude air. A very intense action occurs: the chlorine,
quitting the aluminium, seizes on the sodium, and their combination is
attended by an enormous increase of temperature. The fused aluminium is
run off from the furnace together with the slags which are produced by
the operation. In this way, with a furnace having a hearth 16 ft.
square, about 16 lbs. of aluminium can be obtained in one operation.
Rose, the eminent German metallurgist, prefers to obtain aluminium from
cryolite, which is a compound of sodium, aluminium, and fluorine, found
in large quantities in Greenland. It is powdered and mixed with common
salt, and with the mixture a certain quantity of sodium cut into small
pieces is uniformly mingled. The whole is thrown into a heated crucible,
previously lined with a fused mixture of cryolite and salt, and more of
the same mixture is poured upon the contents of the crucible, which is
then covered and exposed to a red heat for two hours. The aluminium
generally collects into buttons, which may be easily melted together by
heating them in a crucible with common salt.
It will be obvious, from the preceding account of the processes of
extracting aluminium, that the cost of the metal must depend upon that
of sodium; and the same remark will apply to the case of magnesium. It
is interesting to observe how the price of the alkaline metals has
decreased as improved processes have been devised, and as the scale of
production has increased with the commercial demand for the article.
Prepared by Gay Lussac and Thenard’s process, these metals were produced
in but small quantities, and were sold at £5 per oz. When the mode of
reducing them by charcoal came into operation, the price fell to 30_s._
per oz.; and the researches of Deville so far improved the processes,
that in 1854 sodium could be procured for 5_s._ per oz. Mr. Gerhard, of
Battersea, subsequently manufactured sodium, so that it can now be
retailed at less than 1_s._ per oz. The price of aluminium before
Deville’s investigations was about 24_s._ per oz., but now the metal can
be purchased at about one-eighth of that cost. [1875.]
Aluminium is a white malleable metal, in colour and hardness not unlike
zinc. Its colour is not so white as that of silver, as it has a marked
bluish tint. It can be rolled into very thin sheets, and by rolling it
becomes harder and more elastic. It can also be drawn into fine wire. It
is remarkably sonorous, and a suspended bar gives out a clear musical
note when struck. Perhaps no property of aluminium more strikes a
person, who examines the metal for the first time, than its lightness.
It is, in fact, only two and a half times as heavy as water, while zinc
is seven times, silver ten and a half times, and gold more than nineteen
times as heavy as water. It retains its lustre in dry or in moist air
for any length of time, and at all ordinary temperatures. It is not
acted upon by nitric or sulphuric acids, but is attacked by hydrochloric
acids and by alkaline solutions with great energy. It has great rigidity
and tenacity, and can be turned, chased, and filed with the greatest
ease, and without clogging the tools. In the Paris Exhibition,[15] M.
Christofle showed spoons and forks and a cup made from it; and it may be
mentioned, as showing the hardness and strength of the metal, that the
cup could be allowed to fall on a stone pavement without being indented.
The metal gives a good impression by casting; and by striking under a
die, some admirable medals have been produced in it. Aluminium has
hitherto been chiefly used for ornamental articles, and for purposes
where lightness and rigidity are desirable, such as in the tubes of
telescopes, opera glasses, beams of balances, &c. Its unalterability and
admirable working qualities have also caused it to be used for cheap
trinkets and ornaments—such as watch-cases, bracelets, combs, seals,
penholders, candlesticks, &c. It is, however, incapable of receiving the
lustrous polish of silver, as it has a decidedly blue tint, so that it
will probably never replace silver for ornamental plate; but it would be
a good material for egg and mustard-spoons, as it is quite unaffected by
the sulphur compounds which so readily tarnish silver. It has been
suggested that if aluminium could be procured cheaply enough, “its
hardness, lightness, and incapability of rusting would render it
admirably adapted for the helmets and cuirasses of the cavalry; it would
make splendid field-guns, as strong as the present ones, and not
one-third of their weight; and, in sheets, it might serve as an
incorrodible roofing, far lighter and more durable than zinc. It would
admirably replace copper, if not silver, for the purpose of coinage. A
crown-piece in aluminium would hardly weigh more than a shilling in
silver, or a piece the size of a penny about as much as a copper
farthing. The same qualities of lightness, hardness, and incorrodibility
also excellently fit it for the beams of delicate balances, and for the
small weights used by the analytical chemist. It would make admirable
utensils for the more delicate operations of cooking—replacing the
copper ones, which render pickles and soups so poisonous. It is
extremely sonorous, and would make capital bells.”
Footnote 15:
Of 1867.
Some difficulty in working the metal has occurred from the want of any
suitable solder. This difficulty has been overcome by electrolytically
coating the metal with copper at the place where it has to be united
with others, and then soldering the copper in the ordinary manner.
Aluminium readily forms alloys with copper, silver, and iron. The alloys
with copper vary in colour from white to golden yellow, according to the
proportion of the metals. Some of these alloys are very hard and possess
excellent working qualities. The alloy of copper with 10 per cent. of
aluminium, which is called _aluminium bronze_, has been manufactured by
Messrs. Bell in considerable quantities. It is made by melting a
quantity of very pure copper in a plumbago crucible, and when the
crucible has been removed from the furnace, the solid aluminium is
dropped in. An extraordinary increase of temperature then occurs: the
whole mass becomes white hot, and unless the crucible be made of a
highly refractory material, it is fused by the heat developed in the
combination of the two metals, although it may have stood the heat
necessary for the fusion of copper.
The qualities of aluminium bronze have been investigated by Lieut.-Col.
Strange, who finds that the alloy possesses a very high degree of
tensile strength, and also great power of resisting compression, its
rigidity, or power of resisting cross strains, is also very great; in
other words, a bar of the alloy, fixed at one end and acted on at the
other by a transverse force tending to bend it, offers great
resistance,—namely, three times as much as gun-metal. An advantage
attending the use of the alloy for many delicate purposes is found in
its small expansibility by heat; it is therefore well adapted for all
finely-graduated instruments. It is very malleable, has excellent
sounding properties, and resists the action of the atmosphere. It works
admirably with cutting tools, turns well in the lathe, and does not clog
the files or other tools. It is readily made into tubes, or wires, or
other desired forms. The elasticity it possesses is very remarkable; for
wires made of it are found to answer better for Foucault’s pendulum
experiment than even those of steel. These admirable qualities would
seem to recommend the alloy for many applications in which it might be
expected to excel other metals. It appears, however, that the demand for
it has not met the expectations of the manufacturers, and the production
has been somewhat diminished of late, although it is used to some extent
for chains, pencil-cases, toothpicks, and other trinkets. When more than
10 per cent. of aluminium is added to the copper, the alloy produced is
weaker; and if the proportion is increased beyond a certain extent, the
bronze becomes so brittle that it may be pulverized in a mortar.
The metal _magnesium_ was first prepared, in 1830, by the French chemist
Bussy, by a process similar to that by which Deville obtained aluminium.
Bussy heated anhydrous magnesium chloride with potassium in a porcelain
crucible; and when the vessel had cooled, and the soluble residue had
been dissolved out by water, the metal was found as a grey powder, which
could be melted into globules. The recognition of the metal as the base
of magnesia is, however, due to Davy. About a quarter of a century after
Bussy’s discovery Deville having shown that sodium could be substituted
for potassium in such reductions, the metal became more cheaply
producible, and soon afterwards Bunsen and Roscoe pointed out its value
as a source of light. Mr. Sonstadt devoted himself to the elaboration of
a method of working Deville’s process on the large scale, and he
succeeded in establishing a company in Manchester for the manufacture.
The process as carried on at the company’s works in Salford is thus
described in the “Mechanics’ Magazine,” 30th August, 1867:
“Lumps of rock magnesia (magnesium carbonate) are placed in large jars,
into which hydrochloric acid in aqueous solution is poured. Chemical
action at once ensues: the chlorine and the magnesium embrace, and the
oxygen and carbon pass off in the form of carbonic acid. The result is
magnesium in combination with chlorine, and the problem now is how to
dissolve this new alliance—to get rid of the chlorine and so obtain the
magnesium. First, the water must be evaporated, which would be easy
enough if not attended with a peculiar danger. To get the magnesium
chloride perfectly dry it is necessary to bring it to a red heat; but
this would result in the metal dropping its novel acquaintance with
chlorine and resuming its ancient union with oxygen. To avert this
re-combination, the magnesium chloride whilst yet in solution is mixed
with sodium chloride (_i.e._, common salt), and thus fortified, the
aggressions of oxygen whilst drying are kept off. The mixture is exposed
in broad open pans over stoves, and when sufficiently dry, the double
salt is scraped together and placed in an iron crucible, in which it is
heated until melted, whereby the last traces of water are driven off. It
is then stowed away until required in air-tight vessels, to prevent
deliquescence. Here comes in that curious metal, sodium, also discovered
by Davy. Five parts of the mixed magnesium and sodium chlorides, mingled
with one part of sodium, are placed in a strong iron crucible with a
closely-fitting lid, which is then screwed down. The crucible is heated
to redness in a furnace, and its contents being fused, the sodium takes
the chlorine from the magnesium. When the crucible has been lifted from
the fire and allowed to cool, the lid is removed and a solid mass is
discovered, which, when tumbled out and broken up, reveals magnesium in
nuggets of various sizes and shapes, bright as silver.”
The crude metal also presents itself in the crucible as small grains,
and even as a black powder. The whole is carefully separated from the
refuse; it is purified by distillation in a current of hydrogen gas; and
it is afterwards melted and cast into ingots. Magnesium is a very light
metal, its specific gravity being only 1·743; that is, it is only one
and three-quarter times heavier than water. When heated in the air it
takes fire, and is rapidly converted into the oxide, magnesia. In the
form of wire or of narrow ribbon, it burns easily in the air, producing
a light of dazzling brilliancy, which among artificial modes of
illumination is rivalled only by the electric light. This is the chief
use at present made of the metal. Lamps have been contrived for burning
the wire in such a manner as to obtain a steady light, the wire being
pushed forward at a regulated rate by clockwork. The magnesium light is
rich in the rays which act upon sensitive photographic plates, and it
has been successfully employed in obtaining photographs of dark
interiors, such as vaults or caverns, and for the exploration of mines
and other dark places. The brilliancy of the firework displays which can
be produced by magnesium far surpasses that obtainable by any other
material used by the pyrotechnist. In such exhibitions balloons are sent
up having burning magnesium attached to them; and the metal in the state
of filings is also mixed with other materials. But magnesium is still a
very costly metal, and while the firework-makers find it too expensive
for common use, they complain that its brilliancy in occasional displays
dulls by contrast the effect of the ordinary fireworks, with which the
spectators are no longer satisfied.
Magnesium wire is not produced by drawing, as the metal is not ductile.
The wire is formed by a method identical with that used in the
fabrication of the leaden rope for making bullets (p. 330); that is to
say, the metal is forced in a heated and softened state through a small
opening in an iron cylinder. The intensity of the magnesium light has
been measured by Bunsen and Roscoe. They say that 72 grains of
magnesium, when properly burnt, evolve as much light as 74 stearine
candles burning for ten hours, and consuming 20 lbs. of stearine. Lamps
in which magnesium may be steadily burnt are made by Mr. F. W. Hart, of
London. In the more elaborate forms of these lamps, there are springs
and wheels for pushing forward the magnesium ribbon, or a strand of
magnesium wire, into the flame of a spirit-lamp; while at the same time
the magnesium wire is made to revolve on its axis, in order to overcome
its tendency to bend down, which would be a great disadvantage when the
light is used for optical apparatus. But for ordinary purposes a much
simpler arrangement suffices: the magnesium ribbon or wire is coiled on
a drum, from which it is drawn off by passing between two little
rollers, which are turned by hand. The wire or ribbon is drawn off the
drum by the rollers, and pushed forward through a guiding tube, which
brings it into the apex of the flame of a spirit-lamp. In this simpler
form of lamp the rate is, of course, directly dependent on the person
who turns the winch of the feeding-rollers; but in the automatic lamp
there are appliances for adjusting the rate; the suitable speed must be
first found by trial, and then the apparatus is to be regulated
accordingly. By means of these lamps photographs can be taken as quickly
as with sunlight, on account of the abundance of chemically-active rays
given out by the burning magnesium. It has been found that an equivalent
of magnesium, in combining with oxygen, liberates a larger amount of
heat than the equivalent quantity of any other metal, not excluding even
potassium. Magnesium forms alloys with several other metals, such as
lead, tin, mercury, gold, silver, platinum. All these alloys are
brittle, and have a granular or crystalline fracture. They are too
readily acted on by air and moisture to be of any service in the arts.
The alloy of 85 parts of tin with 15 of magnesium is hard and brittle;
its colour is lavender, although both constituents are white, or nearly
so; and it decomposes water at ordinary temperatures. Both metallic
magnesium and aluminium furnish useful re-agents to the scientific
chemist. The latter metal, when fused, dissolves boron, silicon, and
titanium, and on cooling deposits these elements in the crystalline
form, this being the only known process for artificially preparing them
in the crystalline state.
Since the above paragraphs were written, the price of sodium has been
further greatly reduced, and it can now (1890) be purchased in bulk at
about 4_s._ per lb. This cheapness has brought the substance into use
for the reduction of other metals and one consequence has been a great
fall in the price of aluminium. At Salindres, in France, the process of
obtaining this metal that has been described on page 587, has been in
use for many years, during which considerable quantities of aluminium
have been produced, the output for 1882 being stated as 5,280 lbs.
Aluminium has lately been prepared by a company at Wallsend-on-Tyne from
_cryolite_, a mineral which is found only in Greenland, but occurs there
in great abundance. Cryolite is a double fluoride of aluminium and
sodium, and the processes for its reduction consist in fusing it with
common salt in a reverberatory furnace, drawing off the mixture into an
iron vessel, and stirring into the fused mass a certain quantity of
sodium. This produces a violent action, on the cessation of which the
slag is poured off, and the metallic aluminium is found as a “button” at
the bottom of the converter. For obtaining a purer metal, the fusion is
made in crucibles, and the sodium is added in two operations without
removing the crucible. The yield of aluminium is about 8 per cent. of
the weight of cryolite, and three parts of sodium are required to
furnish one part of aluminium. Another large manufactory of aluminium is
in operation at Oldbury, near Birmingham. There is a special difficulty
in the metallurgy of aluminium, arising from the fact of the qualities
of the metal being much deteriorated by the presence of a very small
amount of foreign matters such as iron, silicon, &c., at the same time
that no process has been found for purifying the product from these
substances. If the aluminium is to be pure it must be so prepared at the
first. Electrolysis has been proposed as a means of reducing the
compounds, and obtaining the metal free from admixtures. Experiments
seem to show that the dynamo-electric machine may be applied to this
purpose, as well as to the reduction of sodium compounds, when certain
practical difficulties arising from the chemical energies of the
liberated substances have been overcome. What is called the “electric”
furnace has been successfully used in the production of aluminium
bronze. It is a rectangular iron box, 5 feet long, 1 foot deep, and 15
inches wide, with electrodes formed of rods of carbon 30 inches long and
3 inches in diameter. It is charged with a mixture of 25 parts of
corundum (native crystallized oxide of aluminium), 12 parts of carbon,
and 50 parts of granulated copper. This is covered at the top by lumps
of charcoal, and a lid is fastened over the whole. The current from a
powerful dynamo is sent through the carbons, and in about ten minutes
the copper is melted, when the electrodes, at first only a few inches
apart, are moved to an increased distance, and the strength of current
increased. The corundum is reduced, the aluminium alloying itself with
copper, and the oxygen combining with the carbon to form carbon
monoxide, which is driven off. The resulting alloy is cast into ingots,
its percentage of aluminium ascertained, and then it is melted with
enough copper to produce aluminium bronze (page 719). The price of
aluminium, which was as already stated about 3_s._ per ounce in 1875,
has been so much reduced that the metal may now (1890) be purchased for
11_s._ or 12_s._ per lb. We may therefore expect to see wider
applications of its excellent qualities. Though the price per lb. is
still much higher than that of copper-–22 or more times as much—the
metal is so much lighter that a lb. of aluminium occupies nearly 3⅓
times the space of a lb. of copper, so that, taking bulk for bulk,
aluminium is only about seven times as dear as copper. [1890.]