Aluminum is a shining, white, sonorous metal, having a shade between
silver and platinum. It is a very light metal, being lighter than glass
and only about one-fourth as heavy as silver of the same bulk. It is
very malleable and ductile, and is remarkable for its resistance to
oxidation, being unaffected by moist or dry air, or by hot or cold
water. Sulphureted hydrogen gas, which so readily tarnishes silver,
forming a black film on the surface, has no action on this metal.
Next to silica, the oxide of aluminum (alumina) forms, in combination,
the most abundant constituent of the crust of the earth (hydrated
silicate of alumina, clay).
Common alum is sulphate of alumina combined with another sulphate, as
potash, soda, etc. It is much used as a mordant in dyeing and calico
printing, also in tanning.
Aluminum is of great value in mechanical dentistry, as, in addition
to its lightness and strength, it is not affected by the presence of
sulphur in the food--as by eggs, for instance.
Dr. Fowler, of Yarmouthport, Mass., obtained patents for its combination
with vulcanite as applied to dentistry and other uses. It resists
sulphur in the process of vulcanization in a manner which renders it an
efficient and economical substitute for platinum or gold.
Aluminum is derived from the oxide alumina, which is the principal
constituent of common clay. Lavoissier, a celebrated French chemist,
first suggested the existence of the metallic bases of the earths and
alkalies, which fact was demonstrated twenty years thereafter by
Sir Humphry Davy, by eliminating potassium and sodium from their
combinations; and afterward by the discovery of the metallic bases of
baryta, strontium, and lime. The earth alumina resisting the action of
the voltaic pile and the other agents then used to induce decomposition,
twenty years more passed before the chloride was obtained by Oerstadt,
by subjecting alumina to the action of potassium in a crucible heated
over a spirit lamp. The discovery of aluminum was at last made by Wohler
in 1827, who succeeded in 1846 in obtaining minute globules or beads
of this metal by heating a mixture of chloride of alumina and sodium.
Deville afterward conducted some experiments in obtaining this metal at
the expense of Napoleon III., who subscribed £1,500, and was rewarded by
the presentation of two bars of aluminum. The process of manufacture was
afterward so simplified that in 1857 its price at Paris was about two
dollars an ounce. It was at first manufactured from common clay, which
contains about one-fourth its weight of aluminum, but in 1855 Rose
announced to the scientific world that it could be obtained from a
material called "cryolite," found in Greenland in large quantities,
imported into Germany under the name of "mineral soda," and used as a
washing soda and in the manufacture of soap. It consists of a double
fluoride of aluminum, and only requires to be mixed with an excess of
sodium and heated, when the mineral aluminum at once separates. Its cost
of manufacture is given in this estimate for one pound of metal: 16 lb.
of cryolite at 8 cents per pound, $1.28: 2½ lb. metallic sodium at about
26 cents per pound, 70 cents; flux and cost of reduction, $2.02; total,
$4.
Aluminum is used largely in the manufacture of cheap jewelry by making a
hard, gold-colored alloy with copper, called aluminum bronze, consisting
of 90 per cent. of copper and 10 per cent. of aluminum. Like iron, it
does not amalgamate directly with mercury, nor is it readily alloyed
with lead, but many alloys with other metals, as copper, iron, gold,
etc., have been made with it and found to be valuable combinations.
One part of it to 100 parts of gold gives a hard, malleable alloy of
a greenish gold color, and an alloy of ¾ iron and ¼ aluminum does not
oxidize when exposed to a moist atmosphere. It has also been used to
form a metallic coating upon other metals, as copper, brass, and German
silver, by the electro-galvanic process. Copper has also been deposited,
by the same process, upon aluminum plates to facilitate their being
rolled very thin; for unless the metal be pure, it requires to be
annealed at each passage through the rolls, and it is found that its
flexibility is greatly increased by rolling. To avoid the bluish white
appearance, like zinc, Dr. Stevenson McAdam recommends immersing the
article made from aluminum in a heated solution of potash, which will
give a beautiful white frosted appearance, like that of frosted silver.
F.W. Gerhard obtained a patent in 1856, in England, for an improved
means of obtaining aluminum metal, and the adaptation thereof to the
manufacture of certain useful articles. Powdered fluoride of aluminum is
placed alone or in combination with other fluorides in a closed furnace,
heated to a red heat, and exposed to the action of hydrogen gas, which
is used as a reagent in the place of sodium. A reverberating furnace is
used by preference. The fluoride of aluminum is placed in shallow trays
or dishes, each dish being surrounded by clean iron filings placed in
suitable receptacles; dry hydrogen gas is forced in, and suitable entry
and exit pipes and stop-cocks are provided. The hydrogen gas, combining
with the fluoride, "forms hydrofluoric acid, which is taken up by the
iron and is thereby converted into fluoride of iron." The resulting
aluminum "remains in a metallic state in the bottom of the trays
containing the fluoride," and may be used for a variety of manufacturing
and ornamental purposes.
The most important alloy of aluminum is composed of aluminum 10, copper
90. It possesses a pale gold color, a hardness surpassing that of
bronze, and is susceptible of taking a fine polish. This alloy has found
a ready market, and, if less costly, would replace red and yellow brass.
Its hardness and tenacity render it peculiarly adapted for journals and
bearings. Its tensile strength is 100,000 lb., and when drawn into wire,
128,000 lb., and its elasticity is one-half that of wrought iron.
General Morin believes this alloy to be a perfect chemical combination,
as it exhibits, unlike the gun metal, a most complete homogeneousness,
its preparation being also attended by a great development of heat, not
seen in the manufacture of most other alloys. The specific gravity of
this alloy is 7.7. It is malleable and ductile, may be forged cold as
well as hot, but is not susceptible of rolling; it may, however, be
drawn into tubes. It is extremely tough and fibrous.
Aluminum bronze, when exposed to the air, tarnishes less quickly than
either silver, brass, or common bronze, and less, of course, than iron
or steel. The contact of fatty matters or the juice of fruits does not
result in the production of any soluble metallic salt, an immunity which
highly recommends it for various articles for table use.
The uses to which aluminum bronze is applicable are various. Spoons,
forks, knives, candle-sticks, locks, knobs, door-handles, window
fastenings, harness trimmings, and pistols are made from it; also
objects of art, such as busts, statuettes, vases, and groups. In France,
aluminum bronze is used for the eagles or military standards, for armor,
for the works of watches, as also watch chains and ornaments. For
certain parts, such as journals of engines, lathe-head boxes, pinions,
and running gear, it has proved itself superior to all other metals.
Hulot, director of the Imperial postage stamp manufactory in Paris, uses
it in the construction of a punching machine. It is well known that the
best edges of tempered steel become very generally blunted by paper.
This is even more the case when the paper is coated with a solution of
gum arabic and then dried, as in the instance of postage stamp sheets.
The sheets are punched by a machine the upper part of which moves
vertically and is armed with 300 needles of tempered steel, sharpened in
a right angle. At every blow of the machine they pass through the
holes in the lower fixed piece, which correspond with the needles, and
perforate five sheets at every blow. Hulot now substitutes this piece by
aluminum bronze. Each machine makes daily 120,000 blows, or 180,000,000
perforations, and it has been found that a cushion of the aluminum alloy
was unaffected after some months' use, while one of brass is useless
after one day.
Various formulæ are given for the production of alloys of aluminum, but
they are too numerous and intricate to enter into here.
* * * * *