but its prospective value for application in the arts was recognised,
and in two or three years afterwards it was put on the market at 40_s._
per lb. It was then, as already remarked, applied to many purposes where
lightness is desirable, such as for the tubes of telescopes,
opera-glasses, the mounting of photographic lenses, &c. And in 1888,
when the production of sodium had been cheapened and applied to the
separation of aluminium, the price of the latter metal fell to 18_s._
per lb. In the meantime, the cheap electricity of the dynamo caused
attention to be again directed to the original electrolytic method; but
many difficulties in detail had to be overcome in applying this process
on the commercial scale. At length the sodium process was superseded;
and by the beginning of 1890, a Swiss company was producing aluminium at
11_s._ per lb. In the course of the following year they succeeded in
bringing the price down to 2_s._ per lb.; and again three years later,
namely at the beginning of 1894, they could offer the metal at 1_s._
7_d._ per lb. The conditions required for effecting this great reduction
were found in driving the dynamo machinery by water-power, and in an
abundant supply of cryolite at moderate cost. This cheapness of
production at once placed the Swiss company in the position of being the
largest and most successful aluminium manufacturers in the world, so
that in 1892 they had realised a net profit of £21,563, paying their
shareholders 8 per cent., and, further, in 1893, the net profit was half
as much again, and the dividend was increased to 10 per cent. A British
aluminium company has recently been formed in London for acquiring the
rights of working all the processes of the successful Swiss company,
purchasing outstanding English patents, amalgamating with certain
existing companies, and for working the _bauxite_ deposits in Ireland,
&c., &c. There is every reason to believe that an important result of
this enterprise will be a still further reduction in the price of this
metal, and consequently a great extension of its applications. And now
(September, 1895) we have already heard of a further reduction in the
price of this metal, which, at the present time, can be purchased in
bulk for about 1_s._ 6_d._ per lb.
[Illustration:
FIG. 337.—_Portrait of Mr. Thomas Hancock._
]
INDIAN-RUBBER AND GUTTA-PERCHA.
_INDIAN-RUBBER._
Researches into the history of the human race in remote ages have
revealed the fact, that before man knew how to extract metals from their
ores, his only implements were formed of stone; and before he became
acquainted with iron, there was an intermediate period in which the more
easily obtained metal, copper, had to serve as the material for all
tools and weapons. Hence archæologists speak of the stone age, the
bronze age, and the age of iron. If we were obliged to name the
nineteenth century after the material which distinctively serves in it
for the most extensive and varied uses, surely we should call it the Age
of Indian-rubber!
The industrial application of Indian-rubber is entirely modern. The
substance itself appears, however, to have been known to the natives of
Peru from time immemorial, and to have been used for the preparation of
some kind of garments. Although the first specimens were sent to Europe
so long ago as 1736, and the substance was from that time submitted to
many investigations, no other use was found for it up to the year 1820
than to efface from paper the marks made by pencils. From this it
derives the name by which it is commonly known. It has also been called
“gum elastic,” and _caoutchouc_ from the Indian name. Crude caoutchouc
is the product obtained by the spontaneous solidifying of the milky
juice of certain tropical plants—such as the _Hævea elastica_, _Jatropha
elastica_, and the _Siphonia cautshu_. The first grows chiefly in South
America, and in the basin of the Amazon forms immense forests. At a
certain season each year bands of persons, called “_seringarios_,” armed
with hatchets, visit these forests for the purpose of extracting the
caoutchouc. They make incisions into the trunk, and the milky juice
immediately runs out, and drops into a vessel placed to receive it, and
attached to the tree by means of a lump of clay. In about three hours
the juice ceases to flow, and the _seringario_ collects the products of
the incisions in one large vessel. By dipping a board into this vessel,
it becomes covered with the juice; and when this is allowed to dry, the
caoutchouc remains as a thin brownish yellow layer. The caoutchouc is
not dissolved in the juice, but is merely suspended in it; and to hasten
the drying and coagulation of the liquid, the board is warmed over a
smoky fire made with green wood. When alternate immersions and drying
have covered the board with a sufficient thickness of caoutchouc, the
layer is slit open with a knife, and the board is withdrawn. This is the
best kind of crude caoutchouc, because it is free from all admixture of
foreign bodies except the carbon derived from the smoky flame. The
_bottle_ Indian-rubber is moulded on pear-shaped lumps of clay, which
are covered with successive layers of the milky juice; when a sufficient
thickness has been attained, the clay is removed by soaking in water.
Up to 1820, as already mentioned, Indian-rubber was used only for
effacing pencil-marks, and about that time a piece half an inch square
sold for two shillings and sixpence. But the extreme elasticity and
extensibility of this singular substance was attracting the attention of
practical men in England, Scotland, and France. One of the earliest
patents obtained in this country for applications of caoutchouc was
taken out by Mr. Thomas Hancock, of Newington, in 1820. This gentleman
has written an account of the Indian-rubber manufacture from the
commencement, and the book is extremely interesting from the clear and
simple manner in which the inventor describes how he effected one
improvement after another in his processes and machinery. Mr. Hancock
had, previous to his turning his attention to Indian-rubber, no
acquaintance with chemistry; but he was skilled in mechanical
engineering and the use of tools, and this knowledge proved to be
precisely the kind most valuable for dealing with the first stages of
caoutchouc manufacture. His first patent was for the use of
Indian-rubber for the wrists of gloves, for braces, for garters, for
boots and shoes instead of laces, and for other similar purposes. The
rings for the wrists of gloves, &c., were simply cut from the bottle
Indian-rubber by machinery the patentee himself contrived for that
purpose. Mr. Hancock next arranged an apparatus for flattening the raw
Indian-rubber by warmth and pressure, so as to make it available for the
soles of boots, &c. He relates the practical difficulties he had to
encounter in his operations, and the manner in which he overcame them.
He soon noticed and utilized the fact that two clean freshly-cut
surfaces of caoutchouc, when pressed together, cohere and unite
perfectly. This further led him to devising a machine by which all the
waste cuttings and parings might be worked up. This machine consisted of
a cylinder revolving within a cover, both being provided with steel
teeth, by which the pieces of caoutchouc placed between them were torn
into shreds, and then kneaded into a solid coherent mass of homogeneous
texture. The first machine of this kind made by Mr. Hancock would work
up about 1 lb. of Indian-rubber; but now machines on the same principle
are in use operating on more than 200 lbs. of material at once, and
turning it out on a roll 6 ft. long, and 10 in. or 12 in. in diameter.
While Hancock was thus successful in mechanically working Indian-rubber,
Macintosh, of Glasgow, found means of effecting its solution by
coal-naphtha, and he obtained, in 1823, a patent for the application of
his discovery to the fabrication of waterproof garments. Waterproof
cloth, or “Macintosh,” is prepared by varnishing one side of a suitable
fabric with a solution of caoutchouc, or by covering one side of a cloth
with a thin film, and then bringing it into contact with a second piece
similarly prepared—the two caoutchouc layers becoming incorporated when
the double cloth is passed between rollers. Other solvents for
Indian-rubber have been discovered in ether, chloroform, sulphide of
carbon, and rectified turpentine. By treatment with these liquids it
swells up, and eventually dissolves, producing a viscid ropy mass,
which, by evaporation of the solvent, leaves the caoutchouc with all its
original elasticity. By the use of these last-named solvents the
persistent and disagreeable odour occasioned by coal-oil is avoided. Mr.
Hancock relates that when the manufacturers had overcome all obstacles,
and had succeeded in producing thin, light, pliable, and perfectly
waterproof fabrics, they had to encounter another quite unexpected
difficulty—the tailors set their faces against the new material, and
could not be induced to make it up! The manufacturers were, therefore,
obliged themselves to fashion waterproof garments, and retail them to
the public. This, however, turned out to be a benefit, for the seams
were made waterproof, so as to exclude even the little water which would
otherwise pass in by capillary attraction at the stitches.
It will now be observed that there are two distinct modes of working
caoutchouc: by dealing, viz., with the solid material, or with the
solutions. Thus, from a solid disc of caoutchouc long ribbons of the
material may be cut, and these ribbons, by being passed between a set of
circular knives, may be divided into a number of square threads. These
threads may then be drawn out to six or ten times their length; and, if
wound and maintained in this state of tension for forty-eight hours in a
warm place, they will lose their condition of tension, and their
elongated form will become their natural or unstrained one. In this
manner are the Indian-rubber threads prepared, which, covered with silk
or other material, form elastic fabrics such as those used in the sides
of boots. The circumstance of caoutchouc, when heated for some hours at
a temperature a little above the boiling-point of water, retaining
whatever form it has during the heating, is the basis of methods of
obtaining thin sheets and other forms of the material. Tubes are made by
forcing the heated caoutchouc through an annular opening by application
of great pressure; it sets in cooling, preserving a section
corresponding with the orifice through which it issues. In another mode
of forming tubes, a paste composed of caoutchouc, oxide of zinc, and
lime, is formed into sheets, which are cut into strips. The strips are
folded longitudinally, and the edges are cut together at an angle of 45°
with the surface, so that the cut surfaces may meet each other when the
strip is rolled on a mandril to give it a cylindrical form. A slight
pressure suffices to solder together the cut surfaces, and the tube is
then “vulcanized” by a process to be presently described.
The dissolved caoutchouc serves to prepare waterproof garments, round
threads, sheets of Indian-rubber, &c. Fabrics are coated with the
solution by pouring it on the material as it is passing horizontally
from a roller. A straight-edge, under which the charged cloth passes,
distributes the caoutchouc in a uniform layer, the thickness of which is
regulated by the space between the knife-edge and the fabric. When
sulphide of carbon is the solvent used, its evaporation is complete in
about ten minutes, but with other solvents two or three hours are
required. The caoutchouc is usually mixed with lampblack before being
spread on the cloth, and the article is finished by giving the
Indian-rubber layer a coat of gum-lac varnish. Sheets of Indian-rubber
are obtained by spreading fifteen or twenty layers over a cloth, which
is afterwards detached by wetting it with a solvent.
Threads of circular section are manufactured from a paste of caoutchouc,
made by dissolving that substance in sulphide of carbon mixed with 8 per
cent, of alcohol. This paste is placed in a cylinder, out of which it is
forced by a piston through a number of circular holes, whence it issues
in the form of filaments. These are received upon a stretched cloth,
which moves along, carrying the parallel threads, until the sulphide of
carbon has evaporated.
A modification of caoutchouc, possessing very valuable qualities for
many purposes, was discovered by Mr. Charles Goodyear, and largely
applied by him in the United States to the fabrication of waterproof
boots. In 1842 these boots were imported into Europe, and it was seen
that this form of the material had the advantages of not sticking to
other bodies at any ordinary temperatures, and of preserving its
elasticity even in the coldest weather, whereas ordinary Indian-rubber
becomes rigid by cold. The cut edges of this variety of caoutchouc do
not cohere by pressure. Mr. Goodyear attempted to keep his process a
secret; but Mr. Hancock, having soon detected the presence of sulphur in
the American preparations, set to work to discover how that substance
was made to combine with the caoutchouc. He succeeded, and he obtained a
patent for sulphurizing Indian-rubber before the original inventor had
applied for one. Mr. Hancock found that a sheet of caoutchouc immersed
in melted sulphur at 250° F., gradually absorbed from 12 to 15 per cent,
of its weight of sulphur; and, further, that this does not in any way
alter its properties. When, however, the sulphurated substance was for a
few minutes exposed to a temperature of 300°, it acquired new qualities,
which were precisely those of the modification employed by Mr. Goodyear
for his impervious boots. This transformation effected by sulphur Mr.
Hancock called _vulcanization_; and _vulcanized Indian-rubber_ is now
employed in nearly all the innumerable applications of caoutchouc,
provided the presence of sulphur is not absolutely objectionable.
Goodyear’s process consists in mixing the sulphur with the caoutchouc,
the suitable proportion (7 to 10 per cent.) having been determined
beforehand, and the sulphur ground up with the Indian-rubber in the
masticating machine, or disseminated through the viscid liquid if a
solution is used, or dissolved in the solvent employed. This gives
better results than Hancock’s process, because the sulphurization is
more uniform, and this method is therefore more largely employed. When
the various articles have been fabricated in the ordinary manner from
the mixture of caoutchouc and sulphur, they are enclosed in vessels,
where they are submitted for two or three hours to the action of steam
under a pressure of nearly 4 atmospheres, so that the steam may have a
temperature of about 280° F. A still easier method, due to Mr. Parkes,
consists in steeping the articles (which in this case should be thin) in
a solution of one part of chloride of sulphur in sixty of bisulphide of
carbon. The object becomes vulcanized by simple exposure to the air,
without the aid of heat. But this process is said to be liable to cause
the article afterwards to become brittle. The addition of oxide of zinc,
carbonate of lead, and other substances, is found to yield a product
better adapted for certain purposes than one in which only sulphur is
used.
The list of applications of vulcanized Indian-rubber would be a very
long one; but as a great number of these applications must be known to
everybody, it will be unnecessary to specify them. It has lately been
used for carriage-springs, for the tires of wheels, and for the rollers
of mangles. Its employment in the construction of portable boats,
pontoons, life-buoys, dresses for divers and for the preservation of
life at sea, air-tight bags and cushions, air and water beds, cushions
of billiard-tables, are a few of the thousand instances of its utility
which might be quoted.
When the proportion of sulphur mixed with the caoutchouc is increased to
25 or 35 per cent., another product having qualities entirely different
from those of vulcanized Indian-rubber is obtained when the mixture is
heated. This is the jet-black substance termed _ebonite_ or _vulcanite_,
which is made into such articles as combs, paper-knives, buttons, canes,
portions of ornamental furniture, and plates of electrical machines. It
is in many cases an excellent substitute for horn and for whalebone,
while for insulating supports, &c., in electric apparatus, it is
unrivalled. It has a full black colour and takes a bright polish; and it
may be cut, or filed, or moulded. It is very tough, hard, and durable.
In the transformation of Indian-rubber into vulcanite, the temperature
must be somewhat higher than that required for the production of the
vulcanized Indian-rubber. The caoutchouc used is very carefully purified
before it is incorporated with the sulphur; and the yellow paste formed
by the mixture is subjected to the contact of steam at a temperature of
about 310°.
_GUTTA-PERCHA._
Gutta-percha is a substance very like Indian-rubber in its chemical
properties, having the same composition, although in outward appearance
very different. It was first sent to Europe in 1822, but did not become
an article of commerce until 1844. It is the solidified juice of a tree
(_Isonandra percha_) which abounds in Borneo and Malacca. The trunk of
the tree grows to a diameter of 6 ft., but as timber it is valueless.
When an incision is made through the bark and into the wood, a milky
juice flows out, which speedily solidifies. Gutta-percha is a very tough
substance, but is without the elasticity of Indian-rubber. It differs
from the latter, too, in becoming softened by a gentle heat, and it will
then readily take and retain any impressions with great sharpness and
fidelity. Thus beautiful mouldings and other ornamental objects are
easily made. It also has the valuable quality of welding when softened
by heat. It is a non-conductor of electricity, and it is largely used
for covering telegraph-wires, and especially for forming an insulating
coating in submarine cables. It seems to have become known precisely at
the time it was required for this purpose, and the success of ocean
telegraphy is largely owing to its valuable properties. It is employed
as a substitute for leather in soling shoes and boots, and in forming
straps and bands for driving machinery; also in the preparation of tubes
used for conveying liquids, and for speaking-tubes. Dilute mineral acids
have no action upon it, and hence it is especially valuable for making
bottles to contain hydrofluoric acid, which attacks glass. A drawback to
the use of gutta-percha is its tendency to become oxidized when exposed
to light and air, by which it entirely loses its power of becoming
plastic by heat, and is converted into a brittle substance. But in the
dark, or under water, it may, however, be preserved for an indefinite
period without change.
Mr. Charles Hancock, in 1847, patented a machine for cutting the
gutta-percha into slices. In this machine there is a circular iron
plate, with three radial slots, in which knives are fixed somewhat in
the manner of the cutting tool of a spokeshave. The lumps of
gutta-percha drop against these knives as the plate is driven round, and
the material is cut into slices, which have a thickness determined by
the projection which has been given to the blades. Sometimes an upright
chopper is used, with straight or curved blades. These slices are
immersed in hot water, until they are softened, and they are then
subjected to the action of rollers armed with toothed blades, called
“breakers,” and also to the action of the mincing cylinder, which is
furnished with radiating blades, and revolves partly immersed in the
water. The material is carried out of the hot water to these machines by
endless webs mounted on rollers. The breakers and mincing cylinders make
about 800 revolutions per minute. The gutta-percha, thus reduced to
fragments, is carried forward again by endless webs into cold water,
where it is thoroughly washed and separated from the impurities, which
fall to the bottom, while the lighter gutta-percha floats on the surface
of the water.
Gutta-percha, like caoutchouc, can be combined with sulphur. The best
product is obtained when a small proportion of sulphur is used along
with some metallic sulphide. Mr. C. Hancock uses 48 parts of
gutta-percha, 1 of sulphur, and 6 of antimony sulphide. These
ingredients are thoroughly mixed and put into a boiler, where they are
heated under pressure for an hour or two. Another method of treating
gutta-percha was also devised by Mr. C. Hancock, who found that when
this strange substance was exposed to nitric oxide gas (which is given
off when nitric acid acts on copper) it became quite smooth, and
acquired an almost metallic lustre, losing also all its stickiness.
Another modification is formed by treating gutta-percha with chloride of
zinc; and yet another by the action of a solvent, such as turpentine, a
sulphide, sulphur, and carbonate of ammonia, employed simultaneously.
Mr. Hancock mixes all these materials together in a “masticator,” and
then applies heat to them while confined in a vessel under pressure. The
product of these operations is a very singular modification of
gutta-percha, in which the material assumes a spongy, elastic condition,
and in this form it is used to form the stuffing of sofas, easy chairs,
&c. Among the purposes to which gutta-percha has been applied besides
the general one of waterproof tissues and fabrics, may be named the
formation of straps, belts, bandages, cups, and other vessels, rollers
for cotton-spinning machinery, hammers of pianofortes, cards for
wool-carding, hammercloths, life-preservers, and trusses.
Gutta-percha is made into strips, bands, cords, or threads of any
required section, by passing sheets of suitable thickness between
rollers provided with grooves and cutting edges. For strips and bands
the sheets are passed through the machine cold, and divided by the
cutting-edges. But for round cords or threads sheets are supplied to the
rollers from a receptacle in which they acquire a temperature of about
200° F. The material is forced to take the form of the grooves, the
operation in this case being analogous to that of rolling iron bars. The
gutta-percha cords are received as they issue from the rollers in a tank
of cold water, from which they pass on, to be wound on reels or drums.
It is obvious that cords of any section may be formed by making use of
grooves of suitable shape. Tubes of gutta-percha are made by forcing the
softened substance out of an annular orifice: it is received into
vessels filled with cold water. Telegraph-wires are covered by a similar
process—the copper wire being made to pass through the centre of a
circular opening with the gutta-percha surrounding it. Picture-frames,
&c., are made by forcibly pressing warm gutta-percha into the warmed
moulds. Gutta-percha tubing is largely used everywhere for the
speaking-tubes by which persons in remote apartments of even the largest
building can converse. This is one of the labour-saving inventions of
our day. It must have struck every one who has seen these speaking-tubes
in operation in a large establishment, what a vast amount of running to
and fro they save, and how much they expedite business by the convenient
means they afford of giving orders and directions to persons in distant
apartments. This tubing is also used for the conveyance of liquids, and
it has been proposed to employ it instead of the ordinary leaden piping
used for carrying water. It may seem to the reader that gutta-percha is
too fragile a material to resist the pressure to which water-pipes are
exposed. But, judging from some experiments made by the engineer of the
Birmingham Waterworks, the power of gutta-percha tubing to resist
pressure is quite extraordinary, and far beyond what would be supposed.
The tubes experimented on had diameters of ¾ and ⅞th of an inch
respectively. The water from the main, where the pressure was that
caused by a head of 200ft., was in communication with these pipes for
several weeks, and they were found unaltered in any way. In order to
test the strength of the tubes, and find the greatest pressure they
would bear, the engineer then had them connected with a hydraulic
proving-pump; and here, when exposed to the highest pressure at which
the ordinary water-pipes were tested, namely, to 250 lbs. on the square
inch, they also remained intact. The pressure was afterwards increased
to 337 lbs., but without any damage to the tubes.
The increasing importance of gutta-percha may be inferred from the
continually augmented importation of the crude substance into this
country. In 1850 only 11,000 cwt. were imported, but the quantity has
increased year by year; and in 1872 we received nearly 46,000 cwt. The
demand is still increasing; but there is reason to apprehend that under
the stimulus of a rising market, the producers have collected the
gutta-percha wastefully and with great destruction of the trees, so that
it is not improbable that if the demand still increases, there may be a
gutta-percha famine. The concreted juices of certain other trees have
been proposed as substitutes for gutta-percha. None of these have as yet
come into practical use. The increase in a few years of the quantity of
Indian-rubber imported into the United Kingdom is perhaps more
extraordinary. From the tables given in Mr. Hancock’s book, it appears
that our imports of caoutchouc were 853,000 lbs. in 1850, but by 1855
they had amounted to 5,000,000 lbs.
[Illustration: