GUMS AND RESINS USED IN THE ARTS
Most vegetable juices exposed to the air harden into firm substances,
commonly called _gum_. Some of these dissolve, or at least soften, in
water; these technically are known as "gums," and usually are so
designated in commerce. Others are insoluble in water, but dissolve
readily in alcohol, in naphtha, in turpentine, or in other essential
oils; these are designated as "gum-resins." Still others yield oils or
pitchy substances on distillation; these are known as "oleo-resins."
There are many other dried vegetable juices, however, that in commerce
are not classified among the gums and resins, and of these the most
important is the substance commonly known as india-rubber.
=Rubber and Rubber Products.=--"Caoutchouc" is approximately the name
given by Indians of the Amazon forests to a substance that had also been
found in India. Some of it was brought to Europe from the Amazon region
as early as 1736, and for nearly one hundred years no general purpose
was discovered for which it could be used, except to erase lead-pencil
marks--hence the name india-rubber, which has held ever since.
Common rubber is the prepared juice of a dozen or more shrubs and trees,
all of which grow in tropical regions.[37] The belt of rubber-producing
plants extends around the world and includes such well-known species as
the fig, the manihot (or manioc), and the oleander; indeed, it is a
condition of sap rather than a definite species of plant that produces
rubber, and the latter is a manufactured rather than a natural product.
The process of preparing the juice is practically the same in every part
of the world.
The rubber-gatherer of the Amazon, who is practically a slave, wades
into the swamp, makes several incisions in the bark of the tree,
fashions a rough trough of clay under it, and waits till the sap fills
the clay vessel. When the sap has been gathered he makes a fire of the
nuts of the urucuri palm and places an inverted funnel over it to
concentrate the smoke. He first dips the end of a wooden spindle into
the juice and then holds it in the smoke until the juice coagulates;
this process is repeated until there has formed a ball of rubber
weighing from five to ten pounds. The smoke of the palm-nuts is a
chemical agent that converts the juice into the crude rubber of
commerce.
Crude gum, however, is lacking both in strength and elasticity. The
process that makes it a finished product is known as _vulcanization_.
The crude rubber, having been exported to the manufacturer in the United
States or Europe, is shredded, washed, and cleansed, and partly fused
with varying proportions of sulphur. For a very soft product, such as
the inner surface of tires, only a small proportion is used; where the
wear is considerable, a larger proportion is employed.[38] White clay is
sometimes added to give body to the product; coloring matter is also
sometimes added.
By far the greater part of the crude rubber comes from the Amazon
forests. Brazil produces about one-half, but a considerable quantity is
obtained in Acré, the territory formed where the borders of Brazil,
Bolivia, and Peru meet, and now ceded to Brazil. Nearly all this
product, that of the Ceará region excepted, is marketed at Pará and is
known as Pará rubber. It is the best produced. The African product,
mainly from the forests of the Kongo, and Madagascar, and nearly all the
East Indian product is sent to Europe.
[Illustration: REGIONS YIELDING RUBBER]
The world's product is about one hundred and thirty-three million pounds
of crude rubber. Of this product the United States takes nearly
one-half. The greater part is used in the manufacture of pneumatic
tires, hose, and overshoes. A large part is used for making water-proof
cloth,[39] and considerable is made into the small elastic bands for
which there is a growing use.
=Gutta-Percha.=--Gutta-percha is obtained from the juices of several
plants (chiefly _Dichopsis gutta_ and _Supota mülleri_) both of which
abound in the Malay peninsula and the East Indies. It is prepared in a
manner somewhat similar to that employed in making crude rubber; it is
also easily vulcanized by heating with sulphur. It is used to a limited
extent in the manufacture of golf-balls, but mainly as the insulating
cover of copper wires used in ocean telegraph cables. For this purpose
it has no known substitute, and its essential merit is the fact that it
is not altered by salt water. Nearly all the product is shipped from
Singapore to England.
=Pine-Tree Products.=--The various members of the pine and cone-bearing
trees yield valuable essential oils and oleo-resins that are very
important in the arts and sciences. These, in nearly every instance, are
prepared from the sap of the tree.
_Oil of turpentine_ is known as an "essential oil," and in chemical
structure and properties it does not differ from the various essential
oils, such as lemon, orange, peppermint, etc. Commercial turpentine is
generally made from the sap of the long-leafed pine of the Atlantic
coast-plain.
The bark of the tree is cut near the foot, and the sap that oozes from
the scar quickly hardens into a gum. The gum, generally known as "crude
turpentine," is distilled and yields about one-fourth its weight of oil
or "spirit" of turpentine. It is a staple article of manufacture in
Europe, India, and the United States, and is used chiefly to dilute the
oil paints and varnishes used in indoor work. The United States supplies
about two-thirds of the world's product, a large part of which is
shipped from Savannah and Brunswick, Ga., to Great Britain.[40]
_Resin_ is the substance remaining after the crude turpentine has been
distilled. It is used in the manufacture of varnish, sealing-wax, and
soap. Finely powdered resin is also mixed with wood-pulp in the
manufacture of wrapping-paper. It gives the latter a glazed surface and
renders it almost water-proof. Most of the world's product of resin
comes from the turpentine district of the United States, and about
four-fifths of it is exported to Europe.
When resin is subjected to distillation at a still higher temperature,
_resin oil_, a very heavy turpentine, is given off, and a viscous
substance known as _pitch_ remains. A considerable amount of this is
still made in the United States, but the greater part comes from the
pine-forests of Russia and Scandinavia. When pine-wood is distilled,
_tar_ is the chief product. In Russia tar is generally made by burning
green logs covered with turf, over a pit. _Creosote_, or wood
preservative, is made from tar. The various pine-tree products, creosote
excepted, are commonly known as "naval stores," the tar being used in
water-proofing the rigging of vessels, the pitch in calking the seams in
between planks, in the decks and hulls.
=Other Resins and Gums Used in the Arts.=--Most of the gums and resins
used in the arts and sciences are the hardened sap of plants--in some
cases exuding by natural means from the bark, in others resulting from
the puncture of the bark.
The _lac_ of commerce is due to the puncture of the young branches of a
tree, frequently a fig (_Ficus religiosa_) growing in the tropical
forests of India. The hardened sap incrusts twigs forming _stick-lac_;
when crushed, washed, and freed from the woody matter it is _seed-lac_;
when melted and cooled in flakes it is _shell-lac_, the form best known
in commerce. It is the chief ingredient in sealing-wax, and is
extensively used as a varnish. It is also used in fireworks on account
of its inflammability.
_Dammar_ is the product of a tree growing in the East Indies; it is the
basis of a very fine white varnish. _Copal_ is a term applied to
oleo-resins soluble in turpentine, and used almost universally as
varnishes. They come from the tropical regions of South America, Africa,
and from the East Indies. _Kauri_ is the fossil gum of a cone-bearing
tree dug from the ground in northern New Zealand. _Amber_ is the fossil
gum of extinct cone-bearing trees found mainly along the Baltic coast of
Prussia. It is used chiefly for the mouth-pieces of tobacco-pipes and
cigar-holders; the inferior product is made into varnish. It is sold
wherever tobacco is used. _Sandarach_, found on the north African coast,
is used principally in Europe, being employed as a varnish. The United
States and Great Britain consume most of the foregoing products.
QUESTIONS FOR DISCUSSION
Name any elastic substance you know about that is in every way a
substitute for rubber.
What has been the relation between rubber and good roads?
Describe the structure of a bicycle tire.
Why are tar, pitch, and turpentine called naval stores?--and what
determines the locality in which they are made?
What is varnish, and for what purposes is it used?
FOR STUDY AND REFERENCE
Obtain specimens of crude rubber, vulcanized rubber, and hard rubber;
note carefully the characteristics of each.
Burn a very small piece of cheap white rubber-tubing in an iron spoon or
a fire-shovel; note the character of the residue.
Obtain specimens of gutta-percha, resin, pitch, turpentine, shellac,
copal, dammar, and creosote for study and inspection.