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Tannins are organic compounds, mostly
glucosidal, which have an acid reaction and are very astringent. Their biological function is in
doubt. They may be concerned with the
formation of cork or pigments, or with the protection of the plant. Tannins are of economic interest because
of their ability to unite with certain types of proteins, such as those in animal
skins, to form a strong, flexible, resistant insoluble substance known as
leather. Because of this
characteristic, tannin-containing materials are in great demand. Tannins also react with salts of iron to
form dark-blue or greenish-black compounds, the basis of common inks. Their astringent nature makes them useful
in medicine. Tanning materials are
also utilized in oil drilling to reduce the viscosity of the drill without
reducing the specific gravity. Most plants contain some tannin, but
only a few species have a sufficient quantity to be of commercial
importance. Tannins are found in the
cell sap or in other definite areas in bark, wood, leaves, roots, fruits and
galls. Such structures are of little
value for other purposes, so that the extraction of tannin is usually
incidental to other industries.
Hill (1952) described the tanning industry as very old, possibly
dating to 3,000 B.C. Leather was
being tanned in China before 1,000 B.C.
The Romans used oak bark for tanning animal hides and the Amerindians
used several native plants to cure the hides of buffalo. In the United States the first tannery was
established in Virginia in 1630.
Scarcely 20 years after there were more than 50 tanneries in New
England. The industry centered around
the latter because of the abundant hemlock bark available there. Thereafter the industry gradually shifted
to the west and south when hemlock supplies became scarce. Oak then became the principal tanning
substance. Later on chestnut was used
in the South. When these became
scarce other sources, such as sumac and canaigre, were utilized and some
foreign products were imported. Today
concentrated extracts with high tannin content are generally utilized. Eastern Hemlock, Tsuga
canadensis,
had been the main domestic source of tannin in the United States from the
beginning of the industry. The
pressure on the forests was so great as to practically eliminate the
species. Hemlock bark has 8-14
percent tannin. It was used for
sheepskins and for sole and other leathers, either by itself or in
combination with oak. Later extracts
with 28-30 percent tanning became available.
Some attention was also given to the Western Hemlock, Tsuga heterophylla. Several species of American oaks
used for tanning. The Chestnut Oak, Quercus
montana,
is abundant in Appalachia. It has
tannin content of 6-11 percent. The
extract with 26-30 percent tannin is widely used for heavy leathers. The Black Oak, Quercus
velutina, produces quercitron
extract. Although widely used it does
stain the leather with a yellowish color.
The
California Tanbark Oak, Lithocarpus
densiflora, was utilized since
1850. It has tannin content of as
high as 29 percent. Red Oak, Quercus borealis, and White Oak, Q. alba, are used to some
extent but they have lower amounts of tannin. European oaks are used in England and on continental Europe. Mangrove is a good source of
tannin and gradually became more important as other sources diminished. The Red Mangrove, Rhizophora
mangle,
has been the main source. It is
abundant in tropical swamps of both hemispheres. The bark is very hard and heavy and contains 22-33 percent
tannin. The leaves may also be used
and the extract has been the least expensive of tanning substances. However, it causes the leather to darken
and is therefore rarely used alone. This source is used mainly in
Great Britain. It is obtained from
several species of acacia, mainly Acacia
decurrens and its varieties dealbata
and mollis, and Acacia
pycnantha. These are small trees native to Australia
but have become cultivated in Africa, Ceylon, Brazil and other countries. The bark may have tannin content of 40-50
percent. It is removed from trees
that are 5-15 years old. It is then
ground to a powder. Wattle bark
produces a solid, very firm, and faintly pink leather, which is especially
used for soles. Wattle wood is a good
pulpwood and can be used for timbers, posts and other things. There is also a gum produced by the trees. Other Bark Sources Other barks that are occasionally
used for tanning are the European larch, Larix decidua, Norway Spruce, Picea
abies and
some birches and willows. Birch bark is favored in Russia
and the fragrance of Russian leather is due to the presence of an essential
oil in the bark. Willow bark gives light-colored,
soft pliable leather favored for tanning gloves. Some tropical
barks are
of minor importance. Mallet bark is obtained from Eucalyptus
occidentalis
of
Western Australia, with tannin content of 35-50 percent. Avaram
bark is from Cassia
auriculata,
important in India. Tanekaha bark from Phyllocladus
trichomanoides,
a New Zealand tree is used for gloves because it contains an orange-yellow
dye. Castanea dentata
and Castanea
sativa
contain
tannin that is extracted at high temperatures from wood chips. The final product is 30-40 percent
tannin. Chestnut is used for heavy
leathers. This wood of several South
American trees is a very important source of tannin. Schinopsis
lorentzii
and S.
balansae
are
the main species. The wood from these
trees is some of the very hardest known with a specific gravity of
1.3-1.4. Argentina and Paraguay are
the main producers. Extracts are made
in factories located near the sources.
The logs are chipped and cooked with steam in copper extractors until
the liquor is very concentrated. This
has tannin content of 40-60 percent.
It is very quick in its tanning action and is used either by itself or
in combination for all kinds of leather, especially sole leather because it
imparts extra durable qualities. The dried leaves of three North
American species of sumac, Rhus glabra, R. typhina and R.
copallina,
are important tannin sources. Leaves
are harvested in autumn when they begin to turn red, and are dried and ground
to a powder. The tannin content of
10-25 percent is higher in plants that grow at lower latitudes. Rhus coriaria
has
a content of 20-35 percent The
leather becomes a pale color and is soft in texture. It is especially suited for gloves and
bookbinding. This is one of the few
tanning plants that are cultivated. Gambier or White Cutch is a resinous
substance that is extracted from the leaves and young branches of Uncaria
gambir. It is a climbing shrub of Malaya and
Indonesia. The plants become shrubby
when cultivated. The trees are
cropped four times a year and the tannin is extracted from the tissues with
boiling water. It crystallizes out as
a semisolid white substance and has tannin content of 35-40 percent. Gambier is also used as a dye, masticatory
and in medicine. Myrobalan nuts are the unripe fruits
of two trees of India, Terminalia chebula and T.
bellerica. These trees are grown in India for both
fruit and timber. The nuts have
tannin content of 30-40 percent. When
they are used by alone they yield spongy, light yellow leather, but in
combination they are preferred. They
are used to tan leather of goats, calves, and sheep and are best suited for
soles and harnesses. The tannin is obtained from the
dried twisted seedpods of a small leguminous tree, Caesalpinia
coriaria,
of the West Indies and South America.
The tannin content is 40-50 percent. A stocky shrub or small tree Tara, Caesalpinia
spinosa is widely distributed in tropical America. The fruits are 43-51 percent tannin. The plant is cultivated in Peru and North
Africa. It is used for producing a
high grade of leather as it changes the color only slightly. The fruits are also used for ink and black
dye. Caesalpinia
brevifolia from Chile has pods with high tannin content. It is usually used in combination with
other materials. The sun-dried cups of Turkish oak acorns, Quercus
macrolepis,
have tannin content of 45 percent. It
is used in combination with other materials for fine grades of leather. Rumex hymenosepalus is
a species native to the southern United States and Mexico. It is widely cultivated. The roots are sliced and the tannin
extracted. The tannin content is
about 30 percent and yields a bright orange firm and heavy leather. Sabal palmetto
has been used somewhat as a source of tannin, but the content is only 10
percent. Writing inks date back to ancient
times. In Egypt ink was used on
papyrus before 2,500 B.C. and the oldest writings in China are dated to at
least 2,600 B.C. Originally carbon
ink was used, which is a combination of gums, charcoal and varnish. The charcoal was secured from plant
sources such as charred date seeds or it was of animal origin. The two most important modern inks
are Carbon Inks and Tannin Inks.
These are paint like inks that remain on the surface of paper while
the others are dyelike and soak into the paper where they combine chemically
with the fibers. India or Chinese Ink
is very permanent ink that is made from the carbon
black, lampblack, or soot obtained by burning pinewood or a vegetable oil such as tung
or sesame, mixed with glue, gum arabic, or some similar sizing material. Printing Ink contains carbon obtained from
natural gas, petroleum, or other materials, combined with rosin, a drying oil
such as linseed, some chemical drier and often soap. These utilize the property of
tannin that combines with iron salts to produce a blue-black color. Most are gallotannate in nature, the tannin
being derived from insect galls that also contain gallic acid. Tannin inks were first used in the 11th
Century. Aleppo or nutgalls have been
the main source of the tannin. These
galls are formed on the twigs of the Aleppo oak, Quercus infectoria,
as a response to the injuries caused by egg laying insects. The plant is a small shrub that ranges
throughout the Mediterranean region.
The small spherical or pear-shaped galls form in great quantity and
have very high tannin content. To
make the ink either the galls or an extract of them are combined with ferrous
sulfate, an agglutinant like gum arabic, and a coloring material, like
logwood. Rhus
chinensis and other species of sumac from China and Japan are at times
used to substitute the Aleppo galls, although they are not of as good a
quality. Oak galls are sometimes
used and tannin inks may also be made from other sources of tannin such as
logwood and chestnut. Logwood has the
advantage of containing both tannin and a coloring agent. These are made from natural or aniline dyes in
combination with water, gum and alum.
A high quality red ink is derived from brazilwood. Natural dyestuffs and stains,
obtained from the roots, bark, leaves, fruit or wood of plants, have been in
use worldwide from earliest time. The
cultivation of the plants and the preparation of the dyes have been an
important industry in many areas.
Around the mid 1800’s the natural products began to be supplanted by
synthetic or aniline dyes obtained from coal-tar products. These synthetic dyes are brighter, more
permanent, easier to use, are less costly and afford a wider range of
colors. Their development has
gradually led to the abandonment of most of the plant products.
There are over 2000 different pigments secreted by plants. Most of these are used only locally by
primitive cultures, if at all. A
comparatively small number, about 155, have been of commercial importance,
and of these only a few have been able to compete with the artificial colors. The principal use of dyes has been in
the textile industry. However, before
the fabrics can incorporate them, they must be rendered insoluble so that
they will not run. This is
accomplished by the use of mordants, which are various metal salts. When fabrics are steeped in a solution
containing a weak salt of iron, chromium, aluminum, or tin, a fine layer of
the metallic oxide is deposited on the cloth. The dye forms an insoluble compound with this oxide. Dyes are also used for coloring paints,
varnishes, leather, ink, paper, wood, furs, food, cosmetics and medicines. Many different kinds of dyes have
been in use in the United States alone at various times. The Amerindians made use of many native
species and the early settlers followed their example. In the past dyes that have been important
include butternut bark, which was used for dyeing homespun, and later for
dyeing the uniforms of the Confederate army.
As in other parts of the world plant pigments have been generally
supplanted by aniline dyes. For a
while after 1914 when the First World War reduced the supply of synthetic
colors, 90 percent of which were made in Germany, the United States returned
to the use of the natural products.
However, soon a domestic synthetic dye industry arose which by 1930
was producing products valued at US$83,000.000.
Almost all colors were available that occurred in plant pigments. Red dyes were obtained from alkanna,
barwood, brazilwood, cudbear, logwood, safflower, sappanwood and
sandalwood. Yellow dyes were from
annatto, fustic, gamboge, henna, osage orange, Persian berries, quercitron,
saffron, tumeric and weld. Blues were
from cudbear, indigo, and woad.
Greens were from chlorophyll and lokaio; and brown was from cutch.
This is one of the oldest and most important dyes. It is obtained from the heartwood of Haematoxylon
campechianum, a small, thorny tree
legume of Mexico. It has been
introduced throughout the world tropics.
The trees are propagated from seed when cultivated. They are cut when 10-12 years old and the
bark and sapwood removed. Both the
logs and/or the extracts are exported.
The purple-red dye is known as haematoxylin and is used in its natural
state or with a mordant. The presence
of a large amount of tannin allows logwood extract to react with iron salts
to produce a black color. This is
used especially for dyeing cotton and woolen fabrics, leather, silk and
furs. Haematoxylin stain is widely
used in histological work. A related
species, braziletttte or hypernie (H.
brasiletto) gives a red dye that is used for dyeing leather.
This is the main source of natural yellows, olives and browns and
ranks with logwood in importance. It
is used for leather and in combination with logwood for silk, wool, nylon and
rayon. It comes from the heartwood of
Chlorophora tinctoria, a forest tree of the
West Indies, Central and South America.
The light-yellow wood turns a dark yellow-brown when exposed to
air. Faustic is exported as short
logs, chips, powder or paste. The dye
is frequently called Old Fustic to distinguish it from Young Fustic, once
obtained from the twigs of Cotinus
coggygria.
The term Cutch refers to several kinds of raw materials that are
useful in making dyes and in tanning.
Gambier or white cutch was discussed above. Black cutch or catechu is the source of an important brown
dye. It is from the heartwood of Acacia catechu, a tree native to
Burma and India. Wood pieces are
boiled in water and the extract is evaporated to a purple-black, gummy,
semisolid mass that is then molded into blocks. This is a fast dye and is used for the different shades of
fawn, brown, olive and drab colors, including khaki. Catechu is also used in medicine and as a
masticatory.
Maclura
pomifera is a tree native to
southern Missouri and Texas. It is
frequently planted as an ornamental.
The bright orange wood yields a dye that is used for orange-yellows
and gold and as a base for green colors.
Amerindians used Osage Orange and it is a substitute for fustic and
aniline dyes.
These are soluble red wood dyes that have had an interesting history. One of the first red dyewoods known was
from the heartwood of Caesalpinia
sappan,
or sappanwood. The tree is indigenous
in Malaya and India and cultivated elsewhere in Asia. The wood was introduced into Europe during
the Middle Ages when it was called Bresel Wood. Then the Portuguese discovered a similar wood in South America
to which they applied the name Bresel.
The source of this Western Hemispheric Brazilwood is Caesalpinia echinata, and it ranges
generally in the American tropics.
The heartwood has a red dye that is used for woolen and cotton cloth
and for red ink. The wood is also
valuable for making violin bows. The
bark and pods also serves as a source of the dye. Sappanwood has the same uses as Brazilwood, and in both the
color is fugitive.
These are obtained from several West African trees. Baphia nitida and a related species
are usually the source of Camwood.
Barwood is obtained from Pterocarpus erinaceus and P. soyauxii. They both yield shades of red, brown, and
violet and are used mainly for dyeing wool. Red Sanderswood (Red Sandalwood)
Pterocarpus
santalinus is an East Indian tree with hard, fragrant, reddish wood that
is the source of an insoluble blood-red dye.
Indigo or Anil was known as
the “King of the Dyes,” due to the permanency and strength of its deep-blue
color. Synthetics have largely
replaced it today, however. It is
obtained from Indigofera
tinctoria of Asia and Indigofera
suffruticosa of tropical America, as
well as several other species of the same genus. These are stiff-stemmed, weedy annuals or shrubby
perennials. The dye is not present in
the plant itself. The leaves contain
a soluble colorless glucoside, indican, which oxidizes in water to form the
insoluble indigo. Fresh plants are
gathered in the flowering season, are broken up and steeped in water for 12
or more hours. The liquid is
constantly stirred to bring about complete oxidation, and the indigo
gradually settles out as a blue sediment that is packaged as small cubes for
export. Indigo was used as a dye in
India and other parts of Asia in ancient times. It was introduced into Europe in the 16th Century and from
there it spread worldwide.
Being present in all green plants, chlorophyll is especially typical
of the leaves of the higher plants from which it can be extracted with
different solvents. Chlorophyll is
important as a coloring substance for foods, soap and other products. It is especially valuable by being
harmless and serving as a deodorant.
The leaves and young shoots of Lawsonia inermis provide an orange
dye. It is a small tree 6-8 ft. tall
and native to India, Iran and the Arabian Peninsula. The plant is widely cultivated in the
tropics and subtropics as an ornamental and dye plant. Branches are cut when the tree is only
three years old and from then on twice annually. The leaves are dried and ground to a paste. Henna is a fast dye that was once used
mainly for leather and fabrics. It
also serves for dyeing eyebrows, fingernails, hair and other personal
adornment. Isatis
tinctoria is a blue dye that was used in Europe in early times. It was used by the primitive Britons to
paint their bodies (Hill 1952). The
leaves are moistened, slightly fermented, molded into balls and dried. This is a
deep yellow European dye that has been widely used for silks. It is extracted from the leaves of the
weld, Reseda
luteola, that was formerly
cultivated in Europe. The American
colonists brought the plant to America where it still persists in several
localities. Once widely cultivated in the
Mediterranean region, Madder is still grown in the Levant and in Italy. The dye is found in the roots of Rubia
tinctorum,
which occurs as a wild plant in Asia Minor, the Caucasus and Greece. An infusion is made from 3-4-year old
roots to produce a brilliant scarlet color known as “turkey red.” The coloring agent is a glucosice,
alizarin, which was one of the first synthetically produced dyes. Alkanna tinctoria roots yield a red,
violet or crimson dye that is used for coloring pomades, medicines, oils and
wines and as a histological stain. An important coloring material in
India, turmeric is obtained from the tubers of Curcuma
longa. The color is orange-red or reddish
brown. It is used to give a yellow
color to cloth and foods, such as curries.
Turmeric also is a chemical indicator as it changes color in the
presences of acids or alkalis. The crushed bark of black oak, Quercus
velutina,
of eastern North America yields a bright yellow dye called quercitron. It is used to dye cotton, leather and
wool. It is also a tanning
substance. Flavin is
a strong preparation of quercitron. Often called Chinese
Green, this is one of the few natural green coloring substances. It is the powdered bark of two Chinese
buckthorns, Rhamnus globosa and R.
utilis. It use is in dyeing cotton and silks. Carthamus
tinctorius is a dye plant native to India. The flowers are used to color food and the seeds furnish edible
oil that is low in cholesterol. The
leaves are also used in salads. The
yellow or orange thistle like heads are picked in dry weather dried and
pressed into cakes. An extracted red
dye is used for fabrics and rouge and a yellow dye is for various purposes. Crocus sativus is
the source of an old and strong yellow dye.
Native from Asia Minor to Greece it is cultivated in many parts of
Europe and Asia. The lavender colored
flowers blossom in autumn. The
stigmas and tips of the styles contain the dye. These are clipped as soon as the flowers open and are dried
naturally or by heating. At least
4,000 flowers are required to make one ounce of dye. The coloring substance is soluble in water
so it may not be used for fabrics.
However, it is useful for coloring medicines and food to which it also
imparts a typical flavor. A buckthorn, Rhamnus
infectoria,
of Southern Europe, Asia Minor and Iran, has fruits called Persian
Berries. An extract yields yellow and
green dyes. Rhamnus cathartica of Europe is also the
source of Sap Green. It is a
watercolor pigment obtained from the fruit.
It also has medicinal properties. Seeds of Bixa
orellana are the source of this dye.
The plant is an evergreen shrub or small tree native to tropical
America. Fruit is borne in the second
year and the yield averages 300-600 lbs. per tree. Each spiny pod contains 30-50 seeds surrounded by a scarlet
aril, the source of a bright yellow dye.
Annatto is almost tasteless so that it is adapted for coloring cheese,
butter, margarine and other foods. It
is also sued to dye wool and calico, paint, varnish, lacquer and soap. In South America Amerindians paint their
bodies red with Urucú, which is the same dye. The gamboge tree, Garcinia
hanburyi,
and a few other species, from Thailand exudes a gum resin that furnishes a
yellow dye. Incisions are made in the
bark that yields a yellow viscid fluid, which dries on exposure to air. The yellow dye is soluble in water,
alcohol or oil and is widely used by artists. It also gives a gold tinge to varnishes that are used for
lacquer and metalwork. Gamboge is a
violent cathartic. Several purple or blue dyes are
known by various names such as Archil, Orchil, Orseille or Cudbear. They are obtained from different species
of lichens, mainly Roccella tinctoria. These were used for wool and silks and for
staining wood and coloring wine, but now they are used mainly for drugs in
sauces and bitters. Treating
macerated lichens with ammonia and exposing them to air make the dye. A blue archil liquor is then extracted
with water. When heated to drive off
the ammonia it changes to red archil.
This is evaporated and ground to a fine power or paste known as
cudbear. Litmus is made
from the same lichens by a different process. The lichens are treated with an alkali and allowed to ferment
for a few days. Lime is added and the
dye extracted with water. The liquid
is evaporated and mixed with chalk or powdered gypsum or applied to
paper. Litmus is used as a chemical
indicator for acids and alkalis because its natural purple color is change to
red by acids and to blue by alkalis. |