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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 loka9o; 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.
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 an 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.