CHAPTER 6

BARK

CORK^[23]

Cork is the soft tissue found in the inner bark of the cork oak (Quercus suber - family Fagaceae), an evergreen oak that occurs in the western Mediterranean region. While other trees may contain layers of soft, spongy inner bark, e.g. Douglas fir (Pseudotsuga menziesii), no other tree in the world produces the thick layers of cork that Q. suber produces. Cork is a unique and important non-wood forest product that has a wide range of uses, including wine and champagne stoppers, insulation, floats for fishing nets and bulletin boards.

HISTORICAL ASPECTS

Ironically, the first people who made use of cork lived in the eastern Mediterranean, a considerable distance from where cork oaks occur naturally. In Egypt, tombs dating back thousands of years were found to contain ceramic amphorae with cork stoppers. The ancient Greeks used the bark of cork oak to make buoys to float fishing nets, for sandals and for stoppers of vessels containing wine and olive oil. The Greek philosopher Theophrastus (fourth and third centuries BC) discovered that one layer of cork was stripped from a tree, a new sheath of better quality was quickly formed. Later, the Romans put cork to a wider range of uses. The scholar Marcus Terentius Varron (116-27 BC) and the farmer Lucio Colmuela recommended making beehives out of cork because of its low heat conductivity. Pliny mentioned roofs made of corkwood plank, something that can still be seen in parts of northern Africa today. His writings also mention the use of cork to float anchor ropes and fishing nets, for sealing vessels containing oil or wine and in the production of women’s shoes. Early fishermen in the Mediterranean also used cork to fashion life jackets. Dioscorides, a Greek physician in the second century AD, described some medicinal uses of cork. He recorded a belief that charred cork rubbed on bald patches with the sap of the laurel (Laurus nobilis) would make hair grow again, thicker and darker than before.

In Portugal, where extensive forests of Quercus suber occur, cork was an economic asset of considerable national importance by the fourteenth century. Cork was first exported to England around 1307 during the reign of Dom Dinis. In 1320 this king undertook tough measures against anyone damaging “his” cork oaks. During the reign of the Portuguese king Dom Fernando, cork was one of the main exports to sail out of the port of Lisbon.

One of the most significant discoveries leading toward the development of a modern cork industry took place during the early 1700s by the French Benedictine Dom Pierre Pérignon, the proctor of the Abbey of Hautvillers, near Epernay. Dom Pierre Pérignon is the person who developed the process of champagne production. He observed that the wooden stoppers, wrapped in hemp and soaked with olive oil, used to seal the containers holding the sparkling wine, often popped out. He tested cork as an alternative stopper and had surprisingly good results. Soon cork became essential for all types of wines. Around the mid-1700s, many French wine producers were using cork stoppers for wine containers. The first cork used to make stoppers is believed to have come from the Landes, Var and eastern Pyrennees regions of France. Around 1750, in the village of Angullane, located near the French border in the Catalonian Region of Spain, the first cork stopper factory opened. Cork production increased significantly during the nineteenth century when bottles replaced wooden barrels, a method used since Roman times to store wine. Today, about 25 billion cork stoppers are used worldwide per year [Moussouris and Regato, 1999].

The replacement of corks by other stoppers that were either cheaper or more eye-catching caused a decline in the cork industry and dampened interest in cork oak culture. However, in 1891 an American by the name of John Smith developed a process for making agglomerated cork and opened up a new world of opportunities for this product. This created a demand for waste cork from the stopper industry.

Source: The Economist, 5 June 1999. Plastic wine stoppers - A corking row

CORK OAK

Quercus suber is a large, spreading tree that grows in open woodlands on sandy, chalk-free soils, with low nitrogen and phosphorus levels but rich in potassium and with a pH between 5 and 6. Ideal conditions include an annual precipitation of between 400 to 800 mm, temperatures that never drop below -5^oC and an elevation range between 100 and 300 m. These conditions are found in a relatively narrow band along the western coastal areas that bound the Mediterranean Sea. (Figure 6.1). Within this relatively small area, there are approximately some 2.2 million ha of cork oak stands (Table 6.1). Cork oaks are not found east of the Ionian Sea with Sicily and Calabria (Italy) being the eastern-most locations where cork oak occurs naturally. The eastern-most location where extensive stands of cork oaks occur is on the Italian island of Sardinia [author’s observation].

Cork oak is one of the commonest indigenous trees in Portugal where it is found throughout the country. However, five districts - Beja, Évora, Portalegre, Santarém and Setúbal - have 87 percent of the country’s total cork oak cover. In Spain, cork oak covers extensive areas of the provinces of Badajoz, Cáceres, Cadiz, Huelva, Málaga and Seville in southwestern Spain. The exploitation of cork oak in Spain began in Catalonia, in northeastern Spain, around 1790. By the first half of the nineteenth century, Catalonia’s cork oak forests were reduced by 45 percent and have never recovered. In France, the most valuable cork oak forests are found in the Var, between Cannes and Toulon. The French island of Corsica also has extensive forests of this tree. In Italy, the major cork oak resources are found on the islands of Sardinia and Sicily. On the Italian mainland, there are small areas of cork oak in the regions of Lazio, Tuscany and in parts of Calabria.

Cork oak occurs on the African continent in Algeria, Morocco and Tunisia. Algerian cork oak forests are confined to the coastal region between Algiers and Cape Roux and can be found at elevations as high as 1 500 m. In Morocco, the cork oak is found along the entire Mediterranean coast and along the Atlantic coast as far south as Marrakech. In the southern-most part of its range in Morocco, it can be found at elevations of up to 2 200 m. One of the most outstanding Moroccan cork oak forests lies just to the east of the capital city of Rabat. Tunisian cork oak forests are a continuation of the Algerian coastal strip and are concentrated in large stands in Nefza-Mogode and Khroumiria. The unique products of the cork oak and their high value have led to the introduction of Q. suber to countries outside of the western Mediterranean region with limited success. These include Russia, where the tree has been introduced into the Crimean Region; in California and other southern states of the United States; Argentina; Australia; Israel; Japan; South Africa; Turkey; and Uruguay.

Table 6.1 Area of Quercus suber forests by country^[24]

STRUCTURE AND CHARACTERISTICS

Cork is a plant tissue composed of dead cells, generally 14-sided polyhedrons, and an intercell space filled with gas virtually identical to air but lacking carbon dioxide (CO₂). It has a honeycomb-like structure that has a minimal quantity of solid matter and a maximal quantity of gaseous matter. The combination of cellular membranes and the honeycomb-like structure gives cork its unique properties.

Density

Since approximately 89.7 percent of cork tissue consists of gaseous material, the density of cork is extremely low, in the order of 0.12 to 0.20. Cork is lightweight and will float on water. For thousands of years, this has been the cork’s most evident and celebrated characteristic. In Ancient Greece and Rome, for example, cork was used in fishing equipment.

Figure 6.1 Harvesting cork on a large Quercus suber tree in Portugal.

Elasticity

The cellular membranes of cork are flexible. This makes the cork both compressible and elastic and allows it to return to its original shape after being subjected to pressure. This is the reason why cork has become an indispensable material for stoppers. The cork can be fitted perfectly against the walls of the neck of a bottle. When the cork is subjected to strong pressure, the gas in the cells is compressed and reduces in volume. When released from pressure, the cork immediately recovers its original volume and bears no trace of having been subjected to appreciable deformation.

Impermeability

The presence of suberin, a complex mixture of fatty acids and heavy organic alcohols, renders cork impermeable to both liquids and gases. Therefore, it is not subject to decay and may be considered the best seal in existence. The presence of tannins and the scarcity of albumenoids make it even more effective as a seal because it is both decay resistant and unaltered by moisture. Pieces of cork exist that have been submerged for centuries without rotting.

Low conductivity

The value of cork is further enhanced by its low conductivity of heat, sound or vibrations. This is because the gaseous component of the cork is sealed in tiny impervious compartments that are insulated from one another by a moisture-resistant material of low specific gravity. Therefore, cork has one of the best insulating capacities of any natural substance.

Other desirable characteristics

Cork is also remarkably resistant to wear and has a high friction coefficient because of its honeycomb structure. It does not absorb dust and, consequently, does not cause allergies or pose a risk to people who suffer from asthma or other respiratory diseases. Cork is also fire resistant and is a natural fire retardant.

HARVESTING AND PROCESSING

Cork oak forests are managed as either high forests or coppice. Trees must be from 15 to 20 or 30 years old (depending on different sources) to be of sufficient size and maturity before the first harvesting of cork can occur [Moussouris and Regato, 1999]. The first harvest is a low-quality cork known as borniz or virgin cork that is used exclusively for agglomerate. Subsequent harvests produce what is known as reproduction cork, which is used for bottle stoppers, washers and related products. The second harvest is usually of somewhat better quality than the first one, and the best quality cork is obtained from the third and subsequent peelings. Cork is generally only taken from branches greater than 15 cm in diameter. A tree usually stops producing good-quality cork after 13 to14 barkings [Moussouris and Regato, 1999].

Cork from cork oak forests managed as high forests is harvested at 9- to 10-year intervals. Harvesting operations generally take place from June through September. Peeling cork bark requires a high level of skill. A wrong cut that touches the inner core can wound a tree irreparably and reduce the quality of the cork of future harvests. Trees from which cork has been recently harvested have a characteristic reddish-brown coloured inner bark (Figure 6.2). The outer bark regenerates after stripping [Moussouris and Regato, 1999].

Figure 6.2 A cork oak in Tuscany (Italy) from which planks of cork have recently been harvested

For cork oaks managed under the coppice system, the trees are harvested at 10- to 11-year intervals. The logs are taken to the mill where the cork, which is always virgin cork, is separated from the wood and crushed into agglomerate [Iqbal, 1993].

Cork must be treated properly prior to industrial uses. It is only used in its raw form for traditional products such as beehives or roofing for buildings. When the planks of cork are removed from the trees, they are first sorted, and any material deemed unsuitable is set aside. The planks are then stacked and exposed to the open air for about six months. Exposure to rain, sun and wind triggers chemical transformations that improve the cork’s quality. The cork is then layered in large boilers and is boiled in water for 75 minutes in order to remove any extraneous materials and to render the planks softer and more flexible. After boiling, the planks are again stacked for another three weeks. The planks are then trimmed and graded. The best quality planks are used for production of stoppers and related products, while the remainder are used for production of agglomerates (source: http://www.mantoncork.com/aboutcork.htm).

USES

Corkwood planks are used in the manufacture of stoppers, bungs, washers, buoys, floor coverings, facings for walls and ceilings, inner soles of shoes, records, polishing blocks, protector plates and handicraft products (Figure 6.3). One of the earliest uses of cork was in footwear, a use that still is important today.

Figure 6.3 Graded cork slabs ready for next stage of processing.

Figure 6.3 Wine stoppers cut out from cork planks.

Waste cork from the stopper industry and low-quality cork are used to produce ground cork or granules. These are classified according to density and grain size. The finest are used in the production of linoleum, a product that consists of cork, linseed oil, resin, lead oxide or magnesium and colourings. It is extremely resistant to wear, easy to clean and is widely used for flooring and table coverings. Granulated cork is the base material for agglomerated cork. Its uses include champagne corks, more advanced footwear, wall coverings, flooring in the automobile and aeronautical industry, musical instruments and, most recently, as components in spacecraft. Cork granules are also used in building construction as a thermal insulator in double walls, where it can reduce heat loss by 36 percent, and in roofing, where it can reduce heat loss by 53 percent.

Other cork products include bulletin boards, drink coasters, tile floors, decorative panels, linings of safety helmets and sports equipment such as shuttlecocks and fishing rod handles.

PRODUCTION AND TRADE

Average productivity of cork oak forests, based on extraction at ten-year intervals, is approximately 150 kg of cork/ha and can be as high as 2 000 to 5 000 kg/ha in forests of large trees. Today, the centre of the world's cork oak forest is concentrated in southern Europe - France, Italy, Portugal and Spain account for 90 percent of cork oak production. North Africa has the remaining 10 percent.

Actual production of cork in the Mediterranean forests^[25] reaches 375 000 t/a, which is much less that potential production estimated at 913 500 t/a [Moussouris and Regato, 1999]. Some 600 cork-producing factories have been built in the western Mediterranean region, and they employ approximately 14 000 workers [Cesaro et al., 1995; Moussouris and Regato, 1999].

Portugal, with only 30 percent of the world’s cork oak forests, accounts for more than half of the world’s cork production. During the decade of the 1990s, Portugal’s average production was 170 000 t/a. Although Portuguese cork is exported to more than 100 countries, most of its production goes to EU countries. Annually cork and cork products sold to Europe and the United States correspond to more than US$ 1.5 billion. Cork stoppers account for US$ 1.1 billion, while the sale of agglomerated cork, cork flooring, and other cork products are valued at US$ 400 million.^[26]

During the same period, Spain’s cork production averaged 110 000 t/a; Italy, 20 000 t/a; Morocco, 15 000 t/a; Tunisia 9 000 t/a; Algeria, 6 000 t/a; and France, 5 000 t/a. In 1993 the estimated market value of cork in Spain was Ptas 4 647 million, 80 percent of which was produced on farmlands.

PESTS AND DISEASES

There have been recent reports of decline and mortality of cork oak in portions of Italy, Morocco, Portugal, Spain and Tunisia. Symptoms include death of roots and rootlets, epicormic shoots, a tarry exudation and tree death within one or two growing seasons after the onset of symptoms. In southern Spain, dying trees often occur in groups and are associated with streams, depressions or areas where standing water is common. The root fungus (Phytophthora cinnamomi) has been isolated from the root systems of symptomatic trees occurring on moist sites. It has been suggested that this fungus, which is associated with a number of tree declines [Ciesla and Donaubauer, 1994], has been recently introduced into this region and is interacting with winter drought and changing land-use patterns to bring about a decline of both Quercus suber and Q. ilex [Brasier et al., 1993].

TANNIN

HISTORICAL ASPECTS

Tanning animal hides with extracts of bark from trees is an ancient technique dating back at least 5000 years. The oldest evidence of tanning, a tanning yard with tools, pieces of skin and leather, acacia seed pods and fragments of oak bark, was discovered by the Italian Egyptologist C. Schiaparelli, and shows that the Egyptians used a vegetable tanning process similar to that used today. Tanning was depicted in Egyptian tomb paintings from 3000 BC and was known to the Chinese as early as 1000 BC. The Romans tanned with the bark from oak trees. Native Americans used a variety of local plants to make leather from hides of the American bison. The neolithic people of Europe are believed to have tanned hides by immersing them in water holes filled with bark high in tannin content.

Although tanning is an ancient industry, the actual chemicals that cause tanning were not discovered until 1790-1800 in France, when tannins were isolated as distinct chemical compounds [Prance and Prance, 1993].

COMPOSITION AND PROPERTIES

The tanning process is possible because of a property of chemicals known as tannins that allows them to combine with the protein of animal skins, known as collagen, to produce leather. This product is tougher and more permanent than unprocessed (untanned) skins.

Tannins are chemically classified into two groups: hydrolysable tannins and condensed or nonhydrolysable tannins. Hydrolysable tannins (gallotannins) are glucosides. They contain a central core of glucose or other polyhydritic alcohol with gallic acid residues attached out from the core. Condensed tannins (polyphenols) are compounds of high molecular weight. They are polyphenolic polymers apparently lacking sugars.

Tannins are acidic and astringent. This property has made them an important ingredient of traditional medicines. In addition to the production of leather, they are used in food processing, fruit ripening and are an ingredient of many beverages (e.g. cocoa, tea and red wine). When mixed with iron salts, tannins produce a black colour that has been used for ink (see Chapter 9). Tannins are also used as mordants in dye [Prance and Prance, 1993].

Tannins are derived primarily from the bark of trees and are considered to be among the most important products from tree bark. Tannins are widely distributed in the plant kingdom. About 500 plant species in 175 families are known to contain varying amounts of tannins. These compounds are particularly abundant in various species of acacias (Acacia spp.),^[27] hemlock (Tsuga spp.), oaks and related genera (Quercus, Castanea, Lithocarpus), and certain mangrove species.

PRINCIPAL SOURCES

Among temperate broad-leaved hardwoods, members of several genera of the family Fagaceae (Castanea, Lithocarpus and Quercus) have the highest bark tannin content.

Until the early part of the twentieth century, the primary source of tannins in the United States was the bark of the eastern hemlock (Tsuga canadensis), a conifer. The hemlock tannin industry was destructive and led to the devastation of hemlock forests in the northeastern United States. During the early years of this industry, only the bark of giant hemlock trees was used because there was no demand for hemlock lumber, and the trees were left in the forests to decay. As the area of hemlock forest declined, the tannin industry moved south and used the bark of Castanea dentata and various oaks (Quercus spp.) as the primary tannin source [Hergert, 1983; Prance and Prance, 1993]. In the western United States, the bark of the tan oak (Lithocarpus densiflorus) was heavily exploited for tannins [Peattie, 1953]. Eventually the American leather tanning industry relied on importation of tannin from foreign sources, or tanning was done by alternative chemical processes [Hall, 1971; Hergert, 1983].

The inner bark of the cork oak (Quercus suber) is an important tannin source, and the inner bark of large trees, which have never been stripped for cork, have the highest tannin content. The high demand for cork oak as a tannin source led to the cutting of large numbers of trees, many of which were centuries old. The fine quality of Moroccan leather can be related to the practice of using tannin extracted from the inner bark of this tree (Figure 6.4). There was such a great interest in harvesting this material that large area of Morocco’s cork oak forests suffered irreversible damage toward the end of the nineteenth century and the beginning of the twentieth century.^[28]

Figure 6.4 A leather merchant in Rabat, Morocco. The fine quality of Moroccan leather is attributed to the use of tannins from cork oak during the curing process.

In India, several species of the family Fagaceae are tannin sources. These include Lithocarpus fenestrata (common name, kala chakma), Quercus floribunda (common name, kilonj), Q. lamellosa (common name, bujrat), Q. leucotrichophora (common name, gray or ban oak) and Q. semicarpifolia (common name, kharshu oak). These trees are components of the Himalayan moist temperate forests. European and Near Eastern oaks with a high bark tannin content that have been used in tanning include Q. ilex, Q. infectoria, Q. macrolepis and Q. suber.^[29]

The bark of Eucalyptus astringents is the only eucalypt species in Australia that is sufficiently rich in tannin to warrant export. Other eucalypts that are tannin sources include E. accedens and E. wandoo. These are the principal sources of tannin in western Australia [Jacobs, 1979].

MINOR SOURCES

The bark of some species of Alnus is high in a tannin that has similar characteristics to oak tannin. The bark of European alder (A. glutinosa), for example, contains about 20 percent tannin and has been used as a tannin source in Europe, the Near East, Siberia and North America [Hora, 1981], where it has become naturalized [Duke, 1983; Harlow and Harrar, 1950]. The North American speckled alder (A. rugosa = A. incana) has also been used as a tannin source [Hora, 1981]. The bark of the common birch of Europe (Betula alba) contains only about 3 percent tannic acid but has been used extensively for tanning throughout northern Europe. It gives a pale colour to skins and is used for the preliminary and final stages of tanning [Grieve, 1931]. The bark of the European service tree (Sorbus domestica) has been used for tanning leather [Hora, 1981].

DYES

The bark of several species of temperate broad-leaved trees is the traditional source of dyes. One species of oak, the American black oak (Quercus velutina), is the source of a once important commercial dye. Procedures for dyeing with bark called for stripping it from trees, chopping it into fine pieces and boiling it [Adrosko, 1971].

QUERCITRON

Quercitron is a brilliant yellow dye that occurs in the mid and inner bark of Quercus velutina, commonly known as black oak, a tree found in eastern North American broadleaf forests. It is recognized by its thick, nearly black bark and the orange-coloured inner bark. The latter is very rich in tannic acid. An Englishman, Dr Edward Bancroft, first reported the occurrence of this dye after returning from a journey to the New World during the latter part of the eighteenth century. He named the dye material “quercitron” and suggested that it might be an inexpensive alternative to weld, a yellow dye extracted from Reseda lutea, a herbaceous plant indigenous to parts of Europe. In 1785, the British Parliament favoured Bancroft’s idea and awarded him an exclusive right to apply quercitron to dyeing and calico printing in England [Adrosko, 1971; Bancroft, 1814; Wickens, 1986].^[30]

Even before Bancroft published his discovery, American home dyers probably used the bark of Q. velutina for dyeing woollens, cottons and silks bright yellow. However, it was not until this dyestuff was introduced to Europe that quercitron took its place among the important vegetable dyes. Quercitron remained in commercial use until the second quarter of the twentieth century when it was replaced by aniline dyes [Adrosko, 1971]. An extract of quercitron, known as flavine, is free from tannin and produces brighter colours than the pulverized bark [Wickens 1986].

In 1817, quercitron bark priced for export in New York was valued at US$ 45-60/t. Apothecaries and druggists in Pennsylvania sold the dye for US$ 0.125 per pound (US$ 0.275 per kg). Quercitron also contains tannin and was used by tanners as well as dyers. Therefore, home dyers were able to purchase the material from tanners for as little as US$ 0.05 per pound (US$ 0.11 per kg) [Adrosko, 1971].

A wide range of colours can be produced on wools, cottons and silks from quercitron by combining it with other dyestuffs or by using different mordants. For example, a mixture of quercitron with cochineal, a red dye extracted from the female adult of a scale insect (Dactylopius cocus), which infests prickly pear cactus (Opuntia spp.), produces a brilliant orange dye [Adrosko, 1971; Wickens, 1983] (Table 6.2). Quercitron has also been used to dye splints and reeds for baskets [Bliss, 1981].

Unfortunately, this dye is no longer produced, not even for home dyers who prefer to work with natural rather than synthetic dyes [author’s observation].

Table 6.2 Range of colours available from quercitron, a commercial dye from the inner bark of Quercus velutina

Source: Lust, 1990

TRADITIONAL DYES

The bark of a number of trees of the family Fagaceae were traditional sources of dye and used by dyers in various parts of the world (Table 6.3). Several native North American oaks, in addition to Quercus velutina, were traditional dye sources and used by dyers in the eighteenth and nineteenth centuries. For example, the bark of northern red oak (Q. rubra) produces a yellow dye; chestnut oak (Q. montana) produces red colours; and the bark of Q. alba was used to colour wool brown or a tea colour. The dye produced from the bark of Q. alba reportedly does not fade when exposed to the sun [Adrosko, 1971]. In Japan, extracts of the nutshell, burr and bark of Castanea crenata are used for dyeing or staining. When mordanted with water containing iron, a grey colour is obtained. When ash extract is added to the iron water, a chestnut-brown colour is obtained [Kamazaki, 1984]. Other traditional dyes produced from the bark of trees of the family Fagaceae are summarized in Table 6.3.

The bark of red maple (Acer rubrum) was used during the eighteenth century in North America in combination with a copperas mordant to dye worsted and linen fibres a slate blue-grey colour. When used with an alum mordant, it produced a cinnamon-brown colour. This dye bath was used to colour woollen and cotton fabrics [Adrosko, 1971].

The Navajo Indians of the southwestern United States used the bark of alder (Alnus tenufolia) to produce dyes ranging in colour from golden tan to dark olive green [Bliss, 1993]. The bark, as well as the nut husks, of various species of Juglans is known to produce a rich brown dye (see chapter on Fruits and Nuts) [Adrosko, 1971].

The root bark of the Osage orange (Maclura pomifera, family Moraceae), a small to medium-sized tree indigenous to the south-central United States, produces a dye with colours ranging from tan to olive green, depending on the mordant, and was widely used during the First World War for dyeing khaki military uniforms [Bliss, 1993].

Table 6.3 Traditional dyes produced from the bark of trees of the family Fagaceae

CASCARA

Cascara (Rhamnus [=Frangula) purshiana, family Rhamnaceae) is a small- to medium-sized tree found primarily along the Pacific Coast of Canada and the United States. The reddish-brown coloured bark of this tree is considered to be one of the most important natural drugs produced in North America. Cascara is widely used in the production of laxatives and tonics and is marketed under a Spanish name “cascara sagrada” [Lust,1990; Panshin et al., 1950]. It was accepted as an important natural medicine by the medical community in 1877 [Panshin et al., 1950] and is the most widely prescribed naturally derived laxative today [Leung, 1977]. Cascara is found in nearly 200 products sold in Canada including some veterinary medicines [Prescott-Allen and Prescott-Allen, 1986].

Cascara bark collection is a local industry in Oregon and Washington (United States) and British Columbia (Canada). Collecting usually begins in mid-April and extends to late August. Yields vary from about 2.25 kg from a tree 7.5 cm in diameter at breast height to about 80 kg from a tree with a diameter at breast height of 42- 44 cm. Daily yields per harvester range from about 45 kg per day to about 135 kg per day. Cascara bark harvesting is often excessive and has been known to kill trees [Prescott-Allen and Prescott-Allen, 1986].

One estimate places the domestic market for cascara bark in the United States at 2 000 t/a with a wholesale price of US$ 0.05-0.80 being paid to the harvester, depending on the age of the bark, its moisture content, time of year and quantity shipped [Thomas and Schumann, 1992]. The dried bark, aged for a year before being used as a laxative, commands a higher price [Prescott-Allen and Prescott-Allen, 1986].

The bark of a European species (Rhamnus fragula) has essentially the same properties as R. purshiana and is also harvested for its medicinal properties, but to a lesser degree [Grieve, 1931; Panshin et al., 1950].

SAPONIN

The rough bark of Quillaja saponaria (common name quillay), a tree which occurs in the sclerophyllous forests of central Chile, is a locally important source of a product known as saponin [Donosa Zegers, 1983]. Saponin has a variety of domestic and industrial uses, including photographic products, cosmetics, carbonated beverages, dental products and shampoos. The saponin extracted from the quillay is also an essential ingredient in the manufacture of anti-explosives in fuels used to propel space vehicles [Garfias Salinas et al., 1995].

Only the outer layers of bark are harvested. A three-person crew can harvest between 150 and 200 kg of quillay bark per day. When the bark is dried to a moisture content of approximately 15 percent, it is packed in clean sacks and transported to a storage facility that is dark and cool. It is then packed in containers weighing approximately 80 kg. Quillay harvesting occurs primarily in the V to VIII regions and takes place on about 4 000 ha/a [Garfias Salinas et al., 1995].

MINOR AND TRADITIONAL USES

The ability of the bark of willow (Salix spp.) to relieve the suffering of pain and fever has been known for at least 2 000 years. The active ingredient in willow bark is salicin, a glucoside that is converted to salicylic acid in the body. Salicylic acid is a component of acetylsalicylic acid, the active ingredient in aspirin, one of the most widely used medicines in the world (see textbox). Today aspirin is produced from synthetic materials [Lust, 1990].^[31]

The bark of the North American paper birch (Betula papyrifera) (Figure 6.5) is impervious to water and was used by indigenous tribes in the construction of small, narrow boats known as canoes. Canoes were constructed from large sheets of bark tied together with root fibres of white spruce (Picea glauca) and smeared with the resin of balsam fir (Abies balsamea) [Hora, 1981]. Containers made from the bark of paper birch were also used as containers for collecting sap during the early days of maple sugaring (see Chapter 5).

Figure 6.5 The bark of the North American paper birch (Betula papyrifera). Birch bark had many traditional uses in northern Europe, Siberia and across northern North America.

The Anishinaabeg, an indigenous tribe in the Great Lakes Region of North America, shaped birch bark for cups, bowls, baskets and trays by heating the bark and bending it to the shape needed. It would retain its shape when cooled. Folds were pierced and sewn together with bark of basswood (Tilia americana) or roots of spruce (Picea glauca). When a watertight container was needed, the seams were caulked with spruce resin. Food was said to keep better in birch bark containers than in containers made from other materials. Berries and corn were dried on sheets of birch bark. Blankets, supplies and equipment were wrapped and bound in birch bark mats for the long and frequent moves of these people. They also wrapped their dead in birch bark, a practice that was also carried out by certain Siberian tribes [Peyton, 1994].

The aboriginal people make traditional products out of birch bark such as quill baskets, lamp shades, birch bed frames and birch bark desk coverings. The bark is sold in two forms: sheets and tubes. Birch bark sheets are cut in various sizes. Bark is also sold by case.

Birch bark is also used as a component in the floral industry, as well as in the design of artificial trees that are incorporated into large commercial spaces such as convention centres and shopping malls. There are markets for birch bark in Canada, United States and Hong Kong. The estimated demand of the birch bark in Hong Kong as a component in the floral industry is estimated at 62 000 ft² [National Aboriginal Forestry Association, 1999].

In Siberia and North America, strips of birch bark with slits cut just wide enough for the wearer to see through and bent down to produce just below the eyes, were worn to prevent snow blindness before dark sunglasses were invented. Shoes made of birch bark were standard footwear for the poor in medieval northern Europe [Peyton, 1994].

The twigs and bark of the European birch (Betula alba) yield an oil known as oil of birch tar. This oil is used in Russia as a preservative for leather and gives Russian leather its distinct fragrance [Hora, 1981]. This oil also imparts durability to leather. Old books with leather covers that are treated with birch oil will not mould. The production of birch tar oil is a major Russian industry [Grieve, 1931].

The twigs and bark of sweet birch (Betula lenta), a tree native to portions of the eastern United States, yield oil of wintergreen. The twigs of this tree are pleasant to chew or can be used as toothpicks [author’s observation]. This product is still harvested from B. lenta in small quantities; however, a synthetic product has now largely replaced it in commercial trade [Harlow and Harrar, 1950].

According to research results, birch bark has a medicinal potential, more precisely, it is referred to as betulin (15% of the birch bark). This compound can be adapted into betulinic acid and cure and control certain health problems. Betulin in itself may have medicinal properties as well. The University of Minnesota-Duluth (UMD) and its Natural Resources Research Institute (NRRI) hold patents related to the use of betulin from birch bark. Betulin represents a potential weapon against melanoma tumours. Fifty pounds of bark might produce 100 doses of betulinic acid, and a betulin-based treatment might be more effective than the current most popular herpes treatment [Lemay, 1999].

The inner bark of several Asian, European and North American species of Tilia (T. americana, T. cordata, T. japonica and T. tuan) are used for making mats, cordage and, when plaited, shoes [Hora, 1981].

The aromatic bark and roots of sassafras (Sassafras albidum) were at one time widely used in eastern North America for a tea and as a folk medicine. One of its medicinal uses was as a tonic. There was an old saying in the southern Appalachian Mountains: “Drink sassafras during the month of March and you won’t need a doctor all year.” It was also used as a blood purifier and to sweat out fevers [Wigginton, 1973]. Sassafras bark and roots were also used for flavouring tobacco, for a beverage known as root beer and as a treatment for lice and insect bites. The aromatic oil contained in sassafras bark and roots contains 80 percent of the phenolic compound safrole and is a potential carcinogen. The use of oil taken from the roots and bark of sassafras in food for human consumption has been banned for a number of years because of health risks [Coppen, 1995; Hora, 1991].

The inner bark of slippery elm (Ulmus rubra), a species indigenous to eastern North America, is mucilaginous and was chewed by early pioneer woodsmen to quench the thirst [Harlow and Harrar, 1950]. It is also used in medicine as a demulcent to sooth irritations, such as sore throats or inflammations of the digestive tract [Hora, 1981; Lust, 1990]. Slippery elm bark is still gathered for this purpose and there is a domestic market for approximately 100 t/a of this material at a wholesale price to the harvester of US$ 5.50 per kg [Thomas and Schumann, 1992].

Source: http://www.inreach.com/dameron_heart/aspirin.htm