tides flow in and out, so the turbines must be able to function in either direction; second, most power is generated during the two high and two low tides that occur each day, but these variable times are usually not synchronized with the peak demand times. To solve the first problem, the French engineers devised turbines thatcan run ineither direction to accommo- date both incoming and outgoing tides. The second problem was solved by using the turbines as pumps, making it possible to use surplus electricity to fill the reservoir behind the dam to a greater depth than the highest level of the tides. Later, this extra water could be released to turn the turbines at a time when power is most needed, rather than when the ocean recedes. The operating policy of this plant was to optimize the value of the power produced rather than maximizing the amount. The annual energy output from this plant is 600 million kilowatt-hours, about 0.012 per- cent of France’s total energy consumption. In the decades after the Rance tidal generating sta- tion began producing electrical energy, the device not only has been remarkably trouble-free but also has provided four indirect benefits: the development of the large reversible turbines, the perfection of cor- rosion control techniques, a roadway linking St. Malo and Dinard across the dam, and increased tourism. Although the major motivation to build the Rance tidal power station was the fear of future power short- ages, this plant has served as a prototype model for tidal power plants as well as an invaluable source of in- formation for future worldwide development. Other Tidal Power Plants Modeled after France’s Rance tidal station and using the French generators, the first Russian tidal power station was installed in 1968, in the Kislaya Gulf, where the tides can rise as high as 20 meters. This facility was abandoned in the mid-1990’s becasue of financial dif- ficulties, but after a ten-year hiatus, it was modernized and began producing 400 kilowatts of power. China has been developing and installing tidal power stations for several decades. As of 2009, eight stations, with a total capacity of 6,120 kilowatts, have been realized. The largest single unit, an experimen- tal station in Zhejiang Province, which became opera- tional in 1980, is capable of producing 3,200 kilowatts of power. An alternate means of using the tides to create en - ergy at less cost and without the possible environmen - tal impacts of barrage systems is to use underwater generators rotated by the force of the water current as the tides come in and recede. Operating on the same principle as wind turbines, these systems produce rel- atively constant and reliable energy without the argu- able visual pollution of wind turbines. Several Euro- pean countries are presently focused on developing these current-driven devices because they do not re- quire the construction of large, costly dams and seem to have minimal environmental impacts. The requi- site hydrokinetic prototype devices in the 100-kilowatt range have been developed and are being tested and evaluated in European waters. One-megawatt com- mercial units were expected to be available by the 2010’s. Because energy derived from a flowing fluid is di- rectly proportional to the cube of the flow velocity, a flow rate of at least 5 knots is essential for effective power production (doubling the flow velocity pro- duces eight times the power). For these systems the current velocity is more important than the tidal range; a high tidal range complicates these systems because the vertical location of the units would need to be adjusted as the elevation of the sea level varies. After three decades of research, Asia’s first tidal current power station, located in the Zhejiang Prov- ince of China, came online in January, 2006. This sys- tem uses tidal currents typically flowing between 6 and 13 feet per second to produce 40 kilowatts of power. The world’s first substantial tidal current sys- tem, with a total 1.2 megawatts capacity, installed in NorthernIreland’s Strangford Lough, became opera- tional in July, 2008. These systems will serve as proto- types for future large-scale commercial applications and will validate this technology as environmentally benign and economically competitive. Proposed Tidal Power Plants Although about only twenty sites in the world have been identified as viable options for possible tidal power-generating stations of the Rance type, there are hundreds of sites where the water-current type of power stations would be feasible. In the United States, tidal power units of both types are under serious con- sideration. During the 1930’s, the Passamaquoddy Tidal Power Development Project received New Deal development funds for a tidal station based in Cobs- cook Bay, Maine, where the tides typically vary by 6 meters. After several years of construction,the project was abandoned because of economic concerns,politi - cal complications, and potential problems integrat - 1226 • Tidal energy Global Resources ing the system into the existing regional electric grid. The concept for a tidal power plant in this region was briefly re- vived during the energy crises of the 1970’s, but although deemed techno- logically feasible, it was again canceled because of economic concerns. During the first decade of the twenty- first century, two apprehensions rekin- dled interest in this project. One was the rising cost of the fossil fuels, and the second was increased anxiety about fossil-fuel-induced global warming. Consequently, a study commenced on the feasibility of constructing a huge dam with three hundred 16-foot-diame- ter generators for a tidal power plant in northern New England. There is also considerable interest in installing tidal current devices in locations such as the harbors of San Francisco and New York City, where dams are unfeasible. Russia possesses a huge potential for developing tidal energy, theoretically enough to provide for the entire coun- try’s electrical energy requirements. Two regions, Kola Bay and the Okhotsk Sea coast, could provide 100 gigawatts of power by Rance-type tidal stations. Several new tidal plants for industrial purposes are planned to be constructed on the Okhotsk and Beloe seas. The Beloe Sea unit will be a 10-megawatt device, with plans to eventually achieve 20,000 megawatts. In the Okhotsk Sea the tides can reach 17 meters, allow- ing the proposed plant to output 20 gigawatts, with a peak capacity of 90 gigawatts. Because of China’s increasing demands for energy, as of 2009, several large-scale tidal generating stations were under way, while still more were in the plan- ning stages. The proposed 300-megawatt generating station at the mouth of the Yalu River will become the world’s largest tidal plant by exceeding the 240- megawatt capacity of Rance. Because the tidal-barrage approach is not feasible in this region, an approach termed “tidal lagoons” is being used. The tidal la- goons use a rubble mound impoundment structure and hydroelectric generating equipment located at least 1.6 kilometers from the shoreline, thus avoiding the expense of a huge dam as well as possible ecologi - cal disadvantages. Eight possible sites for tidal power stations have been identified in Great Britain; the most feasible and most studied site is the Severn estuary, where the tidal range can be as high as 14 meters. Actualizing this proj- ect would require the construction of a 16-kilometer- long dam, forming a bridge between England and Wales and trapping 420 square kilometers of water, which would make this the United Kingdom’s great- est engineering project since the Channel Tunnel. By incorporating 214 40-megawatt turbines, this power plant would produce an average power of 2 gigawatts, with a peak of 8.6 gigawatts, providing the energy equivalent of eight large coal-fired power plants and potentially reducing carbon dioxide emissions by 16 million metric tons annually. Several environmental organizations are vehe- mently opposed to this project, claiming the irrevers- ible effects would be devastating to migratory birds, which depend on the marshy mudflats; would acceler - ate coastal erosion in some areas; and would increase silting in other regions. This huge dam might also pre - Global Resources Tidal energy • 1227 An underwater turbine used to generate tidal power in the United Kingdom. (PA Photos/Landov) vent salmon and other migratory fish from reaching their spawning grounds. Plans are also in progress to construct a tidal power station in Garolim Bay, on the western shore of South Korea. Although the tides average only 2.4 meters, compensation would be achieved because water would be trapped in a 85-square-kilometers area, four times the area of the Rance estuary. With a generating ca- pacity of 520 megawatts, this would provide twice the maximum power of the Rance power plant. In 1981, the project was assessed as economically feasible, but the reduced cost of oil in the mid-1980’s caused the project to be shelved. When the cost of oil rose rapidly in the early 2000’s, reassessment deemed the proj- ect viable; the fundamental design was completed in 2007. Because Garolim Bay is an important spawning ground for many species of fish, environmentalists and local fishermenopposethis project. Nonetheless, the project continued to progress, as it was included in the government’s 2008 renewable energy strategy. However, the Korean Federation for Environmental Movement censured the project, asserting that the spirit of renewable energy is violated by a power plant that destroys valuable tidal flats and severely disrupts the local ecology. Desirable and Undesirable Aspects of Tidal Power Plants Although tidal energy is a reliable and plentiful alter- nate energy source, convertingit into useful electrical power is not always easy or desirable. The positive at- tributes of tidal energy are that it is renewable, it does not consume nonrenewable resources, it produces no noxious wastes, it does not contribute to global warm- ing, and it does not create thermal pollution. There is no slag or fly ash to dispose of, there areno radioactive waste products to be stored, and there is virtually no aesthetic pollution. Other advantages are the longev- ity of the plants and their reliability. Tidal plants have a practical life of at least seventy-five years, compared with thirty years for a fossil-fuel plant and twenty-five years for a nuclear plant. Tidal power stations are also more reliable, because the tidal cycle is extremely reg- ular. Less reserve equipment is needed because there are many small generating units rather than a few large ones. If one unit goes off line, the power reduc- tion is small. Finally, the water dammed during high tides provides an almost ideal lake for water recre - ation sports. On the negative side, tidal power can provide only a fraction of the world’s energy needs. Tidal dams, ex - cept in special cases, are quite expensive to construct, cannot work continuously, affect a large area, and may cause ecological disruptions to marine creatures and wildlife. Furthermore, large tide differentials are essential for productivity. Although there may be rela- tively few places where tidal dams may be economi- cally constructed, there are many more options for utilizing underwater current flow turbines. Not only are they less expensive; the environmental impact is minimal. There is an urgent need for the development of al- ternate sources of energy,particularly sources that are nonpolluting. Tidal power not only is a realistic and realizable concept but also could be a valuable com- plement to the world’s increasing use of alternate en- ergy sources, such as solar and wind. As dwindling supplies of renewable resources continue to escalate in cost, and as global warming from fossil fuels be- comes a more acute problem, the technological and economic barriers to increased utilization of tidal en- ergy are diminishing rapidly. Tidal generating sta- tions stand posed to become a major contributor to the world’s future alternate-energy mix as humanity overcomes its addiction to fossil-fuel energy. George R. Plitnik Further Reading Charlier, Roger Henri, and Charles W. Finkl. Ocean Energy: Tide and Tidal Power. London: Springer, 2008. Charlier, Roger Henri, and John R. Justus. “Is Tidal Power Coming of Age?” In Ocean Energies: Environ- mental, Economic, and Technological Aspects of Alterna- tive Power Sources. New York: Elsevier, 1993. Clancy, Edward P. The Tides: Pulse of the Earth. Illus- trated by Warren H. Maxfield. Garden City, N.Y.: Doubleday, 1968. Clark, Robert H. Elements of Tidal-Electric Engineering. Hoboken, N.J.: Wiley-Interscience, 2007. Congressional Research Service. Energy from the Ocean. Honolulu, Hawaii: University Press of the Pacific, 2002. Goldin, Augusta. Oceans of Energy: Reservoir of Power for the Future. New York: Harcourt Brace Jovanovich, 1980. Gray, T. J., and O. K. Gashus, eds. Tidal Power: Proceed- ings of the International Conference on the Utilization of Tidal Power, 1970, Nova Scotia Technical College. New York: Plenum Press, 1972. 1228 • Tidal energy Global Resources Peppas, Lynne. Ocean, Tidal, and Wave Energy: Power from the Sea. New York: Crabtree, 2008. Thirring, Hans. Energy for Man: From Windmills to Nu- clear Power. 1958. Reprint. Bloomington: Indiana University Press, 1976. Web Site U.S. Department of Energy Renewable Energy: Ocean Tidal Power http://www.energysavers.gov/renewable_energy/ ocean/index.cfm/mytopic=50008 See also: Hydroenergy; Ocean current energy; Ocean thermal energy conversion; Ocean wave energy; Oceans; Renewable and nonrenewable resources. Timber. See Forests; Timber industry; Wood and timber Timber industry Categories: Obtaining and using resources; plant and animal resources Increasing demand for timber and forest products has resulted in loss of natural forest cover in many regions of the world. As global demand for wood has grown, in- ternational agencies have found that reports of unsus- tainable and illegal harvesting practices have also risen in number. Background The timber industry is composed of a diverse group of companies and organizations utilizing wood and fiber harvested from forests in the production of solid wood products (such as furniture and lumber), reconsti- tuted wood products (such as particleboard),pulp and paper, and chemicals. In addition, many other com- mercial products are derived from forest resources, in- cluding types of medicine, food, specialty items such as Christmas trees, and fuel. A surprisingly high percent- age of wood harvested is used solely for fuel, either as firewood or as charcoal. In 2005, the Food and Agri - culture Organizationof the UnitedNations (FAO) es - timated that approximately one-half of all harvested wood in the world is used for fuel and that the major - ity of energy needs in many developing countries is met by fuel wood, although this number declined slightly after the mid-1990’s. While the amount of fuel wood harvested may have declined, global demand for timber overall continues to rise, with China emerg- ing as the world’s leading consumer. Historical Significance The development of the forest products industry par- allels the development of Western civilization. From Robin Hood to Paul Bunyan, the utilization of forest products is ingrained in Western mythology and cul- ture. Development of the first forest management techniques in the Middle Ages was motivated by secu- rity interests related to the continued availability of wood for shipbuilding. Royal foresters planned for needs that might not arise for centuries. Forest man- agers in Great Britain, for example, planted oak trees in the sixteenth century to ensure that a supply of ships’ timbers would be available one hundred years or more into the future. In the Americas, the westward movement of Euro- pean settlement was accompanied by, and in some cases motivated by, the development of the forest- products industry. The first products shipped back to Europe included forest products such as ships’ masts, ships’ timbers, potash, and tannin. When explorers and settlers encountered the old-growth forests of North America, conservation principles that had been emerging in the Old World were ignored in the New World. The forests of New England, the Deep South, and other sparsely populated areas seemed so inex- haustible that loggers clear-cut and then moved on with little thought that the resource might ever be ex- hausted. Eventually, first in Europe and then in North Amer- ica, the realization that natural forests could indeed be depleted came to the forefront. Government and industry realized that it was necessary to develop tech- niques for regenerating and managing forest ecosys- tems to ensure a continued supply of wood products to meet human needs. This process is still occurring in many developing countries, as logging advances into areas that had experienced only limited harvest- ing for millennia. Forests also continue to be lost to agriculture, and, indeed, according to FAO assess- ments, clearing land for agricultural development continues to be the leading reason for the loss of trop - ical rain forests in developing nations. Global Resources Timber industry • 1229 Old-Growth Forests All ecosystems develop within the context of natural disturbance cycles. Whether the natural agent is fire, flooding, or windstorms,every hectare on the Earthis subject to periodic disturbance even without the in- fluence of human activity. The disturbance intervals may be very long in some systems; forests consisting of late-successional species that have not been disturbed for an extended interval are referred to in common language as old-growth forests. The forest-products industry developed through the utilization of these natural forests. As these re- sources became scarce, forest management techniques were developed to ensure the restoration of forests following utilization. As old-growth forests containing large trees were depleted, manufacturing technology changed to use smaller material that could be har- vested from second-growth forests. This led to the development of composite wood products such as plywood, oriented strand board (often referred to simply as OSB), medium density fiberboard (MDF), particleboard, and laminated beams. OSB, MDF, and particleboard are often made from the materials, such as sawdust and chips, left after logs are sawn into lumber. Prior to development of these products, waste material in sawmills was generally disposed of by burning. Sustained Yield Humans obtained goods and services from natural forests for millennia before increasing population, the development of agriculture, and improvements in technology allowing larger and faster harvests began to lead to the depletion of natural forests. Fear of the depletion of natural forests and an impending timber famine led to development of the sustained-yield con- cept, which holds that forests should be managed to produce wood products at a rate approximately equal to the natural rate of biological growth. The develop- ment of the sustained-yield concept was associated with the belief that properly managed forests could produce a continuous, never-ending flow of wood and fiber. This concept is still evolving to include recogni- tion that the continued survival of all species and the maintenance of ecosystem structure and function, as well as the production of goods and services, are of vital interest to human society. In addition to managing natural forests to provide for sustained yield, researchers have developed hy- brids and fast-growing strains of desirable species, such as loblolly pine in the United States and eucalyp- tus in Australia, for use in plantations. The 2002 FAO Global Forest Resources Assessment noted that be - tween 1990 and 2000, the number of hectares world - wide devoted to tree farming increased 428 percent, 1230 • Timber industry Global Resources U.S. Timber-Based Industries, 2002 Paid Employees Payroll (millions of dollars) Shipment Value (billions of dollars) Millwork 151,245 4,416 22.6 Paper mills 102,571 5,700 45.2 Paperboard 184,884 7,091 43.5 Paperboard mills 48,005 2,667 21.2 Pulp mills 8,043 487 3.7 Sawmills 95,724 3,124 21.4 Veneer, plywood, & engineered wood products 114,300 3,681 20.2 Wood preservation 12,321 372 4.5 Other converted paper products 43,042 1,611 14.0 Source: U.S. Census Bureau, 2002 Economic Census, Manufacturing, General Summary, 2005. from 43.6 million to 187 million hectares. Plantation forests have the advantage of allowing for ease in har- vesting and for providing for a predictable volume of timber. There are, however, a number of disadvan- tages to the typical forest plantation, including loss of diversity in the plantation (both floral and faunal) and an increased risk of disease and insect infesta- tions. Effects of Timber Harvesting Harvesting forest products in such a way as to mimic natural disturbance and to ensure the continued func- tioning and survival of all ecosystem components is possible. However, many examples exist of harvesting that have led to long-term disruption and alteration of ecological processes. Nutrient loss, erosion, and loss of species following poorly designed or improp- erly implemented harvesting operations can result in the loss of biodiversity and a reduction in long-term productive capacity. The removal of forest-canopy trees, whether through harvesting or natural disturbance, leads to increased soil temperature, increased decomposition, increased leaching of nutrients and soil carbon, and, if extreme, a reversion to an early-successional plant community. Removal of the canopy trees will usually lead to increased erosion, which, if harvesting is not properly implemented, can be severe and result in degradation of water quality and aquatic habitat. In- creased runoff from lack of forest cover can also lead to flooding downstream from cleared areas. Devastat- ing floods in Bangladesh, for example, may be becom- ing more frequent and severe because of clear-cutting of forests in the Himalayas in India. Practices meant to improve forest health and en- courage sustainable harvesting have had unintended consequences. For example, twentieth century pro- grams promoting fire protection resulted in the inter- ruption of natural disturbance cycles in many ecosys- tems. In these cases, artificial disturbance through harvesting may be the only way to ensure the contin- ued presence of early-successional species in the land- scape. In many cases, these early-successional tree species are fast growing, straight, and relatively easy to artificially plant and regenerate. These early- successional forests are ideally suited for the produc- tion of pulp and paper, fuel wood, and such products as posts and poles. The challenge to industrial and public-land managers is to develop the appropriate mix of all successional stages in the landscape in order to ensure the continued survival of all species and the maintenance of ecosystem structure and function, while allowing for utilization to meet the needs of the globally expanding human population. As the global demand for timber continues to rise, illegal and unsustainable logging practices have also risen. International agencies such as United States Agency for International Development and FAO have documented numerous cases of the illegal harvesting of timber in national parks and preserves, particularly in developing nations such as Gabon and Indonesia. David D. Reed, updated by Nancy Farm Männikkö Further Reading Baldwin, Richard F. Maximizing Forest Product Resources for the Twenty-first Century: New Processes, Products, and Strategies for a Changing World. San Francisco: Miller Freeman Books, 2000. Bettinger, Peter, et al. Forest Management and Planning. New York: Academic Press, 2008. Bowyer, James L., Robin Shmulsky, and John G. Haygreen. Forest Products and Wood Science: An Intro- duction. Drawings by Karen Lilley. 5th ed. Ames, Iowa: Blackwell, 2007. Ellefson, Paul V., and Robert N. Stone. U.S. Wood- Based Industry: Industrial Organization and Perfor- mance. New York: Praeger, 1984. Evans, Julian, and John W. Turnbull. Plantation For- estry in the Tropics: The Role, Silviculture, and Use of Planted Forests for Industrial, Social, Environmental, and Agroforestry Purposes. New York: Oxford Univer- sity Press, 2004. Gane, Michael. Forest Strategy: Strategic Management and Sustainable Development for the Forest Sector. New York: Springer, 2007. Klemperer, W. David. Forest Resource Economics and Fi- nance. New York: McGraw-Hill, 1996. Peck, Tim. The International Timber Trade. Cambridge, England: Woodhead, 2001. Richards, E. G., ed. Forestryand the Forest Industries, Past and Future: Major Developments in the Forest and Forest Industry Sector Since 1947 in Europe, the U.S.S.R., and North America. Boston: M. Nijhoff for the United Nations, 1987. Sills, Erin O., and Karen Lee Abt, eds. Forests in a Mar- ket Economy. Boston: Kluwer Academic, 2003. Smith, Wynet, et al. Canada’s Forests at a Crossroads— An Assessment in the Year 2000: A Global Forest Watch Canada Report. Washington, D.C.: World Resources Institute, 2000. Global Resources Timber industry • 1231 United Nations Food and Agriculture Organization. FAO Yearbook of Forest Products, 2002-2006. Rome: Author, 2008. _______. State of the World’s Forests 2009: Adapting for the Future—Society, Forests. and Forestry. Rome: Author, 2009. Web Sites United Nations Food and Agriculture Organization State of the World’s Forests 2009 http://www.fao.org/docrep/011/i0350e/ i0350e00.htm U.S. Forest Service, U.S. Department of Agriculture U.S. Forest Products Annual Market Review and Prospects, 2004-2008 http://www.fpl.fs.fed.us/documnts/fplrn/ fpl_rn305.pdf See also: American Forest and Paper Association; Forest management; Forestry; Forests; Land ethic; National parks and nature reserves; Sustainable de- velopment; United Nations Framework Convention on Climate Change; Western Wood Products Associa- tion; Wood and charcoal as fuel resources; Wood and timber. Tin Category: Mineral and other nonliving resources Where Found Although tin is widely distributed throughout the Earth’s crust, the average concentration is very low, less than 0.001 percent. The primary ore mineral is cassiterite (SnO 2 ) which generally occurs in granitic igneous rocks, associated hydrothermal veins near ig- neous rocks, or the weathered and eroded debris of granitic rocks. The major producers of tin are China, Indonesia, and Peru. Primary Uses Traditionally, tin has been used in the production of tin plate, used in making food containers. Tin is also al - loyed with lead to make solders; with copper to make bronze;and with lead, brass, or copper to make pewter. Technical Definition Tin (chemical symbol Sn) is a silver-white metal that belongs to Group VIA of the periodic table. Tin has an atomic number of 50 and an atomic weight of 118.65. Tin comes in two forms (allotropes): gray (alpha) tin and white (beta) tin. Gray tin, a face-centered crystal- line structure with a density of 5.75 grams per cubic centimeter, changes into white tin at 13.2° Celsius. White tin has a body-centered tetragonal structurewith a density of 7.28 grams per cubic centimeter. The melt- ing point of tin is 231.97° Celsius, and the boiling point is 2,270° Celsius, giving this element one of the largest temperature ranges for a liquid metal. Tin has ten sta- ble isotopes, the highest number of any element. Description, Distribution, and Forms Tin is a widely distributed elementin the Earth’s crust and can form both inorganic and organic com- pounds. Tin generally forms two series of inorganic compounds. Since tin has two valence states, II and IV, the inorganic compounds are built using these two states of tin. Some of the more commercially impor- tant compounds of tin (II) include stannous chloride (SnCl 2 ), stannous oxide (SnO), and stannous fluo- ride (SnF 2 ). The most common tin (IV) compounds are stannic oxide (SnO 2 ) and stannic chloride (SnCl 4 ). Tin can also form compounds with carbon; more than five hundred organotin compounds are known. Some of these compounds are nontoxic and are used extensively as stabilizers for polyvinyl chlo- ride. Other organotin compounds are toxic and are used as biocides in fungicides and disinfectants. Although tin is widely distributed throughout the crust of the Earth, it is concentrated in ore deposits in three main regions. The primary tin regions are from the Korean Peninsula through China to Southeast Asia; Thailand to Indonesia; and Peru, Bolivia, and Brazil. The tin deposits found in these regions are as- sociated with granitic igneous rocks, related hydro- thermal veins, or stream deposits (placers) that con- tain the eroded tin minerals. More than 99 percent of the tin produced through history has been derived directly or indirectly from granitic rocks. The tin- bearing granitic rocks were formed near convergent plate boundaries in or near subduction zones. Some of the granitic rocks produced in the subduction zones contain the tin ore mineral cassiterite; com- monly the associated hydrothermal veins also carry cassiterite. These hydrothermal veins tend to be rich in fluorine and boron and often contain minerals 1232 • Tin Global Resources such as tourmaline,apatite,fluorite, and topaz. Cassit - erite is also found associated with the minerals ar- senopyrite, molybdenite, and wolframite. Lode de- posits of tin are found primarily in Bolivia and in England. The Bolivian lode tin ores also contain the rare tin minerals stannite (CuFeSnS 4 ) and cylindrite (PbSn 4 FeSb 2 S 14 ). These vein deposits are worked by underground mining techniques used in mining many other base metals. History Early civilizations used tin as an alloy with copper to make bronze. The earliest bronzes appear to have been an alloy of copper and arsenic, but the later dis - covery of tin/copper bronzes yielded a safer and better material. The earliest tin bronze items were probably produced by the peoples of the Middle East as long ago as 3500 b.c.e., and Egyptian bronze arti- facts date back to at least 3000 b.c.e. Although these items contain about 10 percent tin, it is unlikely that these early people knew of tin as a distinct and sepa- rate metal. No evidence of smelting of pure tin from ore or use of the metal by itself has been discovered. Rather, tin ores may have been added to the copper ores when smelting the copper, and the resultant liq- uid contained both copper and tin. Global Resources Tin • 1233 Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009.Source: Mineral Commodity Summaries, 2009 3,000 100,000 2,000 38,000 100 2,000 100 3,500 4,000 Metric Tons of Tin Content 175,000150,000125,000100,00075,000 50,000 25,000 Vietnam Portugal Peru Malaysia Indonesia Congo, Democratic Republic of the Russia Thailand Other countries 2,000 150,000China Brazil Bolivia Australia 16,000 12,000 Tin: World Mine Production, 2007 The Romans learned how to “roast” the tin from ore deposits, and they used tin as a coating on iron ob- jects. Because tin is easy to apply to other metals, and because it does not corrode under normal condi- tions, this use of tin has continued to the present. Food canning was developed in 1812, and much of the tin mined today is used to coat steel food contain- ers. Tin was also used to produce pewter as far back as 1500 b.c.e., and this alloy was used extensively by the Romans. In the early 1800’s, tin was first mixed with copper and antimony to make a babbitt metal that re- duced friction of bearings in machinery. This alloy, created by Isaac Babbitt, was an important contribu- tion to the Industrial Revolution. Obtaining Tin Deposits of cassiterite are concentrated in the upper levels of the granitic intrusions and associated hydro- thermal veins, and they are commonly exposed to weathering and erosional forces. As a result, most of the currently operating mines are working tin depos- its less than 300 million years old. The older deposits have generally been eroded away. The older tin ores that have escaped erosion are found primarily in the central parts of continental masses. The oldest known tin ores are found in South Africa and are more than 2.5 billion years old. Because cassiterite is both heavy (with a specific gravity of 6.8 to 7.1) and hard (with a Mohs scale hard- ness of 6 to 7), it is commonly found as a placer mineral in streams and rivers that drain tin-bearing igneous regions. As the rocks of the tin region are weathered and eroded, the softer minerals are broken down into small clasts, and the lighter materials are carried downstream to the oceans. The heavy miner- als such as cassiterite and wolframite (a tungsten- bearing mineral) are left behind in the alluvial de- posits. Most of the Southeast Asian deposits of tin are alluvial accumulations that formed in ancient rivers that are now exposed on dry land. A small percentage of tin has been produced from base-metal sulfide deposits. These deposits occasion- ally contain cassiterite concentrated in volcanic- sedimentary deposits that were formed in or near high-temperature submarine vents. The major depos- its of this type are found in Canada and Portugal. The Neves Corvo, Portugal, deposit is a massive sulfide ore containing copper, zinc, lead, silver, and tin. Although many of the tin deposits first exploited were located in Europe, the majority of the tin mined now comes from Asia and South America. World pro - duction of tin is approximately 280,000 metric tons. The largest producer is China, which mines about 40 percent of the world total. Second in mine produc- tion is Indonesia (30 percent of the world total), fol- lowed by Peru (16 percent), Bolivia (6 percent), and Brazil (4 percent). Additional countries—including Australia, Malaysia, Vietnam, and Russia—also mine tin. The last operational tin mine in the United States closed in 1990. World tin reserves have been esti- mated at about 6 million metric tons, most of which is in Southeast Asia and South America. World con- sumption is about 350,000 metric tons annually. Pure tin can also be obtained by recycling tin-plate scrap and tin-plated steel cans. Other secondary tin can be extracted from other scrap materials and from solutions commonly involved in the manufacturing of electronic equipment. Total production of secondary tin from recycling averages about 15,000 metric tons per year, and the United States is the largest producer. The vein deposits of South America, England, and Australia are mined by the same techniques used in hard-rock base-metal mining throughout the world. Alluvial sands are generally mined by surface mining methods. The sands can be cleared of any barren overburden and then excavated by directing high- pressure water jets that disaggregate the sands. These sands are then washed over a series of baffled sluices that will retain the heavy minerals such as cassiterite. The cassiterite and other heavy minerals are periodi- cally removed from behind the baffles. This ore is then sent to smelters for refining. Some alluvial cassit- erite has been washed out to the oceans, and in some Southeast Asian areas the shallow ocean floor is dredged to recover the ore. Stream tin accounts for about 80 percent of the tin recovered each year. Although cassiterite is almost 79 percent tin, the tin ores and concentrates vary in the amounts of im- purities they contain. Generally the ores are first roasted to drive off any sulfur in the associated miner- als, and then the tin minerals are heated in the pres- ence of carbon (coke) to reduce the cassiterite to liq- uid tin with the release of oxygen, which reacts with the carbon to form carbon dioxide. Commonly, lime- stone is used as a flux to reduce the temperatureof the reaction to approximately 1,400° Celsius. The liquid tin is extracted and the floating slag removed and re- processed. The smelted tin is then refined either elec - trolytically or by fire. Electrolytically refined tin can reach a purity as great as 99.999 percent. 1234 • Tin Global Resources Uses of Tin Tin and its alloys have many important commercial and industrial uses. In the past, primary use was in the production of tin plate, which is used in the produc- tion of food containers. Tin plate is made by coating steel with a thin (1 micrometer thick) layer of tin. Un- til the middle of the twentieth century, tin plate was manufactured by immersing the steel in a hot bath of molten tin, but most tin plate is now made by electro- lytically plating the tin onto the steel. The tin plate is used primarily to make cans for food and beverages. However, aluminum has become the metal of choice in the production of food cans. Tin alloys are also used in coating steel and other metals. Zinc, nickel, copper, and lead are each alloyed with tin to produce coatings with specific properties. Terneplate is a tin/lead coating for steel that is com- monly used to retard corrosion. Many gasoline tanks are also made of terneplate. Because of its low melting temperature and its abil- ity to alloy with many metals at different concentra- tions, tin has long been a major component of solders used to join metal parts. The most common solder is an alloy of tin and lead. Tin and lead can be alloyed at all possible relative concentrations, but most com- monly the tin in solder ranges from 30 percent to 70 percent. Tin/lead solders soften over a range of tem- peratures, and this allows for use in a variety of differ- ent soldering techniques. Because of the toxic effects of lead, much research has been concentrated on pro- ducing lead-free solders. Tin/silver solders and tin/ zinc solders that melt at low temperatures have been developed. In addition, silver and indium are usable alloy metals for lead-free solders. Babbitt metals are important tin, antimony, and copper alloys that are commonly manufactured into bearings that run against steel shafts. This white metal alloy is soft enough to allow for irregularityin the steel shafts. Babbitt metal can also embed any loose metal particles that arise from the running of the machine, which helps to reduce scratching or scoring of the other machine parts, thus prolonging the life of the machines. Babbitt’s discovery of these alloys was an important ingredient in the Industrial Revolution. Al- though Babbitt and others experimented with various concentrations of tin, antimony, and copper, the best white metals contain about 7 percent antimony and 3 percent copper. These tin alloys do not bear heavy loads well, however, and are replaced by a tin/alumi - num alloy when necessary. This alloy, composed of about 80 percent aluminum, is used in diesel engines and some automobiles. Heavy-duty bearings are com- posed of leaded bronzes. Two traditional uses of tin are in the production of bronze and pewter. Tin bronzes today are composed of tin and copper or tin, copper, and lead. The bronze used in the making of bells and musical instruments contains up to 20 percent tin to give it the correct tonal qualities. The pewter of ancient Roman times was an alloy of tin and lead, but the recognition of the toxic nature of lead has caused a shift in the composi- tion of modern pewter. Most pewter today is an alloy containing mostly tin, with only minor amounts of copper and antimony. The copper and antimony give strength to the weak tin and allow it to be used in plate ware as well as in jewelry. Tin is utilized in many other ways. It is used in small amounts to soften some metal alloys, making the met- als easier to machine. It is also alloyed with aluminum and titanium for use in the aerospace industry. Silver and tin are alloyed to make one of the most commonly used dental fillings. Tin has also replaced the lead and tin/lead cap - sules that surround corks on wine bottles. Research Global Resources Tin • 1235 Cans & containers 26% Electrical 24% Construction 11% Transportation 11% Other 28% Source: Mineral Commodity Summaries, 2009 Data from the U.S. Geological Survey, .U.S.GovernmentPrinting Office, 2009. U.S. End Uses of Tin . survival of all species and the maintenance of ecosystem structure and function, while allowing for utilization to meet the needs of the globally expanding human population. As the global demand. A Global Forest Watch Canada Report. Washington, D.C.: World Resources Institute, 2000. Global Resources Timber industry • 1231 United Nations Food and Agriculture Organization. FAO Yearbook of. 70 percent. Tin/lead solders soften over a range of tem- peratures, and this allows for use in a variety of differ- ent soldering techniques. Because of the toxic effects of lead, much research has