According to their chemical compositions, there are seven types of oxides: A 2 O, AO, A 2 O 3 , ABO 3 , AB 2 O 4 ,AO 2 , and A m B n O 2(m+n) , where A and B are met- als and m and n are integers. Cuprite (CuO 2 ), an im- portant ore of copper, belongs to type A 2 O. Both periclase (MgO) and tenorite (CuO) belong to type AO. Corundum (Al 2 O 3 ), hematite, and ilmenite be- long to type A 2 O 3 . Corundum can be utilized as an abrasive and a gemstone. Hematite is an important ore of iron. Ilmenite is an ore of titanium. Perovskite (CaTiO 3 ) belongs to type ABO 3 . Spinel, chrysoberyl, and gahnite (ZnAl 2 O 4 ) belong to type AB 2 O 4 . Pyrolusite, cassiterite, and rutile (TiO 2 ) belong to type AO 2 . Pyrolusite is an ore of manganese. Cassiter- ite isan important source oftin. Rutileis anore oftita- nium. Columbite-tantalite (4[(Fe,Mn)(Nb,Ta) 2 O 6 ]) belongs to type A m B n O 2(m+n) . Besides chemical compositions, the structural, op- tical, and physical properties of oxides are studied. Structural information can be revealed by X-ray dif- fraction. Optical properties include color appearance, reflection, and transmission. Physical properties in- clude density, mechanical strength, and thermal ca- pacitance. Some of the oxides are distributed throughout the world, while others are limited to a few regions. For example, magnetite can be found in the United States; hematite can be found in the United States, Venezuela, Brazil,Canada, andAustralia; and cassiter- ite can be found in Malaysia, Bolivia, and other coun- tries. Xingwu Wang See also: Aluminum; Beryllium; Copper; Igneous processes, rocks, and mineral deposits; Iron; Manga- nese; Oxygen; Pegmatites; Quartz; Sand and gravel; Silicon; Tin; Titanium. Oxygen Category: Mineral and other nonliving resources Where Found Oxygen is the most abundant element in the Earth’s crust (46.6 percent by weight), occurring mainly as oxides and silicates of metals. The earth’s waters are 85.8 percent oxygen by weight, and the atmosphere is 23.0 percent oxygen. The combined weight of oxygen in the crust,hydrosphere, and atmosphere is about 50 percent. Primary Uses In addition to its importance in the combustion of food for energy by living organisms, oxygen has many commercial applications. It is used in theiron and steel industry, in rocket propulsion, in chemical synthesis, and to hasten the aerobic digestion of sewage solids. Technical Definition Oxygen (abbreviated O), atomic number 8, belongs to Group VI of the periodic table of the elements. Its chemical properties are somewhat similar to those of sulfur. It has an average molecular weight of 15.9994 and six naturally occurring isotopes, three of which are radioactive with half-lives on the order of seconds and minutes. At ordinary temperatures, oxygen is a colorless, odorless gas.Its liquidformis paleblue. Ox- ygen melts at −218° Celsius and boils at −183° Celsius. Oxygen can form compounds with all other elements except the low-atomic-weight elements of the helium family. Description, Distribution, and Forms The total content of oxygen in the Earth’s air, crust, and oceans is approximately 50 percent by weight. In chemically combined form, it is found in water and in the clays and minerals of the lithosphere. Despite the fact that it is an active element, forming oxides easily by the process ofcombustion, elemental oxygen makes up about 23 percent of the atmosphere.Dissolved gas- eous oxygen is found in the waters of the Earth,where it provides for the respiration of most marine animals and for the gradual oxidation of waste materials in lakes and rivers. Elemental oxygen is found in three allotropic forms: the ordinary diatomic molecule found in the atmo- sphere (O 2 ), ozone (O 3 ), and the unstable, nonmag- netic, and rare pale blue O 4 form, which decomposes easily to O 2 . Unstable atomic oxygen is a short-lived species that results from the absorption of ultraviolet radiation by ozone in the upper atmosphere or from electrical discharges. The solvent properties of water are attributable to the great difference in the strength of attraction for the bonding electrons between hydrogen and oxy- gen, which makes the resulting molecule very polar. The H 2 O molecules are attracted to both cations and anions, surrounding them by the attraction of the 898 • Oxygen Global Resources negative oxygen or the positive hydrogen, respec - tively. Water also dissociates slightly into H+ and OH− ions. These processes allow water to form hydrates with, and to react with, many compounds. History Most chemists agree that the discovery of oxygen was made independently by Carl Scheele in Sweden and Joseph Priestley in England at about the same time. In 1774, Priestley heated mercuric oxide and collected the liberated gas over water. He showed that the “dephlogisticated air” (oxygen) was capable of sup- porting burning and was respirable. Scheele prepared oxygen in 1771-1772 by heating various carbonates and oxides. Although his experiments were performed earlier than those of Priestley, the latter published his results first. The great French chemist Antoine- Laurent Lavoisier was the firstto recognize thatoxygen is an element, and he was able to explain the combus- tion process correctly. This explanation revolution- ized the field of chemistry and provided the stimulus for the discovery of many new elements. Obtaining Oxygen For many years the only means of obtaining oxygen was by the fractional distillation of liquid air. A varia- tion of this basic process is still used when high-purity oxygen is needed. In 1971, an ambient temperature process was introduced by the Linde Division of Union Carbide Corporation. The process uses a pressure cy- cle in which “molecular sieves” are used to selectively absorb nitrogen from the air. The resulting product contains about 95 percent oxygen and about 5 per- cent argon and is economically preferable in situa- tions where the argon will not interfere. Uses of Oxygen The greatest consumers ofoxygen are the steel, chem- ical, and missile industries. The oldest use of oxygen is in the welding of steel by means of a hot acetylene- oxygen torch. Thicknesses of steel of up to 0.6 meter can be cut by a high-pressure oxygen stream after heating with an acetylene torch. An oxygen stream passed through molten iron can remove carbon im- purities by means of combustion to carbon dioxide. In the chemical industry, oxygen is used for the production of hydrogen from natural gas or “synthe- sis gas”: CH 4 + 0.5 O 2 → CO + H 2 Other important industrial processes are the manu - facture of hydrogen peroxide, sodium peroxide, eth- ylene oxide, and acetylene. Large rockets are propelled from their launch pads by the combustion of a fuel similar to kerosene. The fuel and oxygen are kept in liquid form in separate tanks until ignition. (In some rockets the second stage is propelled by the combustion of hydrogen.) Oxygen has limited but important uses in the health-care industry in the treatment of pneumonia, emphysema, and some heart problems. Hyperbaric chambers provide high-pressure, oxygen-rich atmo- spheres for the treatment of both carbon monoxide poisoning and decompression sickness (“the bends”). Grace A. Banks Further Reading Ardon, Michael. Oxygen: Elementary Forms and Hydro- gen Peroxide. New York: W. A. Benjamin, 1965. Gilbert, Daniel L., ed. Oxygen and Living Processes: An Interdisciplinary Approach. New York: Springer,1981. Greenwood, N. N., and A. Earnshaw. “Oxygen.” In Chemistry of the Elements. 2d ed. Boston: Butterworth- Heinemann, 1997. Hayaishi, O., ed. Molecular Oxygen in Biology: Topics in Molecular Oxygen Research. New York: American Elsevier, 1974. Jackson, Joe. A World on Fire: A Heretic, an Aristocrat, and the Race to Discover Oxygen. New York: Viking, 2005. Lane, Nick. Oxygen: The Molecule That Made the World. New York: Oxford University Press, 2002. Lewis, Bernard, and Guenther von Elbe. Combustion, Flames, and Explosions of Gases. 3d ed. Orlando, Fla.: Academic Press, 1987. Massey, A. G. “Group 16: The Chalcogens—Oxygen, Sulfur, Selenium, Tellurium, and Polonium.” In Main Group Chemistry. 2d ed. New York: Wiley,2000. Scott, Gerald. Atmospheric Oxidation and Antioxidants. New York: Elsevier, 1965. Weeks, Mary Elvira. Discovery of the Elements. 7th ed. New material added by Henry M. Leicester.Easton, Pa.: Journal of Chemical Education, 1968. Web Site Universal Industrial Gases, Inc. Oxygen (O 2 ) Properties, Uses and Applications: Oxygen Gas and Liquid Oxygen http://www.uigi.com/oxygen.html Global Resources Oxygen • 899 See also: Atmosphere; Fuel cells; Minerals, structure and physical properties of; Oxides; Ozone layer and ozone hole debate; Water. Ozone layer and ozone hole debate Categories: Ecological resources; environment, conservation, and resource management; social, economic, and political issues Ozone, a form of the element oxygen, forms naturally in the stratosphere and provides the Earth with a filter from ultraviolet radiation. Some human activities cause a decrease in the amount of ozone present, an ef- fect that has been described as a hole in (more correctly a “thinning” of ) the ozone layer. Background Ozone is a highly reactive form of oxygen. It is com- posed of three oxygen atoms in a molecule (O 3 ) rather than the more usual two atoms (O 2 ). Ozone is formed from diatomic oxygen where high energy is present. Near the Earth, ozone forms in high-temperature combustion processes, such as in automobile engines and in elec- trical sparks. In the stratosphere it forms be- cause of high-energy ultraviolet radiation. Once formed, ozone is quick to react with other molecules. Near the Earth there are many molecules with which to react, and the ozone concentration remains low. In the stratosphere there are few molecules present, so the ozone concentration builds up and forms what is termed the ozone layer.Ozone also disappears naturallyby de- composing to ordinary oxygen, so there is a natural limit to the concentration that ac- cumulates, and a steady state occurs. The ozone layer is actually quite diffuse, and the ozone concentration is never very high. Description, Distribution, and Concentrations Since the mid-1950’s, measurements of ozone concentrations in the atmosphere have been made regularly. In the early 1970’s, analysis of the measurements sug - gested that something was causing a reduc - tion in the concentration of ozone in the strato - sphere, particularly in the region over the South Pole. Continued measurements confirmed a similar lower- ing over the North Pole areaand a spreading of theef- fect over a larger area. Laboratory experiments show that molecular fragments containing unpaired elec- trons are effective in speeding the decomposition of ozone. This catalytic effect is particularly strong in the presence of small ice crystals, which are present in the stratosphere in the polar regions in winter. Chlorofluorocarbons Chlorofluorocarbons (CFCs) are a class of chemicals that have found wide use as propellants in aerosol cans, cleaning solvents for electronic circuit boards, and working fluids in air-conditioning and refrigera- tion. The stability of these molecules is a prime factor in their utility, but this property also allows the mole- cules to drift into the stratosphere when they are re- leased. Most other escaping molecules react or are washed out by precipitation before they gain much height in the atmosphere. In the stratosphere, CFCs decompose by irradiation and form molecular frag- ments to which ozone is sensitive. CFCs are not the 900 • Ozone layer and ozone hole debate Global Resources The Antarctic hole in the ozone layer from 2000 data provided by the TotalOzone Mapping Spectrometer earth probe. (UPI/Landov) only artificial cause of ozone depletion, but they have been recognized as a major contributor.Much of what is known about the way the ozone layer forms and de- composes comes from the work of Paul J. Crutzen, Mario J. Molina, and F. Sherwood Rowland, who re- ceived the 1995 Nobel Prize in Chemistry for their work on this subject. The Importance of Ozone Ozone is decomposed when the energy available in part of the ultraviolet region of the spectrum is ab- sorbed by the molecule. When the energy is used in such a fashion, it is no longer present in the sunlight that comes through the stratosphere to the Earth. This type of energy, if it does make it to the Earth, is capable of causing the reaction of other molecules, including those of biological importance. The evi- dence is overwhelming that the primary cause of nonmelanoma skin cancers is chronic long-term ex- posure to ultraviolet light. Australia has the highest incidence of skin cancer in the world. Other human interactions may lead to melanoma skin cancers and cataracts. Increased ultraviolet levels also cause cellular modifications in plants, including food crops, which may lead totheir death. Of particular concernis the in- hibition of photosynthesis in the phytoplankton that forms the base of the ocean food chain. The ozone layer acts as a filter to limit the Earth’s exposure to high-energy light. With a diminishing level of filtering, one would expect that there would be a global increase in the effectsof overexposure to ultraviolet radiation. The Ozone Debate Some scientists contend that ozone depletion is a part of a natural cycle related to sunspot activity. Knowl- edge of what has happened in the distant past is cir- cumstantial and not easy to interpret, but most scien- tists agree that human activities play a significant role in the current decrease in the ozone layer. In terms of the human contribution, CFCs have received the major attention, and their production was severely limited by international agreement in the 1987 Mon- treal Protocol and later revisions. CFCs are no longer used for propellants, and their role as cleaners is all but over. However, their use as refrigerant fluids continues while economically viable, safe substitutes are being sought. People in developed countries have become extremely dependent on air-conditioning (nearly all large buildings are designed to be air- conditioned rather than open to the outside). The search for substitutes has proved difficult, with eco - nomic, safety, and environmental concerns all plac- ing limits on what is acceptable. Part of the controversy concerning banning CFCs is based on ethical considerations. Developed coun- tries utilized CFCs to gain their positions; should they then prohibit the use of CFCs in developing coun- tries? Should these countries not be allowed to reap the same advantages as others even if there is an envi- ronmental price to be paid? There are no easy, satis- factory answers to such questions. International Day for the Preservation of the Ozone Layer In 1985, the Vienna Convention was signed by twenty- two countries. Two years later, the Montreal Protocol was signed on September 16, a day which has been designated by the United Nations as International Day for the Preservation of the Ozone Layer. The theme for the day in 2008 was “Montreal Protocol: Global Partnership for Global Benefits.” On Interna- tional Day 2008, the World Meteorological Organiza- tion (WMO) released several statements on ozone and ozone-related matters, including the following by Ban Ki-moon, the secretary general of the United Na- tions. After decades of chemical attack, it may take an- other fifty years or so for the ozone layer to recover fully.As the Montreal Protocol has taught us, when we degrade our environment too far, nursing it back to health tends to be a long journey, not a quick fix. According to WMO, the 2008 Antarctic ozone hole was largerthan the one of 2007. The observed changes in the stratosphere could delay the expected recovery of the ozone layer. It is therefore vital that all member states with stratospheric measurement programs con- tinue to support and enhance these measurements. Routine ozone measurements in all parts of the world, using surface-based spectrophotometers, balloon-borne sensors, aircraft, and satellites, have been made by the National Meteorological and Hy- drological Services of WMO members and partners worldwide since the 1950’s. In the 1980’s, compre- hensive measurements started under coordination of the WMO Global Atmosphere Watch (GAW). These measurements have been critical to the series of Scien - tific Assessments of Ozone Depletion published since the mid-1980’s by WMO and the Ozone Secretariat of Global Resources Ozone layer and ozone hole debate • 901 the United Nations Environment Programme, docu - menting progress made under the Vienna Conven- tion for the Protection of the Ozone Layer (signed in 1985 by twenty-two countries). The most recent of these assessments came out in the spring of 2007. The work on the following ozone science assessment be- gan in the middle of 2009. The Montreal Protocol on Substances That De- plete the Ozone Layer underpins efforts to combat depletion of the Earth’s fragile protective shield. It also contributes to combating climate change, be- cause many of the chemicals controlled under the treaty also contribute to global warming. By phasing out CFCs and deciding to accelerate a freeze and phase-out of hydrochlorofluorocarbons (HCFCs), the treaty has provided two benefits at once. The U.N. secretary-general expressed the hope that “Govern- ments will look at such results and feel empowered to act across a wide range of environmental challenges, and not only in prosperous times.” In August, 2008, WMO released its first of the 2008 series biweekly Antarctic Ozone Bulletin on the current state of stratospheric ozone in the Antarctic. These bulletins use provisional data from the WMO/GAW stations operated within or near the Antarctic, where the most regular and dramatic decreases in ozone occur. According to the 2008 bulletin, the vortex was more circular than at the same time in 2007. The me- teorological conditions observed indicate that the 2008 ozone hole was smaller than that of 2006 but larger than that of 2007. The Antarctic ozone hole reached its maximum in- tensity in late September/early October. In 2008, the ozone hole appeared relatively late. On September 13, 2008, the ozone hole covered an area of 27 million square kilometers. The maximum area reached in 2007 was 25 million square kilometers. WMO and the scientific community continue to make ozone obser- vations from the ground, from balloons, and from sat- ellites, together with meteorological data, to keep a close eye on the ozone development and depletion. Ozone Depletion and Climate Change Many scientists are increasingly aware of the possible links between ozone depletion and climate change. According to many studies, increased atmospheric concentrations of greenhouse gases (GHGs) may lead to warmer temperatures in the troposphere and atthe Earth’s surface. However, in the stratosphere, at alti - tudes where we find the ozone layer, there will be a cooling effect. A cooling of the stratosphere in winter over the latter decades of the twentieth century and the first decade of the twenty-first century has indeed been observed, both in the Arctic and in the Antarc- tic. Lower temperatures enhance the chemical reac- tions that destroy ozone. At the same time, the amount of water vapor in the stratosphere has increased at the rate of about 1 percent per year. A wetter and colder stratosphere means more polar stratospheric clouds, which may lead to more severe ozone loss in both po- lar regions. Together with the International Council for Sci- ence (ICSU), WMO coordinated the International Polar Year 2007-2008. Thousands of scientists collabo- rated to increase understanding of processes that take place in polar regions, including those of strato- spheric ozone and ultraviolet radiation. In February, 2009, WMO and ICSU celebrated the closure of the International Polar Year in Geneva and released WMO’s State of Polar Research. Kenneth H. Brown, updated by W. J. Maunder Further Reading Andersen, Stephen O., and K. Madhava Sarma. Pro- tecting the Ozone Layer: The United Nations History. Edited by Lani Sinclair. Sterling, Va.: Earthscan, 2002. Asimov, Issac. What’s Happening to the Ozone Layer. Mil- waukee, Wis.: Gareth Stevens, 1993. Booth, Nicholas. How Soon Is Now? The Truth About the Ozone Hole. New York: Simon & Schuster, 1994. Christie, Maureen. Ozone Layer: A Philosophy of Science Perspective. New York: Cambridge University Press, 2001. Dessler, Andrew. The Chemistry and Physics of Strato- spheric Ozone. New York: Academic Press, 2000. McElroy, Michael B. The Atmospheric Environment: Ef- fects of Human Activity. Princeton, N.J.: Princeton University Press, 2002. Parker, Larry, and Wayne A. Morrissey. Stratospheric Ozone Depletion. New York: Novinka Books, 2003. Parson, Edward A. Protecting the Ozone Layer: Science and Strategy. New York: Oxford University Press, 2003. Reid, Stephen J. Ozone and Climate Change: A Beginner’s Guide. Amsterdam: Gordon and Breach, 2000. Roan, Sharon. Ozone Crisis: The Fifteen-Year Evolution of a Sudden Global Emergency. New York: Wiley, 1989. Somerville, Richard C. J.“The Ozone Hole.” In The For - giving Air: Understanding Environmental Change.2d ed. Boston: American Meteorological Society,2008. 902 • Ozone layer and ozone hole debate Global Resources Zerefos, Christos, Georgios Contopoulos, and Greg - ory Skalkeas, eds. Twenty Years of Ozone Decline: Pro- ceedings of the Symposium for the Twentieth Anniversary of the Montreal Protocol. New York: Springer, 2009. Web Sites National Oceanic and Atmospheric Administration The Ozone Layer http://www.oar.noaa.gov/climate/ t_ozonelayer.html U.S. Environmental Protection Agency Ozone Layer Depletion http://www.epa.gov/ozone/strathome.html See also: Aerial photography; Agenda 21; Air pollu- tion and air pollution control; Antarctic treaties; At- mosphere; Biosphere; Clean Air Act; Climate Change and Sustainable Energy Act; Earth Summit; Gore, Al; Greenhouse gases and global climate change; Indus- trial Revolution and industrialization; Kyoto Proto- col; Landsat satellites and satellite technologies; Mon- treal Protocol; Oxygen; United Nations Framework Convention on Climate Change. Global Resources Ozone layer and ozone hole debate • 903 P Paper Category: Products from resources The pulp and paper industry produces a wide variety of primary products, including newsprint, printing and writing papers, packaging and industrial papers, corrugated containers, gray and bleached boxboards, bags, dissolving pulps, and wood pulp. All pulping processes involve tremendous amounts of water and timber. Background Before the invention of paper, written wordswere pre- served on fabric in the form of scrolls. The Chinese are credited with inventing paper around 105 c.e. Historians note that this date was chosen somewhat subjectively, as early experiments in the process of papermaking probably stretched over a long period of time before the process was perfected. No records exist that indicate how the Chinese first made paper, but it is believed that this early paper was made by pouring fibrous pulp onto flat cloth-covered molds, then drying it—essentially the same way paper is pro- duced today. Once the pulp had dried, an interlock- ing matrix of fibers created the paper. Early forms of paper were not as well processed as modern paper products. In fact, early forms of paper had more in common with the fabrics they replaced than with modern paper. They were coarse in nature, but they did lie flat. This quality made it possible for the first real books to be produced. Over the following five hundred years, the Chinese papermaking process slowly spread throughout Asia, In this 1936 photograph, factory workers add pulp to a machine as part of the papermaking process. (SSPL via Getty Images) from Vietnam and Tibet to Korea and eventually to Japan in the sixth century. The Japanese refined the process and continued to produce high-quality paper varieties for centuries. The process moved west through Nepal and India. Several papermaking de- vices were captured by Islamic warriors, thus moving the technology furtherwestthrough the Muslim world. It went to Baghdad into Egypt and across North Af- rica. The technology finally entered Europe in the twelfth century when the Moors invaded Spain and Portugal. In 1456, the German printer Johannes Gutenberg successfully printed a Bible on his movable-type press, making it possible for the written word to move out to a much larger population. Industrial papermaking and printing grew from this point. The Fourdrinier Machine The first major improvement in papermaking was dipping the molds directly into the fibrous pulp (the exact date of this improvement is unknown). Dipping the molds allowed artisans to produce a greater quan- tity of high-quality paper. Paper was made by hand until the early nineteenth century, when the Fourdrinier brothers, Henry and Sealy, introduced the first machine designed specifi- cally for the manufacture of paper. The Fourdrinier brothers were the financiers of the first modern pa- permaking machine, which was designed by Nicholas Louis Robert in Essonnes, France. Robert received a patent for the continuous papermaking machine in 1799. Unable to afford the cost of development and implementation of his machine, Robert and his part- ner, Saint-Léger Didot (who often claimed the contin- uous papermaking machine was of his own inven- tion), sent Didot’s brother-in-law, John Gamble, to England to find financial backing. A British patent was awarded in October, 1801. The first continuous paper machine was installed and made operational in Hertfordshire, England, in 1803. The next year, an- other machine followed. Robert sold the rights to his invention to the Fourdrinier brothers in England. The principle of Robert’s machine was to construct the paper on an extensive woven-wire cloth that re- tained the matted fibers while allowing the excess water to drain through—this same principle holds with all modern papermaking machines. In the United States, the first documented paper - making machine was installed in 1817, in Brandywine Creek, Delaware, by the Thomas Gilpin Mills. This machine differed from the Fourdrinier device in that it was a cylindrical mold. The first Fourdrinier device was installed in the United States in 1827. Production of Pulp and Paper Paper production has changed significantly since the early industrial days and even the boom manufactur- ing years of the 1960’s and 1970’s. The recycling of paper products has become commonplace, as have government-mandated levels of postconsumer fiber content. A single sheet of paper could contain fibers from hundreds of different trees around the world. These fibers travel thousands of kilometers from the forest to the office printer. While recycling technolo- gies have greatly improved in the twenty-first century, there is still only a 10 percent chance that the com- mon paper used in personal printers contains post- consumer recycled fibers. On average, office employ- ees in the United States use almost ten thousand sheets of paper, roughly 12 kilograms of paper per person per year. In 2005, the average North American created 302 kilograms of paper waste per year com- pared to 231 kilograms for citizens of high-income countries other than the United States and Canada, or 39 kilograms for citizens of middle-income coun- tries, or 4 kilograms for citizens of low-income coun- tries. The manufacturing of paper and paperboard in- volves the production and conversion of pulp from some fibrousfurnish.“Furnish” is any blend of fibrous materials (such as timber, wood chips, or recycled pa- per) used to produce pulp. Wood is the most com- monly used furnish—roughly 95 percent of all pulp and paper manufacturers use wood in some form. The second most widely used form of furnish is sec- ondary fibers from either mill waste or postconsumer fibers, such as newsprint and corrugated boxes. The usage of secondary fibers grows as consumer and commercial demand increases for products made from recycled paper. Pulp Production The production of pulp once involved the breaking down of homogeneous furnish feedstock into its fi- bers, often bleaching to increase the whiteness of the paper fibers, and mixing with water to produce a slurry. In August, 1998, the Environmental Protection Agency (EPA) passed a regulation called the cluster rule. This rule requires the pulp industry to stop the use of bleaches in paper production and imposes the Global Resources Paper • 905 use of chlorine-free colorants instead. These chlorine dioxide derivatives are created from sodium chlorate instead of chlorine. A totally chlorine-free future is be- ing sought by the EPA for paper production in the United States and other countries. There are four types of pulping processes: chemi- cal, semichemical, mechanical, and secondary fiber pulping. Chemical pulping includes the kraft (sulfate) process, soda pulping, sulfite pulping, and neutral sul- fite chemical pulping. Mechanical pulping includes chemi-mechanical, thermo-mechanical,chemi-thermo- mechanical, refiner mechanical pulping, and stone groundwood pulping. The type of pulping process affects the durability, appearance, and intended use of the resulting paper product. Regardless of the pulping method employed, pulping is “dirty.” During the pulping stage of production, nuisance odors may be released into the air, and dioxins from kraft chemi- cal bleaching may be released into wastewater. Thus the pulping process is a major concern to the EPA in the United States and equivalent agencies in Europe. Chemical pulping liberates the fibers from the fur- nish by dissolving the lignin bonds, which hold the cellulose fibers together,by cooking wood chipsin liq- uid chemical solutions at extremely high tempera- tures and pressures. Kraft pulping is by far the domi- nant form of chemical (and nonchemical) pulping because of its early development in the 1800’s, its abil- ity to use nearly every species of wood as furnish, and the fact that its resulting pulps are markedly stronger than those of other chemical processes. However, chemical pulp yields are roughly 45 to 50 percent. In other words, roughly 50 percent of the furnish is con- verted into pulp. Semichemical pulping produces verystiff pulp and is used mainly for corrugated containers. The semi- chemical process consists of the partial digesting of hardwood furnish in a diluted chemical solution be- fore it is mechanically refined to separate the fibers from the weakened furnish. Pulp yields range be- tween 55 percent and 90 percent, depending on the process employed. Mechanical pulping processes involve the reduc- tion of furnish to fiber by either beating or grinding. This is the oldest known method of releasing the cel- lulose fibers from wood furnish. The pulp yields are high, up to 95 percent, especially when compared with chemical pulping yields of 45 to50 percent. How - ever, the mechanicallyproduced pulp is of low strength and quality.Thus, mechanicalpulp is often combined with chemical pulp to increase both its strength and quality. Finally, secondary fiber pulping relies on recov- ered (recycled) papers as furnish. Typically, second- ary fibers are presorted and preprocessed before they are sold to a pulp and paper mill. If the recovered pa- pers have not been preprocessed, then they must first be treated to remove common contaminants, such as adhesives, coatings, inks, and dense plastic chips. The most common technique of secondary fiber pulping involves mixing the recycled furnish in a large con- tainer of water, which is sometimes heated. Pulping chemicals may be added to induce the dissolution of paper or paperboard. The mix is then stirred by a ro- tor to produce the pulp. Pulping processes involve tremendous amounts of water, and most require large amounts of timber. Of all the wood harvested globally for industrial pur- poses, 42 percent goes into the production of paper. Latin America is a growing supplier of harvested wood for paper manufacturing. Furthermore the in- ternational Organization of Economic Cooperation and Development indicates that paper and pulp in- dustries are the largest consumers of water of all the major industrial sectors. The papermaking process generates large amounts of air and water pollutants, especially during the pulping stage. It ranks third be- hind the chemical and steel industries in greenhouse emissions. In 2000, the world’s largest producers of paper pulp were the United States, at 57,002 metric tons, and Canada, at 26,411 metric tons, followed by China, Finland, Sweden, Japan, Brazil, Russia, In- donesia, and Chile. Manufacturing Paper There are two general steps in the process of making paper and paperboard: wet-end operations and dry- end operations. During the wet-end operations, pro- cessed pulp is transformed into a paper product via a paper machine, the most common of which is the Fourdrinier paper machine. Pulp slurry (more than 90 percent water at the start) is deposited on a rapidly moving wire mesh for removal of the water by gravity, vacuum chambers, and vacuum rolls. After vacuum rolling, a continuous sheet is left, which is then pressed between a progres- sion of rollers to extract any additional water and to compress the fibers. The sheet is then ready for dry- end operations. During this stage, the sheet enters a drying area, where the paper fibers start to bond as 906 • Paper Global Resources they are compressed by steam-heated rollers. The sheets are then pressed between massive rollers to re- duce paper thickness and to produce a smooth sur - face. After a smooth thin sheet of paper is produced, coatings may be applied to improve the color, luster, printing detail, and brilliance. Finally,the paper prod- uct is spooled for storage. From there, the process of bringing the consumer a standard 8.5-inch-by-11-inch sheet of paper involves nothing more than loading the spool of oversized pa - Global Resources Paper • 907 Pulp Processes Pulp Process Description/Principal Products Dissolving kraft Highly bleached and purified kraft process wood pulp, suitable for conversion into products such as rayon, viscose, acetate, and cellophane. Bleached paper-grade kraft and soda, unbleached kraft Bleached or unbleached kraft process wood pulp, usually converted into paperboard, coarse papers, tissue papers, and fine papers such as business, writing, and printing papers. Dissolving sulfite Highly bleached and purified sulfite process wood pulp, suitable for conversion into products such as rayon, viscose, acetate, and cellophane. Paper-grade sulfite Sulfite process wood pulp with or without bleaching, used for products such as tissue papers, fine papers, and newsprint. Semichemical Pulp processed by chemical pressure and (sometimes) mechanical forces with or without bleaching, used for corrugating medium (for cardboard), paper, and paperboard. Mechanical pulp Pulp manufacture by stone groundwood, mechanical refiner, thermochemical, chemi-mechanical, or chemi-thermomechanical means for newsprint, coarse papers, tissue, molded fiber products, and fine papers. Nonwood chemical pulp Production of pulp from textiles (e.g., rags), cotton linters, flax, hemp, tobacco, and abaca to make cigarette wrap papers and other specialty products. Secondary fiber deink Pulps from waste papers or paperboard using a chemical or solvent process to remove contaminants (such as inks, coatings, and pigments), used to produce fine, tissue, and newsprint papers. Secondary fiber non-deink Pulp production from waste papers or paperboard without deinking processes to produce tissue, paperboard, molded products, and construction papers. Fine and lightweight papers from purchased pulp Paper production from purchased market pulp or secondary fibers to make clay-coated printing, uncoated free sheet, cotton fiber writing, and lightweight electrical papers. Tissue, filter, nonwoven, and paperboard from purchased pulp Paper production from purchased market pulp to make paperboard, tissue papers, filter papers, nonwoven items, and any other products other than fine and lightweight papers. Source: U.S. Environmental Protection Agency. Development Document for Proposed Effluent Limitations Guidelines and Standards for the Pulp, Paper, and Paperboard Point Source Category, October, 1993. . These measurements have been critical to the series of Scien - tific Assessments of Ozone Depletion published since the mid- 1980 ’s by WMO and the Ozone Secretariat of Global Resources Ozone layer and ozone hole. decomposition of ozone. This catalytic effect is particularly strong in the presence of small ice crystals, which are present in the stratosphere in the polar regions in winter. Chlorofluorocarbons Chlorofluorocarbons. not interfere. Uses of Oxygen The greatest consumers ofoxygen are the steel, chem- ical, and missile industries. The oldest use of oxygen is in the welding of steel by means of a hot acetylene- oxygen