Amorphous carbon is not generally called a third allotrope because it is a form of graphite consisting of microscopic crystals. Amorphous carbon is obtained by heating any of a variety of carbon-rich materials to 1,200. to 1,800. F (650. to 980. C) in a limited amount of air so that complete combustion does not occur. Coal, for example, is heated to give coke; natural gas or petroleum to give carbon black (also called lampblack and channel black); wood to give charcoal; bone to give bone char; petroleum coke or coal to give baked carbon, carbon arcs, or carbon electrodes.
In 1785 it was discovered that activated carbon from the carbonization of wood and charcoal removes color from solutions for example, the brown color from raw sugar solutions. Activated carbon is still used in the beet sugar industry and bone char is favored for the same purpose in the cane sugar industry. Other foodstuffs commonly decolorized by activated carbon include vinegar, soup stock, whiskey, gelatin, and oils and fats. Activated carbon is also used to adsorb the toxic gases used in chemical warfare, to adsorb organic vapors, and to reclaim solvents. All these uses depend on the adherence of impurities to the enormous surface area of the finely divided carbon.
Most carbon black is used in the manufacture of tires; it improves the strength of rubber and resists scraping. The rest is used in making printing inks for newspapers and magazines, and in paints, lacquers, enamels, and carbon paper.
Fullerene, a hollow cluster of carbon atoms that resemble the geodesic domes made by architect R. Buckminster Fuller, was first postulated to exist in 1985. In 1990 its existence was confirmed, and methods of synthesizing it in mass quantities were invented. The most studied form, known as buckminsterfullerene or the buckyball, has 60 carbon atoms arranged into a five-sided and six-sided geometry to resemble a soccerball. It is suited for use as a lubricant, superconductor, radioactive shield, hard coating, battery, and ball bearing. Through experimentation, scientists concluded that fullerene exists in interstellar space and in soot from the burning of certain gases on Earth. In 1992 it was found for the first time in rock sediments formed more than 600 million years ago. Because fullerene is unstable when exposed to air it is not found in large quantities naturally.
The synthesis of carbon-containing compounds starts from carbon compounds available in nature. The sources of the starting compounds are petroleum for aliphatic hydrocarbons (straight-chain molecules of carbon and hydrogen) and coal or petroleum for aromatic hydrocarbons (rings of carbon and hydrogen). Limestone, from which carbon-containing calcium carbide and acetylene can be made, is extremely important to the chemical industries in countries that have no native petroleum.
Carbon compounds containing boron and silicon are among the hardest substances known. On a standardized scale of hardness called the Mohs scale, where diamond is 10, SiC, silicon carbide (or Carborundum), is 9.15 and B4C, boron carbide, is 9.32. These carbides are used as abrasives on emery wheels. They are chemically inert and nearly indestructible. Carbides formed by the more metallic elements such as iron, cobalt, and nickel, in contrast, are easily decomposed by acids to give hydrocarbons, chiefly methane and hydrogen.
An ordinary carbon atom has six protons and six neutrons in its nucleus; so the atom is called C12. Another isotope, or type, of carbon atom has six protons and seven neutrons in its nucleus and is called carbon-13, or C13. The relative abundance of C12 and C13 in natural sources is 98.89 percent and 1.11 percent respectively. In the air, however, the fast-flying neutrons from cosmic rays keep hitting nitrogen atoms (N14, with seven protons and seven neutrons). Each time a neutron hits, it drives a proton from the nitrogen atom s nucleus and takes its place. Since the atom now has six protons, it is an atom of carbon. It has 14 particles (six protons and eight neutrons), however, in the nucleus; so it is called C14.
This form of carbon decays radioactively. The production and decay are balanced so that C12 and C14 remain always at the same ratio to each other in carbon dioxide. Since the two forms are the same chemically, plants use them for photosynthesis in this same ratio. Because animals eat plants the ratio is found in all living organisms.
Fossils, mummies, and wooden relics, however, no longer exchange carbon with the air. The carbon (C12) that was present at death remains, but the C14 decays radioactively and becomes less in ratio to C12. The changing ratio can be detected easily with a Geiger counter or a scintillation counter; and the amount of change tells the age of the specimen. For example, suppose that the percentage of C14 in a specimen is only half that in the air. Since C14 undergoes a half-life of decay in about 5,730 years, the specimen must be this old.
Radioactive carbon-13 is used as a tracer for many chemical reactions. Chemists can introduce it into food, for example, and then trace the course of the food through the body with a special type of Geiger counter or scintillation counter. This method has been used to trace the steps in photosynthesis.
Leallyn B. Clapp
see Pict 5 & 6
: Potassium Dating
The chemical element potassium is essential to life. In higher animals potassium ions together with sodium ions act at cell membranes in transmitting electrochemical impulses in nerve and muscle fibers and in balancing the activity of food intake and waste removal from cells.
Discovered in 1807 by the English chemist Humphry Davy, who obtained it from molten potassium hydroxide (KOH), potassium, a soft, silver-white metal, was the first metal to be isolated by electrolysis. It belongs to the family of elements known as the alkali metals. It oxidizes rapidly in air and also reacts violently with water, yielding potassium hydroxide and hydrogen gas (which ignites). Because of this, potassium is stored submerged in mineral oil. It is never found alone and is difficult to isolate from its compounds. (See Properties of Potassium table page )
The seventh most abundant element in the Earth s crust, potassium occurs in many silicate rocks and minerals. The major commercial source is salt deposits, but a small fraction is obtained from plant and animal sources. Water-soluble potassium compounds are economically recovered. They are frequently found as dry mineral deposits and as brines. Most potassium is present in insoluble minerals, making it difficult to obtain, but it can be prepared commercially by electrolysis from some refinable minerals.
- Properties of Potassium
Potassium compounds have many commercial uses. Potassium chloride (KCl) is used in preparing other potassium compounds and in fertilizers. Electrolysis of potassium chloride yields potassium hydroxide, also called caustic potash, a water-absorbing substance used in making soaps and detergents. Caustic potash is also used for preparing many potassium salts, such as potassium carbonate (K2CO3), a water-absorbing substance used in making glass and textile dyes and for cleaning and electroplating metals.
Potassium nitrate (KNO3), also known as niter or saltpeter, has wide use as a fertilizer and in fireworks and explosives. It also serves as a food preservative. Potassium chlorate (KClO3), as a source of oxygen, is used in fireworks, matches, and explosives. The iodide of potassium (KI) is added to table salt and animal feed to protect against iodine deficiency. It is also used to treat goiter and certain fungal infections. Applications for potassium sulfate (K2SO4) include use as a laxative and in the production of fertilizer, rubber, and potassium carbonate. Potassium cyanide (KCN) is a poison used in some insecticides and is a source for the fumigant hydrogen cyanide. It is also used to extract gold and silver from their ores.
: Contextual Analysis:
Determining the chronology of an artifact is only half of the archaeologist s task; the other half is reconstructing the ancient culture from which the artifact came. This process is called contextual analysis.
The lowest, or most basic, level of contextual analysis consists of analyzing a culture s systems of subsistence and technology that is, the ways in which ancient people adapted to their environment. The next level involves reconstructing their social structures and settlement patterns. Finally, archaeologists try to reconstruct a culture s ethos, or guiding beliefs.
Each of these levels requires different analytical methods. Archaeologists may start reconstructing an ancient subsistence system by determining what the people ate. They may do this through coprology, the examination of fossilized feces, or by analyzing human bones for the presence of certain forms of carbon and nitrogen. The study of the plant remains found in a dig can also provide clues to a people s diet.
By studying ancient tools such as arrow tips, butcher knives, and grinding stones archaeologists can find out how people obtained and prepared their foods. Archaeologists may also be able to determine how ancient people made and used their tools. Studying the work of a modern flint knapper, for instance, may show an archaeologist how ancient people made flint tools. (In archaeology, this type of reasoning or interpretation is called ethnographic analogy.)
When archaeologists attempt to reconstruct ancient social structures, they often use data gathered by ethnographers, social anthropologists, and historians. The excavated materials themselves may also provide hints of ancient social organization. Specialized artifacts that are found concentrated in certain areas may indicate that the ancient culture had full-time craft specialists, and different types of burial arrangements may indicate that social classes existed.
Reconstructing the highest level of a culture, including its values, ethos, or religion, is the most difficult type of contextual analysis. Such items as statues or paintings of figures that appear to be supernatural, buildings that may have been temples, and evidence of religious ceremonies can all be used to help reconstruct ancient systems of beliefs.
The goal of chronological and contextual analysis is to write and publish records of ancient history. Excavated materials have value only if the information gained from them is disseminated through books, magazines, and other publications. Such publications not only keep track of how techniques have changed but also record great archaeological discoveries.
: Historical Particularism
By the beginning of the 20th century, anthropologists in Great Britain, Germany, and the United States were questioning the belief that all societies developed in much the same way. They suggested that each culture was unique because each had its separate geography, history, creativity, and degree of contact with its neighbors.
One of the first to reject evolutionism was a German-born American anthropologist, Franz Boas. Boas emphasized the importance of fieldwork and observation. Fieldwork involves seeking information about a particular group s behavior by gathering data and recording observable behaviors in that group s natural environment.
Boas believed that every aspect of a culture should be recorded and that the anthropologist studying a native culture should not only learn its language but should attempt to think like its people. Boas emphasized the importance of collecting information that described the individuals and their interrelationships in a particular culture. Such information was gathered through the recording of life histories and folklore, and then connecting these details with archaeological and historical data. Boas also believed that similarities among different cultures were the result of similar outside influences rather than to the similarity in thought processes or to any universal laws of development. He stressed the importance of analyzing a culture within its historical context.
Boas is known as the founder of the culture history school of anthropology, which dominated American cultural anthropology for much of the 20th century. Anthropologists who followed Boas theories included Ruth Benedict, Alfred L. Kroeber, Margaret Mead, and Edward Sapir.