Ceramics, Metals Ceramics are probably the earliest man-made material. The biggest uses of ceramic

2.3.1 Ceramics, Metals Ceramics are probably the earliest man-made material. The biggest uses of ceramic

materials are in construction materials and household goods, and more recently for high technology space age applications. Mud and clay, the precursor to ceramics, have long been used for walls and buildings, but they are vulnerable to erosion by rain and water. Bricks have a history dating from 7500 BCE, made by mixing clay with water and sand to the desired consistency, and then pressing into molds to dry in the sun. Bricks are much better than clay at resisting compression but are vulner- able to tension or shear. Their construction was important enough that even the Bible discussed the reinforcement of bricks with straw to strengthen the product.

With improvements in kilns some 4000 years later, superior kiln dried bricks were made that could withstand more weight and resist erosion from water. In con- struction, masons lay down the bricks in rows and bind them together with mortar, which is made from water, sand, and a binder such as lime or gypsum. Masonry, one of the oldest professions, was held in high regard in both the ancient and the medie- val world. The fraternal organization of the Freemasons uses the square and the compass as their symbol, with a distinguished list of members that included George Washington.

Cement is also a ceramic material used in construction. The Romans prepared dry cement with crushed rock, volcanic ash, and burnt lime as a binder; subse- quently cement was used to make mortar and concrete. Modern hydraulic cement, known as Portland cement, sets after mixing with water. The manufacturing of Port- land cement begins by burning limestone (calcium carbonate) with clay or shale containing silica in a 1450

C rotary cement kiln to drive off carbon dioxide and produce calcium silicates. The rotary kiln can be 60 m long, and is set at a slight angle so that solids can slowly slide down from the high inlet end to the low outlet end, while air and fuel enter from the low end and flow in the opposite direction. The rotary kilns run 24 h a day, typically stopping only once a year for maintenance. The resulting “clinker” is discharged from the low end and then ground up with a small amount of gypsum (calcium sulfate) to make ordinary Portland cement, which is used to make mortar, grout, and concrete. Cement kilns are among the most energy-intensive processes in use today, and generate a tremendous quantity of carbon dioxide, soot, and dust.

Finer ceramics are used as kitchen utensils, to store oil and wine, and to cook and serve food. The making of fine ceramics requires a complex set of skills and tools, including the judicious blending of minerals, and the building and tending of fires. An enclosed insulated kiln was introduced to make the heating more even, to decrease heat loss and to increase the temperature; the introduction of bellows and charcoal made the fire burn even hotter. These kilns may have originated in the

73 Middle East around 6500 BCE, as indicated by the age of the oldest burnt clay

2.3 MATERIALS

objects that have been discovered. As the kiln temperature increases, ceramics grow stronger. Earthenware are made of mixtures of clay, kaolin, quartz, and feldspar, and are fired in a kiln at 800– 1200

C. They are less strong and have porous surfaces. Stoneware fired from 10 to 1300

C is stronger. Surface porosity can be reduced by applying a glass-like glaze, which is made of high melting silicon and aluminum oxides, then added to a mixture of sodium and calcium oxides to act as a flux that melts at a lower temperature.

C is stronger still and less porous. Early porce- lain began in China around 1600 BCE, and reached a high level of excellence dur- ing the Tang Dynasty (618–906), and was a main article for trade on the Silk Route. The secret of porcelain manufacturing was discovered in Europe around 1700, which led to the Meissen and other works.

Porcelain fired at 1300–1400

Most modern glass is based on ordinary sand and the mineral silicon oxide, which has a very high melting point of 1700

C. The melting point of glass is low- ered by mixing it with soda and lime; this molten material is cooled quickly to form

a noncrystalline solid. The most important glass found in common windowpanes and bottles is soda–lime glass, made with a mixture such as 16% soda and 10% lime, which lowers the melting point to a manageable 700

C. Glassmakers from ancient times to the present dope their glass mixture with other elements to create attractive tints and dazzling colors. Optical glass, such as that used for eyeglasses, binoculars, or luxury Waterford and Swarovski glassware, has as much as 35% lead oxide to create a higher refraction index that bends light more strongly into rain- bows. Some wine and whiskey decanters contain so much lead that they have been accused of causing health problems such as gout.

High-temperature ceramics are used in the most demanding applications that are subject to high temperatures and erosion, such as the tips of turbine blades in jet aircrafts and electric power generation stations, the nose cones of rockets, and the heat shield panels of space shuttles. Flak jackets use ceramic plates to resist bullets, and glass fibers form the core of the global communications network in the form of fiber optics.

The Stone Age faded away with the arrival of the Bronze Age. When ancient man first turned to metals, he used what could be found in nature in elemental form: copper, silver, gold, and meteoric iron. However, elemental metals are usually found in quantities too small to make a major impact on the quality of life. In addi- tion, although native copper flakes can be hammered together to make ornaments, it is too soft for tools and weapons.

The major advance in the technology of metalworking was the invention of smelting, which frees the metals from the much more abundant ores. Metallic ores are found in many places in the form of oxides or sulfides, such as the copper ore cuprites (copper oxide), chalcopyrite (copper iron sulfide), and the much more abun- dant iron ores of hematite and magnetite (forms of iron oxide). The metals are tightly bound to oxygen or sulfur in these ores, and the separation requires a lot of energy.

The earliest discovery of smelting was most likely accidental, involving lead and tin ores in wood fires sufficiently hot for these low melting metals. Evidence of

74 CHAPTER 2 INVENTIONS FOR WORK

smelting has been dated back as early as 6500 BCE in Catalhuyuk of Anatolia. The mastery of bronze and iron required advances in the art of making hotter fires in furnaces and crucibles that may have originated in the Middle East for the purpose of making ceramic pottery.

The Bronze Age began around 30 BCE in the Near East, initially using cop- per alloyed with its natural impurity—arsenic, and much later by deliberately adding tin to copper. The production of bronze objects required careful planning, as copper and tin ores were seldom found in the same place. The bronze maker had to import these ores over great distances by trade and transport. Sometime around 3000 BCE it was discovered that mixing copper with tin over a hot kiln fire created molten bronze at 10

C. The liquid could then be poured into molds to make tools and weapons and then hammered on a forge over a hot fire to harden and achieve the final shapes. The Bronze Age was at its zenith around 10 BCE, around the time that The Iliad and The Odyssey were written. Bronze is an alloy of copper mixed with tin—the best bronze has about 10–15% tin. Bronze is much harder than copper, and highly prized for weapons and ornaments. Bronze has many advantages in comparison with stone, as it dents and bends after a heavy blow but does not shatter; blunted blades can be resharpened, and a broken blade can be reforged. It is excellent for making sculp- tures, as it expands slightly just before it sets, thus filling in the finest details of a mold. It also makes excellent ornamental and ritual vessels for food and drinks, as well as musical instruments such as bells. It oxidizes only superficially, and the thin oxide layer protects the underlying metal from further corrosion. Bronze tripods and cups were used in the Shang dynasty in China, for religious ritual and royal banquets, which were distinct from the ceramic bowls for everyday use.

Iron was known to appear in elemental form in meteors since a very early age—the Egyptians made weapons from meteoric iron as early as 3000 BCE. The Sumerian and Hittite name for iron meant “fire from heaven,” and both Attila and Timur had swords from heaven which gave them a mystic aura and psychological advantages. Iron has the disadvantage of quick oxidation leading to rust, in compar- ison with the more stable bronze.

A more reliable source of iron is in the form of the abundant ores of hematite, magnetite, and pyrite. The mastery of iron required the separation of iron from its ores, and then melting the iron to pour into molds; this required extremely high heat of around 1500

C, which could not be achieved without insulated kilns and bellows. The Iron Age began in the Near East, Iran, and India around 1200 BCE. Despite the need for the more demanding high temperature kilns, iron eventually replaced bronze as the primary metal for weapons and tools, because iron is harder than bronze and iron ores can be found almost everywhere.

Elemental iron is quite soft and unable to keep an edge. What we think of as useful “iron” is therefore an alloy containing some percentage of other elements, typically carbon. Since iron ore is smelted over charcoal, and charcoal is nearly pure carbon, all forms of refined iron contain at least some carbon. The simplest and oldest form of iron is known as “pig iron,” due to the resemblance of the mold- ing configuration to nursing piglets.

Pig iron can be exceedingly brittle, as it contains significant (up to 5%) quan- tities of carbon, and undesirable impurities of silicon, sulfur, and phosphorous.

75 When pig iron is remelted in the presence of air, much of the phosphorous and sul-

2.3 MATERIALS

fur burns off, producing the more useful product known as cast iron. When the car- bon content of cast iron is reduced to a very low level of 0.5% by further heating and burning in air, then removing the silica as slag, the final product is wrought iron—soft and ductile but too soft to keep an edge. All of these alloys offer some advantages over bronze, but none have the versatility of steel.

Steel is a form of refined iron that contains about 1–2% carbon. It can be forged, hammered, tempered, and annealed to become harder and more ductile. We use steel today in everything from weapons, tools, bridges, and buildings, to pots and pans. The most prized steel swords in history were manufactured in Damascus in Syria, Toledo in Spain, Solingen in Germany, and Sheffield in England. To this day, the steel Japanese samurai sword has few equals in sharpness and flexibility.

Swords need hardness to grind into and maintain a keen edge, as well as requiring flexibility to avoid shattering upon impact. For a sword to have the neces- sary hardness at the surface with flexibility at the core, it needs a higher carbon content on the cutting edge and lower carbon content in the core. Swords were heated to red hot in a charcoal fire, then hammered, and then rapidly cooled by quenching in water, to become harder and more brittle in a form called martensite. The opposite manufacturing technique is tempering, which involved allowing the hot forged steel to cool slowly thus yielding a less brittle form known as austenite. Hand forging is a technology sufficient to produce a small supply of steel for aristo- cratic weapons, but not for producing the large volume needed for buildings and structures.

The modern industrial use of steel traces its history to an invention in 1855 of Sir Henry Bessemer, a British industrialist who created a crucible to reduce the car- bon content in molten iron by a blast of hot air. This was a very fast method to make steel, which made it economical and widely used for low cost applications such as nails and beams for construction. Since then, the process has been replaced, first by the more advanced Siemens open hearth process in 1865, and then by the Basic Oxygen Process in 1952. Engineers continue to tinker with the composition of steel, sometimes adding materials such as nickel or manganese to increase its strength. The familiar stainless steel of pots and pans is an alloy with 18% chromium and 10% nickel added to inhibit corrosion. In 1977, US consumers and manufacturers used 131 million tons of steel, which averages to 397 kg of steel per person per year.

SCIENCE AND TECHNOLOGY: MATERIAL SELECTION The most important properties of materials are usually their density, mechanical strength,

melting point, and durability. The mechanical strength of material is measured by the stress–strain curve, shown in Figure 2.5. When a rod is stretched, the stress is measured by the force per area, and the strain is the percent elongation of the rod. A brittle material such as a rock would elongate very little, and would suddenly break at the fracture. A tough material such as steel would elongate much more and recover when the stress is removed, till the elastic limit when the elongation can become permanent, and then break at the ultimate strength. The areas under these two curves are the energies required to break the materials. So a material can be considered “strong” with two different

76 CHAPTER 2 INVENTIONS FOR WORK

meanings, one is high resistance to elongation like a rock, and the other is high energy required to break like steel.

Besides mechanical properties, materials have other properties that led to their use in critical technologies.

Electrical Conductivity. Silver and copper have the highest electri- cal conductivity, silicon and gallium–arsenide are semiconductors used for signal, information, displays, and computers. Superconduc- tors have essentially no electrical resistance when cooled below a critical temperature. Heat Resistance. Nickel–chromium alloys and ceramics have very high melting points, and are used for turbine blades and for heat shields for space capsules. Adhesion. Superglues are used to glue airplanes together, Teflon does not adhere to anything and is used for frying pans. Magnetic Properties. Magnetized compass is used for navigation, iron oxide is used to store information on tapes and computers, magnetized rails are used for magnetic levitation for rapid trains. Optical Properties. High refractive index glasses and polymers with optical clarity are used for lenses for eye glasses, cameras, telescopes, microscopes; optical fibers are used for transmission of information, and for light and sight in organ surgery; LED or light emitting diodes are used for computer and television dis- plays and for lighting; solar cells are used to convert sunlight into electricity. Biological Properties. Titanium and ceramics are biocompatible with human tissue and used in biomedical devices to replace hip and knee joints, and in implanted heart pacers; biodegradable poly- mers are used for the controlled release of drugs, and for the surgi- cal suture or stitches that do not have to be removed.

Subsequent developments in metallurgy continued to explore new methods to extract metals from their ores, and to convert them into alloys with useful propert- ies. The Hall–Heroult process for making aluminum relied on electrolysis rather than high heat to remove oxygen from aluminum oxide, which was a major advance that transformed aluminum from one of the most expensive metals to one of the most common. Titanium is one of the best space age metals which is as strong as steel but much lighter. Today, in their search for new metal technologies, materials scientists are extending the legacy of the alchemists who searched, unsuccessfully, for ways to turn lead into gold.