Cologne is the largest city of Germany's most populous federal state of North Rhine-Westphalia, its 1 million+ inhabitants make it the fourth most populous city in Germany after Berlin and Munich. The largest city on the Rhine, it is the most populous city both of the Rhine-Ruhr Metropolitan Region, Germany's largest and one of Europe's major metropolitan areas, of the Rhineland. Centred on the left bank of the Rhine, Cologne is about 45 kilometres southeast of North Rhine-Westphalia's capital of Düsseldorf and 25 kilometres northwest of Bonn, it is the largest city in the Central Ripuarian dialect areas. The city's famous Cologne Cathedral is the seat of the Catholic Archbishop of Cologne. There are many institutions of higher education in the city, most notably the University of Cologne, one of Europe's oldest and largest universities, the Technical University of Cologne, Germany's largest university of applied sciences, the German Sport University Cologne, Germany's only sport university.
Cologne Bonn Airport lies in the southeast of the city. The main airport for the Rhine-Ruhr region is Düsseldorf Airport. Cologne was founded and established in Ubii territory in the 1st century AD as the Roman Colonia Claudia Ara Agrippinensium, the first word of, the origin of its name. An alternative Latin name of the settlement is Augusta Ubiorum, after the Ubii. "Cologne", the French version of the city's name, has become standard in English as well. The city functioned as the capital of the Roman province of Germania Inferior and as the headquarters of the Roman military in the region until occupied by the Franks in 462. During the Middle Ages it flourished on one of the most important major trade routes between east and west in Europe. Cologne was one of the leading members of the Hanseatic League and one of the largest cities north of the Alps in medieval and Renaissance times. Prior to World War II the city had undergone several occupations by the French and by the British. Cologne was one of the most bombed cities in Germany during World War II, with the Royal Air Force dropping 34,711 long tons of bombs on the city.
The bombing reduced the population by 95% due to evacuation, destroyed the entire city. With the intention of restoring as many historic buildings as possible, the successful postwar rebuilding has resulted in a mixed and unique cityscape. Cologne is a major cultural centre for the Rhineland. Exhibitions range from local ancient Roman archeological sites to contemporary graphics and sculpture; the Cologne Trade Fair hosts a number of trade shows such as Art Cologne, imm Cologne and the Photokina. The first urban settlement on the grounds of modern-day Cologne was Oppidum Ubiorum, founded in 38 BC by the Ubii, a Cisrhenian Germanic tribe. In 50 AD, the Romans founded Colonia Claudia Ara Agrippinensium on the river Rhine and the city became the provincial capital of Germania Inferior in 85 AD. Considerable Roman remains can be found in present-day Cologne near the wharf area, where a 1,900-year-old Roman boat was discovered in late 2007. From 260 to 271 Cologne was the capital of the Gallic Empire under Postumus and Victorinus.
In 310 under emperor Constantine I a bridge was built over the Rhine at Cologne. Roman imperial governors resided in the city and it became one of the most important trade and production centres in the Roman Empire north of the Alps. Cologne is shown on the 4th century Peutinger Map. Maternus, elected as bishop in 313, was the first known bishop of Cologne; the city was the capital of a Roman province until it was occupied by the Ripuarian Franks in 462. Parts of the original Roman sewers are preserved underneath the city, with the new sewerage system having opened in 1890. Early medieval Cologne was part of Austrasia within the Frankish Empire. In 716, Charles Martel commanded an army for the first time and suffered the only defeat of his life when Chilperic II, King of Neustria, invaded Austrasia and the city fell to him in the Battle of Cologne. Charles fled to the Eifel mountains, rallied supporters, took the city back that same year after defeating Chilperic in the Battle of Amblève. Cologne had been the seat of a bishop since the Roman period.
In 843, Cologne became a city within the Treaty of Verdun-created East Francia. In 953, the archbishops of Cologne first gained noteworthy secular power, when bishop Bruno was appointed as duke by his brother Otto I, King of Germany. In order to weaken the secular nobility, who threatened his power, Otto endowed Bruno and his successors on the bishop's see with the prerogatives of secular princes, thus establishing the Electorate of Cologne, formed by the temporal possessions of the archbishopric and included in the end a strip of territory along the left Bank of the Rhine east of Jülich, as well as the Duchy of Westphalia on the other side of the Rhine, beyond Berg and Mark. By the end of the 12th century, the Archbishop of Cologne was one of the seven electors of the Holy Roman Emperor. Besides being prince elector, he was Arch-chancellor of Italy as well, technically from 1238 and permanently from 1263 until 1803. Following the Battle of Worringen in 1288, Cologne gained its independence from the archbishops and became a Free City.
Archbishop Sigfried II von Westerburg was forced to reside in Bonn. The archbishop preserv
Woodworking is the activity or skill of making items from wood, includes cabinet making, wood carving, joinery and woodturning. Along with stone and animal parts, wood was one of the first materials worked by early humans. Microwear analysis of the Mousterian stone tools used by the Neanderthals show that many were used to work wood; the development of civilization was tied to the development of greater degrees of skill in working these materials. Among early finds of wooden tools are the worked sticks from Kalambo Falls, Clacton-on-Sea and Lehringen; the spears from Schöningen provide some of the first examples of wooden hunting gear. Flint tools were used for carving. Since Neolithic times, carved wooden vessels are known, for example, from the Linear Pottery culture wells at Kückhofen and Eythra. Examples of Bronze Age wood-carving include tree trunks worked into coffins from northern Germany and Denmark and wooden folding-chairs; the site of Fellbach-Schmieden in Germany has provided fine examples of wooden animal statues from the Iron Age.
Wooden idols from the La Tène period are known from a sanctuary at the source of the Seine in France. There is significant evidence of advanced woodworking in ancient Egypt. Woodworking is depicted in many extant ancient Egyptian drawings, a considerable amount of ancient Egyptian furniture has been preserved. Tombs represent a large collection of these artefacts and the inner coffins found in the tombs were made of wood; the metal used by the Egyptians for woodworking tools was copper and after 2000 BC bronze as ironworking was unknown until much later. Used woodworking tools included axes, chisels, pull saws, bow drills. Mortise and tenon joints are attested from the earliest Predynastic period; these joints were strengthened using pegs and leather or cord lashings. Animal glue came to be used only in the New Kingdom period. Ancient Egyptians invented the art of veneering and used varnishes for finishing, though the composition of these varnishes is unknown. Although different native acacias were used, as was the wood from the local sycamore and tamarisk trees, deforestation in the Nile valley resulted in the need for the importation of wood, notably cedar, but Aleppo pine and oak, starting from the Second Dynasty.
Woodworking was essential to the Romans. It provided, sometimes the only, material for buildings, transportation and household items. Wood provided pipes, waterproofing materials, energy for heat. Although most examples of Roman woodworking have been lost, the literary record preserved much of the contemporary knowledge. Vitruvius dedicates an entire chapter of his De architectura to timber. Pliny, while not a botanist, dedicated six books of his Natural History to trees and woody plants, providing a wealth of information on trees and their uses; the progenitors of Chinese woodworking are considered to be Lu Ban and his wife Lady Yun, from the Spring and Autumn period. Lu Ban is said to have introduced the plane, chalk-line, other tools to China, his teachings were left behind in the book Lu Ban Jing. Despite this, it is believed; this book is filled with descriptions of dimensions for use in building various items such as flower pots, altars, etc. and contains extensive instructions concerning Feng Shui.
It mentions nothing of the intricate glue-less and nail-less joinery for which Chinese furniture was so famous. With the advances in modern technology and the demands of industry, woodwork as a field has changed; the development of Computer Numeric Controlled Machines, for example, has made us able to mass-produce and reproduce products faster, with less waste, more complex in design than before. CNC Routers can carve complicated and detailed shapes into flat stock, to create signs or art. Rechargeable power tools speed up creation of many projects and require much less body strength than in the past, for example when boring multiple holes. Skilled fine woodworking, remains a craft pursued by many. There remains demand for hand crafted work such as furniture and arts, however with rate and cost of production, the cost for consumers is much higher. Woodworkers relied upon the woods native to their region, until transportation and trade innovations made more exotic woods available to the craftsman.
Woods are sorted into three basic types: hardwoods typified by tight grain and derived from broadleaf trees, softwoods from coniferous trees, man-made materials such as plywood and MDF. Hardwoods, botanically known as angiosperms, are deciduous and shed their leaves annually with temperature changes. Softwoods come from trees botanically known as gymnosperms, which are coniferous, cone-bearing, stay green year round. Although a general pattern, softwoods are not always “softer” than hardwoods, vice versa. Softwood is most found in the regions of the world with lower temperatures and is less durable, lighter in weight, more vulnerable to pests and fungal attacks in comparison to hardwoods, they have a paler color and a more open grain than hardwoods, which contributes to the tendency of felled softwood to shrink and swell as it dries. Softwoods have a lower density, around 25-37lb/cu ft, which can compromise its strength. Density, does vary within both softwoods and hardwoods depending on the wood's geographical origin and growth rate.
However, the lower density of softwoods allows it to have a greater strength wi
A clock is an instrument used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, the year. Devices operating on several physical processes have been used over the millennia; some predecessors to the modern clock may be considered as "clocks" that are based on movement in nature: A sundial shows the time by displaying the position of a shadow on a flat surface. There is a range of a well-known example being the hourglass. Water clocks, along with the sundials, are the oldest time-measuring instruments. A major advance occurred with the invention of the verge escapement, which made possible the first mechanical clocks around 1300 in Europe, which kept time with oscillating timekeepers like balance wheels. Traditionally in horology, the term clock was used for a striking clock, while a clock that did not strike the hours audibly was called a timepiece. In general usage today, a "clock" refers to any device for displaying the time.
Watches and other timepieces that can be carried on one's person are distinguished from clocks. Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished; the next development in accuracy occurred after 1656 with the invention of the pendulum clock. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation; the electric clock was patented in 1840. The development of electronics in the 20th century led to clocks with no clockwork parts at all; the timekeeping element in every modern clock is a harmonic oscillator, a physical object that vibrates or oscillates at a particular frequency. This object can be a pendulum, a tuning fork, a quartz crystal, or the vibration of electrons in atoms as they emit microwaves. Clocks have different ways of displaying the time. Analog clocks indicate time with moving hands. Digital clocks display a numeric representation of time. Two numbering systems are in use.
Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays. For the blind and use over telephones, speaking clocks state the time audibly in words. There are clocks for the blind that have displays that can be read by touch; the study of timekeeping is known as horology. The word clock derives from the medieval Latin word for "bell". Clocks spread to England from the Low Countries, so the English word came from the Middle Low German and Middle Dutch Klocke; the apparent position of the Sun in the sky moves over the course of each day, reflecting the rotation of the Earth. Shadows cast by stationary objects move correspondingly, so their positions can be used to indicate the time of day. A sundial shows the time by displaying the position of a shadow on a flat surface, which has markings that correspond to the hours. Sundials can be vertical, or in other orientations. Sundials were used in ancient times. With the knowledge of latitude, a well-constructed sundial can measure local solar time with reasonable accuracy, within a minute or two.
Sundials continued to be used to monitor the performance of clocks until the modern era. Many devices can be used to mark passage of time without respect to reference time and can be useful for measuring duration or intervals. Examples of such duration timers are incense clocks and the hourglass. Both the candle clock and the incense clock work on the same principle wherein the consumption of resources is more or less constant allowing reasonably precise and repeatable estimates of time passages. In the hourglass, fine sand pouring through a tiny hole at a constant rate indicates an arbitrary, passage of time; the resource is not re-used. Water clocks known as clepsydrae, along with the sundials, are the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day counting tally stick. Given their great antiquity and when they first existed is not known and unknowable; the bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC.
Other regions of the world, including India and China have early evidence of water clocks, but the earliest dates are less certain. Some authors, write about water clocks appearing as early as 4000 BC in these regions of the world. Greek astronomer Andronicus of Cyrrhus supervised the construction of the Tower of the Winds in Athens in the 1st century B. C; the Greek and Roman civilizations are credited for advancing water clock design to include complex gearing, connected to fanciful automata and resulted in improved accuracy. These advances were passed on through Byzantium and Islamic times making their way back to Europe. Independently, the Chinese developed their own advanced water clocks（水鐘）in 725 AD, passing their ideas on to Korea and Japan; some water clock designs were developed independently and some knowledge was transferred through the spread of trade. Pre-modern societies do not have the same precise timekeeping requirements that exist in modern industrial societies, where every hour of work or rest is monitored, work may start or finish at any time regardless of external conditions.
Instead, water clocks in ancient societies were used for astrological reasons. These early water clocks were calibrated with a sundial. While never reaching the level of accuracy of a modern timepiece, the water clock was the most a
Late Middle Ages
The Late Middle Ages or Late Medieval Period was the period of European history lasting from 1250 to 1500 AD. The Late Middle Ages followed the High Middle Ages and preceded the onset of the early modern period. Around 1300, centuries of prosperity and growth in Europe came to a halt. A series of famines and plagues, including the Great Famine of 1315–1317 and the Black Death, reduced the population to around half of what it was before the calamities. Along with depopulation came social unrest and endemic warfare. France and England experienced serious peasant uprisings, such as the Jacquerie and the Peasants' Revolt, as well as over a century of intermittent conflict, the Hundred Years' War. To add to the many problems of the period, the unity of the Catholic Church was temporarily shattered by the Western Schism. Collectively, those events are sometimes called the Crisis of the Late Middle Ages. Despite the crises, the 14th century was a time of great progress in the arts and sciences. Following a renewed interest in ancient Greek and Roman texts that took root in the High Middle Ages, the Italian Renaissance began.
The absorption of Latin texts had started before the Renaissance of the 12th century through contact with Arabs during the Crusades, but the availability of important Greek texts accelerated with the Capture of Constantinople by the Ottoman Turks, when many Byzantine scholars had to seek refuge in the West Italy. Combined with this influx of classical ideas was the invention of printing, which facilitated dissemination of the printed word and democratized learning; those two things would lead to the Protestant Reformation. Toward the end of the period, the Age of Discovery began; the expansion of the Ottoman Empire cut off trading possibilities with the East. Europeans were forced to seek new trading routes, leading to the Spanish expedition under Christopher Columbus to the Americas in 1492 and Vasco da Gama’s voyage to Africa and India in 1498, their discoveries strengthened the power of European nations. The changes brought about by these developments have led many scholars to view this period as the end of the Middle Ages and the beginning of modern history and of early modern Europe.
However, the division is somewhat artificial, since ancient learning was never absent from European society. As a result, there was developmental continuity between the modern age; some historians in Italy, prefer not to speak of the Late Middle Ages at all but rather see the high period of the Middle Ages transitioning to the Renaissance and the modern era. The term "Late Middle Ages" refers to one of the three periods of the Middle Ages, along with the Early Middle Ages and the High Middle Ages. Leonardo Bruni was the first historian to use tripartite periodization in his History of the Florentine People. Flavio Biondo used a similar framework in Decades of History from the Deterioration of the Roman Empire. Tripartite periodization became standard after the German historian Christoph Cellarius published Universal History Divided into an Ancient and New Period. For 18th-century historians studying the 14th and 15th centuries, the central theme was the Renaissance, with its rediscovery of ancient learning and the emergence of an individual spirit.
The heart of this rediscovery lies in Italy, where, in the words of Jacob Burckhardt: "Man became a spiritual individual and recognized himself as such". This proposition was challenged, it was argued that the 12th century was a period of greater cultural achievement; as economic and demographic methods were applied to the study of history, the trend was to see the late Middle Ages as a period of recession and crisis. Belgian historian Henri Pirenne continued the subdivision of Early and Late Middle Ages in the years around World War I, yet it was his Dutch colleague, Johan Huizinga, responsible for popularising the pessimistic view of the Late Middle Ages, with his book The Autumn of the Middle Ages. To Huizinga, whose research focused on France and the Low Countries rather than Italy and decline were the main themes, not rebirth. Modern historiography on the period has reached a consensus between the two extremes of innovation and crisis, it is now acknowledged that conditions were vastly different north and south of the Alps, the term "Late Middle Ages" is avoided within Italian historiography.
The term "Renaissance" is still considered useful for describing certain intellectual, cultural, or artistic developments, but not as the defining feature of an entire European historical epoch. The period from the early 14th century up until – and sometimes including – the 16th century, is rather seen as characterized by other trends: demographic and economic decline followed by recovery, the end of western religious unity and the subsequent emergence of the nation state, the expansion of European influence onto the rest of the world; the limits of Christian Europe were still being defined in the 15th centuries. While the Grand Duchy of Moscow was beginning to repel the Mongols, the Iberian kingdoms completed the Reconquista of the peninsula and turned their attention outwards, the Balkans fell under the dominance of the Ottoman Empire. Meanwhile, the remaining nations of the continent were locked in constant international or internal conflict; the situation led to the consolidation of central authority and the emergence of the nation state.
The financial demands of war necessitated higher levels of taxation, resulting in the emergence of representative bodies – most notably the English Parliament. The growth of secular authority was further aided by t
A blacksmith is a metalsmith who creates objects from wrought iron or steel by forging the metal, using tools to hammer and cut. Blacksmiths produce objects such as gates, railings, light fixtures, sculpture, agricultural implements and religious items, cooking utensils and weapons. While there are many people who work with metal such as farriers and armorers, the blacksmith had a general knowledge of how to make and repair many things, from the most complex of weapons and armor to simple things like nails or lengths of chain; the "black" in "blacksmith" refers to the black firescale, a layer of oxides that forms on the surface of the metal during heating. The origin of "smith" is debated, it may come from the old English word "smythe" meaning "to strike" or it may have originated from the Proto-German "smithaz" meaning "skilled worker." Blacksmiths work by heating pieces of wrought iron or steel until the metal becomes soft enough for shaping with hand tools, such as a hammer and chisel. Heating takes place in a forge fueled by propane, natural gas, charcoal, coke or oil.
Some modern blacksmiths may employ an oxyacetylene or similar blowtorch for more localized heating. Induction heating methods are gaining popularity among modern blacksmiths. Color is important for indicating the workability of the metal; as iron heats to higher temperatures, it first glows red orange and white. The ideal heat for most forging is the bright yellow-orange color; because they must be able to see the glowing color of the metal, some blacksmiths work in dim, low-light conditions, but most work in well-lit conditions. The key is to have consistent lighting, but not too bright. Direct sunlight obscures the colors; the techniques of smithing can be divided into forging, heat-treating, finishing. Forging—the process smiths use to shape metal by hammering—differs from machining in that forging does not remove material. Instead, the smith hammers the iron into shape. Punching and cutting operations by smiths re-arrange metal around the hole, rather than drilling it out as swarf. Forging uses seven basic operations or techniques: Drawing down Shrinking Bending Upsetting Swaging Punching Forge weldingThese operations require at least a hammer and anvil, but smiths use other tools and techniques to accommodate odd-sized or repetitive jobs.
Drawing lengthens the metal by reducing one or both of the other two dimensions. As the depth is reduced, or the width narrowed, the piece is lengthened or "drawn out." As an example of drawing, a smith making a chisel might flatten a square bar of steel, lengthening the metal, reducing its depth but keeping its width consistent. Drawing does not have to be uniform. A taper can result as in making a woodworking chisel blade. If tapered in two dimensions, a point results. Drawing can be accomplished with a variety of methods. Two typical methods using only hammer and anvil would be hammering on the anvil horn, hammering on the anvil face using the cross peen of a hammer. Another method for drawing is to use a tool called a fuller, or the peen of the hammer, to hasten the drawing out of a thick piece of metal. Fullering consists of hammering a series of indentations with corresponding ridges, perpendicular to the long section of the piece being drawn; the resulting effect looks somewhat like waves along the top of the piece.
The smith turns the hammer over to use the flat face to hammer the tops of the ridges down level with the bottoms of the indentations. This forces the metal to grow in length much faster than just hammering with the flat face of the hammer. Heating iron to a "forging heat" allows bending as if it were a soft, ductile metal, like copper or silver. Bending can be done with the hammer over the horn or edge of the anvil or by inserting a bending fork into the hardy hole, placing the work piece between the tines of the fork, bending the material to the desired angle. Bends can be dressed and tightened, or widened, by hammering them over the appropriately shaped part of the anvil; some metals are "hot short". They become like Plasticine: although they may still be manipulated by squeezing, an attempt to stretch them by bending or twisting, is to have them crack and break apart; this is a problem for some blade-making steels, which must be worked to avoid developing hidden cracks that would cause failure in the future.
Though hand-worked, titanium is notably hot short. Such common smithing processes as decoratively twisting a bar are impossible with it. Upsetting is the process of making metal thicker in one dimension through shortening in the other. One form is to heat the end of a rod and hammer on it as one would drive a nail: the rod gets shorter, the hot part widens. An alternative to hammering on the hot end is to place the hot end on the anvil and hammer on the cold end. Punching may be done to make a hole. For example, in preparation for making a hammerhead, a smith would punch a hole in a heavy bar or rod for the hammer handle. Punching is not limited to holes, it includes cutting and drifting—all done with a chisel. The five basic forging processes are combined to produce and refine the shapes necessary for finished products. For example, to fashion a cross-peen hammer head, a smith would start with a bar the diameter of the ham
In the history of Europe, the Middle Ages lasted from the 5th to the 15th century. It began with the fall of the Western Roman Empire and merged into the Renaissance and the Age of Discovery; the Middle Ages is the middle period of the three traditional divisions of Western history: classical antiquity, the medieval period, the modern period. The medieval period is itself subdivided into the Early and Late Middle Ages. Population decline, counterurbanisation, collapse of centralized authority and mass migrations of tribes, which had begun in Late Antiquity, continued in the Early Middle Ages; the large-scale movements of the Migration Period, including various Germanic peoples, formed new kingdoms in what remained of the Western Roman Empire. In the 7th century, North Africa and the Middle East—once part of the Byzantine Empire—came under the rule of the Umayyad Caliphate, an Islamic empire, after conquest by Muhammad's successors. Although there were substantial changes in society and political structures, the break with classical antiquity was not complete.
The still-sizeable Byzantine Empire, Rome's direct continuation, survived in the Eastern Mediterranean and remained a major power. The empire's law code, the Corpus Juris Civilis or "Code of Justinian", was rediscovered in Northern Italy in 1070 and became admired in the Middle Ages. In the West, most kingdoms incorporated the few extant Roman institutions. Monasteries were founded; the Franks, under the Carolingian dynasty established the Carolingian Empire during the 8th and early 9th century. It covered much of Western Europe but succumbed to the pressures of internal civil wars combined with external invasions: Vikings from the north, Magyars from the east, Saracens from the south. During the High Middle Ages, which began after 1000, the population of Europe increased as technological and agricultural innovations allowed trade to flourish and the Medieval Warm Period climate change allowed crop yields to increase. Manorialism, the organisation of peasants into villages that owed rent and labour services to the nobles, feudalism, the political structure whereby knights and lower-status nobles owed military service to their overlords in return for the right to rent from lands and manors, were two of the ways society was organised in the High Middle Ages.
The Crusades, first preached in 1095, were military attempts by Western European Christians to regain control of the Holy Land from Muslims. Kings became the heads of centralised nation-states, reducing crime and violence but making the ideal of a unified Christendom more distant. Intellectual life was marked by scholasticism, a philosophy that emphasised joining faith to reason, by the founding of universities; the theology of Thomas Aquinas, the paintings of Giotto, the poetry of Dante and Chaucer, the travels of Marco Polo, the Gothic architecture of cathedrals such as Chartres are among the outstanding achievements toward the end of this period and into the Late Middle Ages. The Late Middle Ages was marked by difficulties and calamities including famine and war, which diminished the population of Europe. Controversy and the Western Schism within the Catholic Church paralleled the interstate conflict, civil strife, peasant revolts that occurred in the kingdoms. Cultural and technological developments transformed European society, concluding the Late Middle Ages and beginning the early modern period.
The Middle Ages is one of the three major periods in the most enduring scheme for analysing European history: classical civilisation, or Antiquity. The "Middle Ages" first appears in Latin in 1469 as media tempestas or "middle season". In early usage, there were many variants, including medium aevum, or "middle age", first recorded in 1604, media saecula, or "middle ages", first recorded in 1625; the alternative term "medieval" derives from medium aevum. Medieval writers divided history into periods such as the "Six Ages" or the "Four Empires", considered their time to be the last before the end of the world; when referring to their own times, they spoke of them as being "modern". In the 1330s, the humanist and poet Petrarch referred to pre-Christian times as antiqua and to the Christian period as nova. Leonardo Bruni was the first historian to use tripartite periodisation in his History of the Florentine People, with a middle period "between the fall of the Roman Empire and the revival of city life sometime in late eleventh and twelfth centuries".
Tripartite periodisation became standard after the 17th-century German historian Christoph Cellarius divided history into three periods: ancient and modern. The most given starting point for the Middle Ages is around 500, with the date of 476 first used by Bruni. Starting dates are sometimes used in the outer parts of Europe. For Europe as a whole, 1500 is considered to be the end of the Middle Ages, but there is no universally agreed upon end date. Depending on the context, events such as the conquest of Constantinople by the Turks in 1453, Christopher Columbus's first voyage to the Americas in 1492, or the Protestant Reformation in 1517 are sometimes used. English historians use the Battle of Bosworth Field in 1485 to mark the end of the period. For Spain, dates used are the death of King Ferdinand II in 1516, the death of Queen Isabella I of Castile in 1504, or the conquest of Granada in 1492. Historians from Romance-speaking countries tend to divide the Middle Ages into two parts: an earlier "High" and late
An abrasive is a material a mineral, used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material means polishing it to gain a smooth, reflective surface, the process can involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut and polish other softer materials are known as abrasives. Abrasives are commonplace and are used extensively in a wide variety of industrial and technological applications; this gives rise to a large variation in the physical and chemical composition of abrasives as well as the shape of the abrasive. Some common uses for abrasives include grinding, buffing, cutting, sharpening and sanding. Files are not abrasives. However, diamond files are a form of coated abrasive. Abrasives rely upon a difference in hardness between the abrasive and the material being worked upon, the abrasive being the harder of the two substances. However, is not necessary as any two solid materials that rub against each other will tend to wear each other away.
Materials used as abrasives are either hard minerals or are synthetic stones, some of which may be chemically and physically identical to occurring minerals but which cannot be called minerals as they did not arise naturally. Diamond, a common abrasive, for instance occurs both and is industrially produced, as is corundum which occurs but, nowadays more manufactured from bauxite; however softer minerals like calcium carbonate are used as abrasives, such as "polishing agents" in toothpaste. These minerals are either crushed or are of a sufficiently small size to permit their use as an abrasive; these grains called grit, have rough edges terminating in points which will decrease the surface area in contact and increase the localised contact pressure. The abrasive and the material to be worked are brought into contact while in relative motion to each other. Force applied through the grains causes fragments of the worked material to break away, while smoothing the abrasive grain and/or causing the grain to work loose from the rest of the abrasive.
Some factors which will affect how a substance is abraded include: Difference in hardness between the two substances: a much harder abrasive will cut faster and deeper Grain size: larger grains will cut faster as they cut deeper Adhesion between grains, between grains and backing, between grains and matrix: determines how grains are lost from the abrasive and how soon fresh grains, if present, are exposed Contact force: more force will cause faster abrasion Loading: worn abrasive and cast off work material tends to fill spaces between abrasive grains so reducing cutting efficiency while increasing friction Use of lubricant/coolant/metalworking fluid: Can carry away swarf, transport heat, decrease friction, suspend worn work material and abrasives allowing for a finer finish, conduct stress to the workpiece. Abrasives may be classified as either synthetic; when discussing sharpening stones, natural stones have long been considered superior but advances in material technology are seeing this distinction become less distinct.
Many synthetic abrasives are identical to a natural mineral, differing only in that the synthetic mineral has been manufactured rather than mined. Impurities in the natural mineral may make it less effective; some occurring abrasives are: Calcite Emery Diamond dust Novaculite Pumice Iron oxide Sand Corundum Garnet Sandstone Tripoli Powdered feldspar StauroliteSome abrasive minerals occur but are sufficiently rare or sufficiently more difficult or costly to obtain such that a synthetic stone is used industrially. These and other artificial abrasives include: Borazon Ceramic Ceramic aluminium oxide Ceramic iron oxide Corundum Dry ice Glass powder Steel abrasive Silicon carbide Zirconia alumina Boron carbide Slags Abrasives are shaped for various purposes. Natural abrasives are sold as dressed stones in the form of a rectangular block. Both natural and synthetic abrasives are available in a wide variety of shapes coming as bonded or coated abrasives, including blocks, discs, sheets and loose grains.
A bonded abrasive is composed of an abrasive material contained within a matrix, although fine aluminium oxide abrasive may comprise sintered material. This matrix is called a binder and is a clay, a resin, a glass or a rubber; this mixture of binder and abrasive is typic