Trencadís known as pique assiette, broken tile mosaics and pieces, shardware, is a type of mosaic made from cemented-together tile shards and broken chinaware. Glazed china tends to be preferred, glass is sometimes mixed in as well, as are other small materials like buttons and shells. Artists working in this form may create random designs, pictorial scenes, geometric patterns, or a hybrid of any of these. Although as a folk art the method itself may be centuries old, the two most used terms are both of modern origin. Trencadís, a Catalan term that means'chopped', is the name for this method as it was revived in early 20th century Catalan modernism, while pique assiette is a more general name for the technique that comes from the French language. In French, pique assiette is a term for a scrounger or sponger, thus as a name for this mosaic technique, it refers to the recycled or'scrounged' nature of the materials. Traditional mosaics, such as classical Roman floors, are made up of individual tesserae small cubes that are uniformly shaped and designed for their intended use.
Trencadís differs in that the tesserae are nonuniform pieces broken from tiles and chinaware made for other uses. Trencadís is thus recycled art. There are two main methods for trencadís. In the first, an initial design is drawn up and the ceramic fragments are fitted into the design. Alternatively, an artist may spontaneously arrange fragments without a prior design; the Catalan modernist architects Antoni Gaudí and Josep Maria Jujol used trencadís in many projects, among which Barcelona's Parc Güell is the most famous. Gaudí's first use of this technique was at the Güell Pavilions, where the sinuous architecture forced him to break the tiles in order to cover the curved surfaces. Gaudí tended to create patterns with his trencadís work, he leaned towards brightly colored glazed ceramic shards, he used discarded pieces of ceramic tile collected from the factory Pujol i Bausis located in Esplugues de Llobregat, as well as pieces of white ceramic from broken cups and plates discarded by other Spanish manufacturers.
The Valencian architect Demetrio Ribes used trencadís extensively for decoration in the hall of Valencia North Station in 1907. In France, the term pique assiette is most associated with Raymond Edouard Isadore a French graveyard sweeper and folk artist. Starting in the late 1930s, he spent 30 years covering both the inside and outside of his house as well as his furniture and his garden walls with mosaics, he found his materials in the surrounding fields and quarries, in the public dump, at auctions. This habit of scavenging earned him the nickname "pique assiette" shortened to "picassiette". Isadore, a religious man, created many of his mosaic scenes with Christian personages and symbols, he built a "sweeper's throne" and a "sweeper's tomb" covered in pique assiette. As the mosaics expanded, the project became more known, in 1954, Pablo Picasso visited Isadore's house. Today, the house is a tourist attraction near Chartres known as "Maison Picassiette"; the Watts Towers in Los Angeles were built over a period of 30 years by Simon Rodia, a construction worker and tile mason.
Begun in 1921, the 17 interconnected towers were decorated with fragments of porcelain, glass and other found objects. Rodia built them without a premade plan, using damaged pieces from local tile companies and materials scavenged by neighborhood children. A contemporary example is the Bridge of the Dragon, which crosses the Guadaíra River at Alcalá de Guadaíra; the bridge's support structure is covered in trencadís. Designed by engineer José Luis Manzanares, it was directly inspired by Gaudí's dragon fountain in Parc Güell. A related form is an American folk art form that memorializes the dead; the memory jug is a vessel with a mosaic-like surface decoration of glass and ceramic shards, trinkets and other small objects objects associated with a specific dead person. Most known examples date back no further than the early 20th century. Fassett and Candace Bahouth. Mosaics Marshall, Marlene Hurley. Making Bits and Pieces Mosaics Wallach, Mara. Making Mosaics with Found Objects Media related to Trencadís at Wikimedia Commons
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
Ancient Greek art
Ancient Greek art stands out among that of other ancient cultures for its development of naturalistic but idealized depictions of the human body, in which nude male figures were the focus of innovation. The rate of stylistic development between about 750 and 300 BC was remarkable by ancient standards, in surviving works is best seen in sculpture. There were important innovations in painting, which have to be reconstructed due to the lack of original survivals of quality, other than the distinct field of painted pottery. Greek architecture, technically simple, established a harmonious style with numerous detailed conventions that were adopted by Roman architecture and are still followed in some modern buildings, it used a vocabulary of ornament, shared with pottery and other media, had an enormous influence on Eurasian art after Buddhism carried it beyond the expanded Greek world created by Alexander the Great. The social context of Greek art included radical political developments and a great increase in prosperity.
The earliest art by Greeks is excluded from "ancient Greek art", instead known as Aegean art. The art of ancient Greece is divided stylistically into four periods: the Geometric, Archaic and Hellenistic; the Geometric age is dated from about 1000 BC, although in reality little is known about art in Greece during the preceding 200 years, traditionally known as the Greek Dark Ages. The 7th century BC witnessed the slow development of the Archaic style as exemplified by the black-figure style of vase painting. Around 500 BC, shortly before the onset of the Persian Wars, is taken as the dividing line between the Archaic and the Classical periods, the reign of Alexander the Great is taken as separating the Classical from the Hellenistic periods. From some point in the 1st century BC onwards "Greco-Roman" is used, or more local terms for the Eastern Greek world. In reality, there was no sharp transition from one period to another. Forms of art developed at different speeds in different parts of the Greek world, as in any age some artists worked in more innovative styles than others.
Strong local traditions, the requirements of local cults, enable historians to locate the origins of works of art found far from their place of origin. Greek art of various kinds was exported; the whole period saw a steady increase in prosperity and trading links within the Greek world and with neighbouring cultures. The survival rate of Greek art differs starkly between media. We have huge quantities of pottery and coins, much stone sculpture, though more Roman copies, a few large bronze sculptures. Missing are painting, fine metal vessels, anything in perishable materials including wood; the stone shell of a number of temples and theatres has survived, but little of their extensive decoration. By convention, finely painted vessels of all shapes are called "vases", there are over 100,000 complete surviving pieces, giving unparalleled insights into many aspects of Greek life. Sculptural or architectural pottery very painted, are referred to as terracottas, survive in large quantities. In much of the literature, "pottery" means only painted vessels, or "vases".
Pottery was the main form of grave goods deposited in tombs as "funerary urns" containing the cremated ashes, was exported. The famous and distinctive style of Greek vase-painting with figures depicted with strong outlines, with thin lines within the outlines, reached its peak from about 600 to 350 BC, divides into the two main styles reversals of each other, of black-figure and red-figure painting, the other colour forming the background in each case. Other colours were limited to small areas of white and larger ones of a different purplish-red. Within the restrictions of these techniques and other strong conventions, vase-painters achieved remarkable results, combining refinement and powerful expression. White ground technique allowed more freedom in depiction, but did not wear well and was made for burial. Conventionally, the ancient Greeks are said to have made most pottery vessels for everyday use, not for display. Exceptions are the large Archaic monumental vases made as grave-markers, trophies won at games, such as the Panathenaic Amphorae filled with olive oil, pieces made to be left in graves.
In recent decades many scholars have questioned this, seeing much more production than was thought as made to be placed in graves, as a cheaper substitute for metalware in both Greece and Etruria. Most surviving pottery consists of vessels for storing, serving or drinking liquids such as amphorae, hydria, libation bowls and perfume bottles for the toilet and cups. Painted vessels for serving and eating food are much less common. Painted pottery was affordable by ordinary people, a piece "decently decorated with about five or six figures cost about two or three days' wages". Miniatures were produced in large numbers for use as offerings at temples. In the Hellenistic period a wider range of pottery was produced, but most of it is of little artistic importance. In earlier periods quite small Greek cities produced pottery for their own locale. These
Italy the Italian Republic, is a country in Southern Europe. Located in the middle of the Mediterranean Sea, Italy shares open land borders with France, Austria and the enclaved microstates San Marino and Vatican City. Italy covers an area of 301,340 km2 and has a temperate seasonal and Mediterranean climate. With around 61 million inhabitants, it is the fourth-most populous EU member state and the most populous country in Southern Europe. Due to its central geographic location in Southern Europe and the Mediterranean, Italy has been home to a myriad of peoples and cultures. In addition to the various ancient peoples dispersed throughout modern-day Italy, the most famous of which being the Indo-European Italics who gave the peninsula its name, beginning from the classical era and Carthaginians founded colonies in insular Italy and Genoa, Greeks established settlements in the so-called Magna Graecia, while Etruscans and Celts inhabited central and northern Italy respectively; the Italic tribe known as the Latins formed the Roman Kingdom in the 8th century BC, which became a republic with a government of the Senate and the People.
The Roman Republic conquered and assimilated its neighbours on the peninsula, in some cases through the establishment of federations, the Republic expanded and conquered parts of Europe, North Africa and the Middle East. By the first century BC, the Roman Empire emerged as the dominant power in the Mediterranean Basin and became the leading cultural and religious centre of Western civilisation, inaugurating the Pax Romana, a period of more than 200 years during which Italy's technology, economy and literature flourished. Italy remained the metropole of the Roman Empire; the legacy of the Roman Empire endured its fall and can be observed in the global distribution of culture, governments and the Latin script. During the Early Middle Ages, Italy endured sociopolitical collapse and barbarian invasions, but by the 11th century, numerous rival city-states and maritime republics in the northern and central regions of Italy, rose to great prosperity through shipping and banking, laying the groundwork for modern capitalism.
These independent statelets served as Europe's main trading hubs with Asia and the Near East enjoying a greater degree of democracy than the larger feudal monarchies that were consolidating throughout Europe. The Renaissance began in Italy and spread to the rest of Europe, bringing a renewed interest in humanism, science and art. Italian culture flourished, producing famous scholars and polymaths such as Michelangelo, Leonardo da Vinci, Raphael and Machiavelli. During the Middle Ages, Italian explorers such as Marco Polo, Christopher Columbus, Amerigo Vespucci, John Cabot and Giovanni da Verrazzano discovered new routes to the Far East and the New World, helping to usher in the European Age of Discovery. Italy's commercial and political power waned with the opening of trade routes that bypassed the Mediterranean. Centuries of infighting between the Italian city-states, such as the Italian Wars of the 15th and 16th centuries, left the region fragmented, it was subsequently conquered and further divided by European powers such as France and Austria.
By the mid-19th century, rising Italian nationalism and calls for independence from foreign control led to a period of revolutionary political upheaval. After centuries of foreign domination and political division, Italy was entirely unified in 1871, establishing the Kingdom of Italy as a great power. From the late 19th century to the early 20th century, Italy industrialised, namely in the north, acquired a colonial empire, while the south remained impoverished and excluded from industrialisation, fuelling a large and influential diaspora. Despite being one of the main victors in World War I, Italy entered a period of economic crisis and social turmoil, leading to the rise of a fascist dictatorship in 1922. Participation in World War II on the Axis side ended in military defeat, economic destruction and the Italian Civil War. Following the liberation of Italy and the rise of the resistance, the country abolished the monarchy, reinstated democracy, enjoyed a prolonged economic boom and, despite periods of sociopolitical turmoil became a developed country.
Today, Italy is considered to be one of the world's most culturally and economically advanced countries, with the sixth-largest worldwide national wealth. Its advanced economy ranks eighth-largest in the world and third in the Eurozone by nominal GDP. Italy owns the third-largest central bank gold reserve, it has a high level of human development, it stands among the top countries for life expectancy. The country plays a prominent role in regional and global economic, military and diplomatic affairs. Italy is a founding and leading member of the European Union and a member of numerous international institutions, including the UN, NATO, the OECD, the OSCE, the WTO, the G7, the G20, the Union for the Mediterranean, the Council of Europe, Uniting for Consensus, the Schengen Area and many more; as a reflection
In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. The cube is one of the five Platonic solids, it has 6 faces, 12 edges, 8 vertices. The cube is a square parallelepiped, an equilateral cuboid and a right rhombohedron, it is a regular square prism in three orientations, a trigonal trapezohedron in four orientations. The cube is dual to the octahedron, it has octahedral symmetry. The cube is the only convex polyhedron; the cube has four special orthogonal projections, centered, on a vertex, edges and normal to its vertex figure. The first and third correspond to the B2 Coxeter planes; the cube can be represented as a spherical tiling, projected onto the plane via a stereographic projection. This projection is conformal, preserving angles but not lengths. Straight lines on the sphere are projected as circular arcs on the plane. For a cube centered at the origin, with edges parallel to the axes and with an edge length of 2, the Cartesian coordinates of the vertices are while the interior consists of all points with −1 < xi < 1 for all i.
In analytic geometry, a cube's surface with center and edge length of 2a is the locus of all points such that max = a. For a cube of edge length a: As the volume of a cube is the third power of its sides a × a × a, third powers are called cubes, by analogy with squares and second powers. A cube has the largest volume among cuboids with a given surface area. A cube has the largest volume among cuboids with the same total linear size. For a cube whose circumscribing sphere has radius R, for a given point in its 3-dimensional space with distances di from the cube's eight vertices, we have: ∑ i = 1 8 d i 4 8 + 16 R 4 9 = 2. Doubling the cube, or the Delian problem, was the problem posed by ancient Greek mathematicians of using only a compass and straightedge to start with the length of the edge of a given cube and to construct the length of the edge of a cube with twice the volume of the original cube, they were unable to solve this problem, in 1837 Pierre Wantzel proved it to be impossible because the cube root of 2 is not a constructible number.
The cube has three uniform colorings, named by the colors of the square faces around each vertex: 111, 112, 123. The cube has three classes of symmetry, which can be represented by vertex-transitive coloring the faces; the highest octahedral symmetry Oh has all the faces the same color. The dihedral symmetry D4h comes from the cube being a prism, with all four sides being the same color; the lowest symmetry D2h is a prismatic symmetry, with sides alternating colors, so there are three colors, paired by opposite sides. Each symmetry form has a different Wythoff symbol. A cube has eleven nets: that is, there are eleven ways to flatten a hollow cube by cutting seven edges. To color the cube so that no two adjacent faces have the same color, one would need at least three colors; the cube is the cell of the only regular tiling of three-dimensional Euclidean space. It is unique among the Platonic solids in having faces with an number of sides and it is the only member of that group, a zonohedron; the cube can be cut into six identical square pyramids.
If these square pyramids are attached to the faces of a second cube, a rhombic dodecahedron is obtained. The analogue of a cube in four-dimensional Euclidean space has a special name—a tesseract or hypercube. More properly, a hypercube is the analogue of the cube in n-dimensional Euclidean space and a tesseract is the order-4 hypercube. A hypercube is called a measure polytope. There are analogues of the cube in lower dimensions too: a point in dimension 0, a line segment in one dimension and a square in two dimensions; the quotient of the cube by the antipodal map yields the hemicube. If the original cube has edge length 1, its dual polyhedron has edge length 2 / 2; the cube is a special case in various classes of general polyhedra: The vertices of a cube can be grouped into two groups of four, each forming a regular tetrahedron. These two together form the stella octangula; the int
A mosaic is a piece of art or image made from the assembling of small pieces of colored glass, stone, or other materials. It is used in decorative art or as interior decoration. Most mosaics are made of small, flat square, pieces of stone or glass of different colors, known as tesserae; some floor mosaics, are made of small rounded pieces of stone, called "pebble mosaics". Mosaics have a long history, starting in Mesopotamia in the 3rd millennium BC. Pebble mosaics were made in Tiryns in Mycenean Greece. Early Christian basilicas from the 4th century onwards were decorated with ceiling mosaics. Mosaic art flourished in the Byzantine Empire from the 6th to the 15th centuries. Mosaic fell out of fashion in the Renaissance, though artists like Raphael continued to practise the old technique. Roman and Byzantine influence led Jewish artists to decorate 5th and 6th century synagogues in the Middle East with floor mosaics. Mosaic was used on religious buildings and palaces in early Islamic art, including Islam's first great religious building, the Dome of the Rock in Jerusalem, the Umayyad Mosque in Damascus.
Mosaic went out of fashion in the Islamic world after the 8th century. Modern mosaics are made by professional artists, street artists, as a popular craft. Many materials other than traditional stone and ceramic tesserae may be employed, including shells and beads; the earliest known examples of mosaics made of different materials were found at a temple building in Abra and are dated to the second half of 3rd millennium BC. They consist of pieces of colored stones and ivory. Excavations at Susa and Chogha Zanbil show evidence of the first glazed tiles, dating from around 1500 BC. However, mosaic patterns were not used until the times of Roman influence. Bronze age pebble mosaics have been found at Tiryns. Mythological subjects, or scenes of hunting or other pursuits of the wealthy, were popular as the centrepieces of a larger geometric design, with emphasized borders. Pliny the Elder mentions the artist Sosus of Pergamon by name, describing his mosaics of the food left on a floor after a feast and of a group of doves drinking from a bowl.
Both of these themes were copied. Greek figural mosaics could have been copied or adapted paintings, a far more prestigious artform, the style was enthusiastically adopted by the Romans so that large floor mosaics enriched the floors of Hellenistic villas and Roman dwellings from Britain to Dura-Europos. Most recorded names of Roman mosaic workers are Greek, suggesting they dominated high quality work across the empire. Splendid mosaic floors are found in Roman villas across North Africa, in places such as Carthage, can still be seen in the extensive collection in Bardo Museum in Tunis, Tunisia. There were two main techniques in Greco-Roman mosaic: opus vermiculatum used tiny tesserae cubes of 4 millimeters or less, was produced in workshops in small panels which were transported to the site glued to some temporary support; the tiny tesserae allowed fine detail, an approach to the illusionism of painting. Small panels called emblemata were inserted into walls or as the highlights of larger floor-mosaics in coarser work.
The normal technique was opus tessellatum, using larger tesserae, laid on site. There was a distinct native Italian style using black on a white background, no doubt cheaper than coloured work. In Rome and his architects used mosaics to cover some surfaces of walls and ceilings in the Domus Aurea, built 64 AD, wall mosaics are found at Pompeii and neighbouring sites; however it seems that it was not until the Christian era that figural wall mosaics became a major form of artistic expression. The Roman church of Santa Costanza, which served as a mausoleum for one or more of the Imperial family, has both religious mosaic and decorative secular ceiling mosaics on a round vault, which represent the style of contemporary palace decoration; the mosaics of the Villa Romana del Casale near Piazza Armerina in Sicily are the largest collection of late Roman mosaics in situ in the world, are protected as a UNESCO World Heritage Site. The large villa rustica, owned by Emperor Maximian, was built in the early 4th century.
The mosaics were covered and protected for 700 years by a landslide that occurred in the 12th Century. The most important pieces are the Circus Scene, the 64m long Great Hunting Scene, the Little Hunt, the Labours of Hercules and the famous Bikini Girls, showing women undertaking a range of sporting activities in garments that resemble 20th Century bikinis; the peristyle, the imperial apartments and the thermae were decorated with ornamental and mythological mosaics. Other important examples of Roman mosaic art in Sicily were unearthed on the Piazza Vittoria in Palermo where two houses were discovered; the most important scenes there depicted are an Orpheus mosaic, Alexander the Great's Hunt and the Four Seasons. In 1913 the Zliten mosaic, a Roman mosaic famous for its many scenes from gladiatorial contests and everyday life, was discovered in the Libyan town of Zliten. In 2000 archaeologists working
The term stained glass can refer to coloured glass as a material or to works created from it. Throughout its thousand-year history, the term has been applied exclusively to the windows of churches and other significant religious buildings. Although traditionally made in flat panels and used as windows, the creations of modern stained glass artists include three-dimensional structures and sculpture. Modern vernacular usage has extended the term "stained glass" to include domestic lead light and objects d'art created from foil glasswork exemplified in the famous lamps of Louis Comfort Tiffany; as a material stained glass is glass, coloured by adding metallic salts during its manufacture. The coloured glass is crafted into stained glass windows in which small pieces of glass are arranged to form patterns or pictures, held together by strips of lead and supported by a rigid frame. Painted details and yellow stain are used to enhance the design; the term stained glass is applied to windows in which the colours have been painted onto the glass and fused to the glass in a kiln.
Stained glass, as an art and a craft, requires the artistic skill to conceive an appropriate and workable design, the engineering skills to assemble the piece. A window must fit snugly into the space for which it is made, must resist wind and rain, especially in the larger windows, must support its own weight. Many large windows have withstood the test of time and remained intact since the Late Middle Ages. In Western Europe they constitute the major form of pictorial art to have survived. In this context, the purpose of a stained glass window is not to allow those within a building to see the world outside or primarily to admit light but rather to control it. For this reason stained glass windows have been described as "illuminated wall decorations"; the design of a window may be figurative. Windows within a building may be thematic, for example: within a church – episodes from the life of Christ. Stained glass is still popular today, but referred to as art glass, it is prevalent in luxury homes, commercial buildings, places of worship.
Artists and companies are contracted to create beautiful art glass ranging from domes, backsplashes, etc. During the late medieval period, glass factories were set up where there was a ready supply of silica, the essential material for glass manufacture. Silica requires a high temperature to melt, something not all glass factories were able to achieve; such materials as potash and lead can be added to lower the melting temperature. Other substances, such as lime, are added to rebuild the weakened network and make the glass more stable. Glass is coloured by adding metallic oxide powders or finely divided metals while it is in a molten state. Copper oxides produce green or bluish green, cobalt makes deep blue, gold produces wine red and violet glass. Much modern red glass is produced using copper, less expensive than gold and gives a brighter, more vermilion shade of red. Glass coloured while in the clay pot in the furnace is known as pot metal glass, as opposed to flashed glass. Using a blow-pipe, a "gather" of molten glass is taken from the pot heating in the furnace.
The gather is formed to a bubble of air blown into it. Using metal tools, molds of wood that have been soaking in water, gravity, the gather is manipulated to form a long, cylindrical shape; as it cools, it is reheated. During the process, the bottom of the cylinder is removed. Once brought to the desired size it is left to cool. One side of the cylinder is opened, it is put into another oven to heat and flatten it, placed in an annealer to cool at a controlled rate, making the material more stable. "Hand-blown" cylinder and crown glass were the types used in ancient stained-glass windows. Stained glass windows were in churches and chapels as well as many more well respected buildings; this hand-blown glass is created by blowing a bubble of air into a gather of molten glass and spinning it, either by hand or on a table that revolves like a potter's wheel. The centrifugal force causes the molten bubble to flatten, it can be cut into small sheets. Glass formed this way can be either coloured and used for stained-glass windows, or uncoloured as seen in small paned windows in 16th- and 17th-century houses.
Concentric, curving waves are characteristic of the process. The center of each piece of glass, known as the "bull's-eye", is subject to less acceleration during spinning, so it remains thicker than the rest of the sheet, it has the distinctive lump of glass left by the "pontil" rod, which holds the glass as it is spun out. This lumpy, refractive quality means the bulls-eyes are less transparent, but they have still been used for windows, both domestic and ecclesiastical. Crown glass is still made today, but not on a large scale. Rolled glass is produced by pouring molten glass onto a metal or graphite table and rolling it into a sheet using a large metal cylinder, similar to rolling out a pie crust; the rolling can be done by machine. Glass can be "double rolled", which means it is passed through two cylinders at once to yield glass of a specified thickness (typically about 1/8" or