Architectural terracotta

Terracotta was made by the ancient Greeks, Babylonians, ancient Egyptians, Romans, Chinese, and the Indus River Valley and Native American cultures. It was used for roof tiles, medallions, statues, capitals and other small architectural details. Ancient Eastern terracotta , Kanpur Indian terracotta manufacturers hand pressed, poured, and double-molded the clay mix. Plaster casts have been found in several ancient sites in Afghanistan, Bangladesh, India and Pakistan. Similarities in motifs and manufacturing processes have caused scholars to note cross cultural pollination between the Hellenic and Indus River Valley sculptural terracotta traditions. Famous early examples include the Bhitargaon temple and the Jain temple in the Mahbubnagar district. Chinese, Korean, and Japanese terracotta making traditions were focused on non-architectural uses such as statuary or cookware but various forms of terracotta tiles were popular roofing materials. Western terracotta Antiquity–1700s Greeks used terracotta for capitals, friezes, and other elements of their temples like at Olympia or Selenius. Domestically they used it for statuary and roof tiles. The Etruscans used terracotta for roof tiles, encased beams, and enclosed brick walls with it. The Roman terracotta innovation was the underfloor or hypocaust heating system that they used for their bath houses. Medieval European architecture did not expand terracotta use beyond the ancients. The manufacture of tile roofs diminished with low cost thatch roofing widely available. in London has an ornate terracotta facade typical of high Victorian architecture – the carvings represent the contents of the museum. 1700s–1880s Great Britain Richard Holt and Thomas Ripley patented an artificial stone recipe in 1722. The business was fairly successful at making small architectural ornaments. Their company was taken over by George and Eleanor Coade in 1769. was one of many manufacturers of architectural terracotta in the United States between the late 1800s and 1920s. 1870s–1930s The Chicago Fire of 1871 destroyed many of the wood and stone-constructed buildings of Chicago, Illinois, and spurred greater interest in fireproof building materials that could enable the elaborate construction of the era. James Taylor, an English-trained ceramicist, played a key role in establishing effective widespread terracotta production in the United States through his work for various firms such as the Chicago Terra Cotta Company, the Boston Terra Cotta Company, and the New York Architectural Terra-Cotta Company. == Manufacturing process ==

Terracotta can be made by pouring or pressing the mix into a plaster or sandstone mold, clay can be hand carved, or mix can be extruded into a mold using specialized machines. Clay shrinks as it dries from water loss therefore all molds are made slightly larger than the required dimensions. After the desired green-ware, or air dried, shape is created it is fired in a kiln for several days where it shrinks even further. The hot clay is slowly cooled then hand finished. The ceramics are shipped to the project site where they are installed by local contractors. The hollow pieces are partially backfilled with mortar then placed into the wall, suspended from metal anchors, or hung on metal shelf angles. Design Academically trained artists were often the designers of the terracotta forms. Their drawings would be interpreted by the manufacturer who would plan out the joint locations and anchoring system. Once finalized, the drawings were turned into a plaster reality by sculptors who would create the mold for the craftsmen. used by the Moravian Pottery and Tile Works in Pennsylvania to refine the clay used for tile production|thumb Clay preparation Clay selection was very important to manufacture of terracotta. Homogenous, finer grain sizes were preferred. The color of the clay body was determined by the types of deposits that were locally available to the manufacture. Sand was added to temper the process. Crushed ceramic scraps called grog were also added to stiffen the product and help reduce shrinkage. Weathering the clay allowed pyrites to chemically change to hydrated ferric oxide and reduced alkali content. This aging minimizes the potential chemical changes during the rest of the manufacturing process. The weathered raw clay was dried, ground, and screened. Later, it would have been pugged in a mill that would mix the clay with water using rotating blades and force the blend through a sieve. Hand pressing terracotta An artist makes a negative plaster mold based on a clay positive prototype. 1–1¼" of the clay/water mixture is pressed into the mold. Wire mesh or other stiffeners are added to create the web, or clay body that surrounds the hollow cell. The product is air dried to allow the plaster to suck the moisture out of the green clay product. It is fired then slowly cooled. Glazing The last step before firing the greenware was glazing. True glazes are made from various salts but prior to the 1890s most blocks were slip glazed or coated with a watered-down version of the clay mix. Liquefying the clay increased the amount of small silica particles that would be deposited on the surface of the block. These would melt during firing and harden. By 1900 almost all colors could be achieved with the addition of salt glazes. Black or brown were made by adding manganese oxide. 's terracotta pediment using polychrome glazing Firing The kiln firing process could take days, up to two weeks. The clay is heated slowly to around 500°C to sweat off the loose or macroscopic water between the molecules. Then the temperature is increased to close to 900°C to release the chemically bonded water in gaseous form and the clay particles will begin to melt together or sinter. If the kiln reaches 1000°C then the clay particles will vitrtify and become glass like. After the maximum temperature was reached then the clay was slowly cooled over a few days. During firing a fireskin is created. A fireskin is the glass-like "bread crust" that covers the biscuit or interior body. Various kilns were used as technology developed and capital was available for investment. Muffle kilns were the most common kiln. They were used as early as 1870. The kilns burned gas, coal, or oil that heated an interior chamber from an exterior chamber. The walls "muffled" the heat so the greenware was not directly exposed to the flames. Down-draught kilns were also widely used. The interior chamber radiated heat around the terracotta by pulling in hot air from behind an exterior cavity wall. Like the muffle wall, the cavity wall protected the greenware from burning. Installation The earliest terracotta elements were laid directly into the masonry but as structural metal became more popular terracotta was suspended by metal anchors. The development of cast and later wrought iron as a structural material was closely linked to the rise of terracotta. Cast iron was first used as columns in the 1820s by William Strickland. Over the course of the 19th century metal became more incorporated into construction but it was not widely used structurally until the late 1890s. A series of disastrous fires (Chicago, 1871; Boston, 1872; and San Francisco, 1906) earned terracotta a reputation for being a fireproof, lightweight cladding material that could protect metal from melting. Holes were bored in the hollow blocks in choice locations to allow for metal 'J' or 'Z' hooks to connect the blocks to the load bearing steel frame and/or masonry walls. The metal could be hung vertically or anchored horizontally. Pins, clamps, clips, plates, and a variety of other devices were used to help secure the blocks. The joints would then be mortared and the block would be partially backfilled. of Long Beach, California == Chemistry ==

Composition Terracotta is made of a clay or silt matrix, a fluxing agent, and grog or bits of previously fired clay. Clays are the remnants of weathered rocks that are smaller than 2 microns. They are composed of silica and alumina. Kaolinite, halloysite, montmorillonite, illite and mica are all good types of clays for ceramic production. When mixed with water they create hydrous aluminum silica that is plastic and moldable. During the firing process the clays lose their water and become a hardened ceramic body. Fluxes add oxygen when they burn to create more uniform melting of the silica particles throughout the body of the ceramic. This increases the strength of the material. Common fluxing materials are calcium carbonate, alkaline feldspars, manganese, and iron oxides. Grog is used to prevent shrinking and provide structure for the fine clay matrix. Causes of failure The most common reasons for terracotta to fail are: poor manufacturing, improper installation, weathering, freeze/thaw cycling, and salt formation from atmospheric pollution. Porosity The porosity of terracotta greatly impacts its performance. The ability or inability for water and pollutants to enter into the material is directly correlated to its structural capacity. Terracotta is very strong in compression but weak in tension and shear strength. Any anomalous material expanding (ice, salts, incompatible fill material, or corroding metal anchors which cause rust jacking) inside the clay body will cause it to crack and eventually spall. Improper molding Inherent faults can severely impact the performance of the material. Improper molding can cause air pockets to form that increase the rate of deterioration. If the block is not fired or cooled properly then the fireskin will not be uniformly adhered to the substrate and can flake off. Likewise, if a glaze is not fired properly it will crack, flake, and fall off. Discolorations can result from mineral impurities such as pyrites or barium carbonates. Handling defects A fair amount of damage comes from clumsy transportation, storage, or installation of the material. If the mortar used around and inside the blocks is too strong then the stress will be translated to the terracotta block which will fail over time. Corroding interior metal anchors expand at a faster rate than the surrounding ceramic body causing it to fail from the inside out. Improper loading of the hollow terracotta blocks can create stress cracks. Flawed repairs Imperfect repair work often exacerbates the underlying problems, speeding up the decay of surrounding elements as well. Making penetrations in terracotta units to attach objects to the outside walls also allows moisture to enter the system, and often crack the terracotta as well. Installing sealant rather than mortar, or applying impervious coating, will trap moisture within the terracotta. Consequences of failure With the majority of terracotta buildings being over one-hundred years old, failing terracotta has become a problem in many cities such as New York. Regular inspections and maintenance and repair programs are required by law, but nonetheless well-publicized incidents such as the death of Erica Tishman after a piece of terracotta fell from a 105-year-old building. ==Manufacturers==

Britain • Royal Doulton (1815 to present) • Fambrini & Daniels (1838 to 1913) • John Marriott Blashfield (1839 to 1878) • Gibbs and Canning (1847 to 1950s) • Burmantofts Pottery (1859 to 1957) • Shaws of Darwen (1897 to 2014) • Darwen Terracotta and Faience (2014–present) United States • Henry Tolman Jr. (1848 to 1855) • Chicago Terra Cotta Works (1868 to 1880) • Gladding, McBean (1879 to present) • Perth Amboy Terra Cotta Company (1879 to 1907) • Boston Terra Cotta Company (1880 to 1893) • A. Hall Terra Cotta Company (1883 to 1887) • New York Architectural Terra-Cotta Company (1886 to 1929) • Los Angeles Pressed Brick Company (1887 to 1916) • Northwestern Terra Cotta Company (1888 to 1954) • Celadon Terra Cotta Company (1888 to 1906) • New Jersey Terra Cotta Company (1888 to 1928) • South Amboy Terra Cotta Company (1903 to 1928) • Denny-Renton Clay and Coal Company (1905 to 1927) • O.W. Ketcham Terra Cotta Works (1906 to 1995) • Ludowici-Celadon Company (1906 to present) • Atlantic Terra Cotta Company (1907 to 1943) • Federal Terra Cotta Company (1909 to 1928) • Moravian Pottery and Tile Works (1912 to present) • Federal Seaboard Terra Cotta Corporation (1928 to 1968) • Boston Valley Terra Cotta (1981 to present) ==References==