Chemistry
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.History
Terracotta was made by theAncient Eastern terracotta
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 theWestern 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 or1700s–1880s
;England Englishmen 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. ee Coade stone, See Eleanor Coade">Coade_stone.html" ;"title="ee Coade stone">ee Coade stone, See Eleanor Coade ] George died a year later, leaving the company to his wife and daughter, both named Eleanor Coade. The Coade ladies popularized the grey mix of terracotta as an alternative to stone with the help of architects like1880s–1930s
Reactions to the Chicago fire in 1871 spurred interest in terracotta as a fireproof building material. In the age of skyscraper construction, the cast iron frame needed to be protected. Terracotta was a lightweight, moldable, fire and pollution resistant material that could be mass-produced. Architects such as Burnham and Root,1930s–present
Economic interest in terracotta plummeted in the 1930s but the industry did not die out. Terracotta could not compete with newer mass-produced materials such as plate glass, new metal alloys, and cement block construction. Changing fashions towards more minimalist, modern styles such as the Bauhaus School and International Style did not help the waning industry. Post-World War II the industry had to face the decline of buildings built during the heyday of the material, 1910–1940. Structural problems resulting from incomplete waterproofing, improper installation, poor maintenance, and interior corroding mild steel made the material unpopular in newer constructions. To combat this accusation against quality, several other industry publications were produced between 1945 and the 1960s. They did not succeed in reviving the artistic architectural terracotta but they were able to turn their attention to public buildings. Advances in machine extruded terracotta made it competitive with other hollow clay tile alternatives at the time. In tiles form, terracotta had a renewal as a hallmark of mid-century public buildings. It lost steam in the 1960s with the introduction of more synthetic materials on the mass market. The industry was sustained by the need for replacement blocks for old buildings, hollow clay tile, and now rainscreen.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.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.Extrusion
Mechanized extrusion was used for the mass-production of terracotta blocks, popular in the 1920s. Prepared clay was fed into a machine that would then push the mix through a mold. The technique required the blocks to be made with simple shapes, so this process was often used for flooring, roofing, cladding, and later hollow clay tiles.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.Firing
TheInstallation
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.Degradation
The most common reasons for terracotta to fail are: poor manufacturing, improper installation, weathering, freeze/thaw cycling, and salt formation from atmospheric pollution. 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) inside the clay body will cause it to crack and eventually spall. 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. 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. Imperfect repair work often exacerbates the underlying problems, speeding up the decay of surrounding elements as well. Installing sealant rather than mortar, or applying impervious coating, will trap moisture within the terracotta. The environment also plays a large role in the survival of terracotta. Different types of air pollution can cause different types of surface problems. When it rains, water and salts get sucked into the voids in and around the terracotta through capillary action. If it freezes then ice forms, putting internal stress on the material, causing it to crack from inside. A similar problem happens with atmospheric pollutants that are carried into the gaps by rains water. The pollution creates a mildly acidic solution that eats at the clay body or a salt crust forms, causing similar issues as ice. With the majority of terracotta buildings being over one-hundred years old, failing terracotta has become a problem in many cities such asSee also
Manufacturers
* John Marriott Blashfield * Burmantofts Pottery * Fambrini & Daniels, Lincoln *References
Bibliography
*Barr, Emily. "PRESSING ISSUES IN-KIND TERRA COTTA REPLACEMENT IN THE 21ST CENTURY." Masters of Science Thesis. Columbia University. 2014 *Dillon M. (1985) ''Bricks, Tiles and Terracotta, An Exhibition on one of the major industries of the Wrexham area'', (Held at the Grosvenor Museum, Chester), 24pp. *Didden, Amanda. "Standardization of terracotta anchorage: an analysis of shop drawings from the Northwestern Terra Cotta Company and the O.W. Ketcham Terra Cotta Works." Masters Thesis, University of Pennsylvania, 2003. *Fidler, John. The Conservation of Architectural Terracotta and Faience. ''Transactions of the Association for Studies in the Conservation of Historic Buildings'', no. 6(1981):3-16. *Fidler, John. Fragile Remains. ''Architectural Ceramics: their History, Manufacture and Conservation''. London: James and James, 1996. * *Gerns, Edward and Joshua Freedland. "Understanding terra-cotta distress: Evaluation and repair approaches." Journal of Building Appraisal. October 2006. *James W P Campbell & Will Pryce, (2003) ''Brick: A World History'', *Jenkins, Moses. "Terracotta and Faience." Historic Scotland, Longmore House. *Mack, Robert C. "The Manufacture and Use of Architectural Terra Cotta in the United States." In ''The Technology of Historic American Buildings'', edited by H. Ward Jandl, 117–51. Washington, D.C.: Foundation for Preservation Technology, 1983. * *Ries, Heinrich and Henry Leighton. ''History of the Clay Working Industry in the United States.'' New York: John Wiley, 1909. * *Stratton, M. (1993) ''The Terracotta Revival : Building Innovation and the Image of the Industrial City in Britain and North America''. London : Gollancz. *Taylor, James. Terra Cotta. ''Architectural Record,'' Vol. 1(July 1891-July 1892):63-68. *Taylor, James. "History of Terra Cotta in New York City." ''Architectural Record'' 2 July 1892-July 1893:136-148. * * * *Wells, Jeremy C. History of Structural Hollow Clay Tile in the United States *''Construction History'', Vol. 22 (2007):27-46. {{Commons category, Terracotta in architecture, position=leftExternal links