Architectural terracotta

Fired clay construction material

Architectural terracotta refers to a fired mixture of clay and water that can be used in a non structural, semi structural, or structural capacity on the exterior or interior of a building. Terracotta pottery, as earthenware is called when not used for vessels, is an ancient building material that translates from Latin as "baked earth". Some architectural terracotta is actually stronger than stoneware. It can be unglazed, painted, slip glazed, or glazed. Usually solid in earlier uses, in most cases from the 19th century onwards each piece of terracotta is composed of a hollow clay web enclosing a void space or cell. The cell can be installed in compression with mortar or hung with metal anchors; such cells are often partially backfilled with mortar. By the late 19th century the version with a ceramic glaze, namely glazed architectural terracotta, became more popular. Terracotta can be used together with brick, for ornamental areas; if the source of the clay is the same they can be made to harmonize, or if different to contrast. It is also often a cladding over a different structural material.

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.

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.

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.

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. Southern German, Italian and Spanish city states kept the tradition alive.

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, H. H. Richardson, Louis Sullivan, and McKim, Mead & White became interested in using terracotta as a building material rather than just as imitation stone. Issues with installation, water penetration, and corroding interior metal caused the industry to organize the National Terracotta Society. They published two widely used standards, in 1914 and 1924, that detailed construction methods, anchoring systems, waterproofing, and maintenance standards.

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.

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.

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 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.

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.

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. 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. 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. 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.