Architectural terracotta

Brick temple in Bhitargaon, 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,[4] India and Pakistan.[5] Similarities in motifs and manufacturing processes have caused scholars to note cross cultural pollination between the Hellenic and Indus River Valley sculptural terracotta traditions.[6] 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.[7]

Greeks used terracotta for capitals, friezes, and other elements of their temples like at Olympia or Selenius.[8] 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.[9] Medieval European architecture did not expand terracotta use beyond the ancients. The manufacture of tile roofs diminished with low cost thatch roofing widely available.[10] Southern German, Italian and Spanish city states kept the tradition alive.

The Natural History Museum in London has an ornate terracotta facade typical of high Victorian architecture – the carvings represent the contents of the museum

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.[10] 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 like Horace Walpole and Sir John Soane. Georgian architectural style was in vogue and demand for repetitive, classically inspired décor was very fashionable. The Victoria and Albert Museum (1867–1880) and the Natural History Museum of London (1879–1880) buildings ushered in an era of mass-produced architectural terracotta.

Trinity Church in New York City, from Robert N. Dennis collection of stereoscopic views 7

United States

Most early terracotta was shipped from Europe or made by small local ceramicists. The first manufacturer of architectural terracotta was opened by Henry Tolman Jr. in Worcester, MA around 1849. The capitals in the Massachusetts State Capitol in Worcester are early examples of locally produced terracotta. In the 1850s, New York architects like Richard Upjohn and James Renwick began to use it as an economical replacement for stone.[11]

The Railway Exchange Building in Chicago is faced entirely with light cream enamel terracotta (drawing by Hugh Ferriss, 1920)

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.[12]

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.[13]

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.[14]

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.[16] Once finalized, the drawings were turned into a plaster reality by sculptors who would create the mold for the craftsmen.[17]

Vertical pugmill used by the Moravian Pottery and Tile Works in Pennsylvania to refine the clay used for tile production

Clay selection was very important to manufacture of terracotta. Homogenous, finer grain sizes were preferred.[18] 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.[19] 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.[15]

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.[20]

A downdraft kiln designed for the Pomona Terra Cotta Manufacturing Company in Guilford County, North Carolina

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.[17]

Philadelphia Art Museum's terracotta pediment using polychrome glazing

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.[17]

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.[16]

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.[16]

Cracking caused by corroding metal anchors at the First Congregational Church of Long Beach, California

• John Marriott Blashfield
• Burmantofts Pottery
• Fambrini & Daniels, Lincoln
• Royal Doulton, UK
• Gibbs and Canning Limited
• Gladding, McBean, California, USA
• Perth Amboy Terra Cotta Company of Perth Amboy, New Jersey, USA