Architectural terracotta

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.

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.

[4] Similarities in motifs and manufacturing processes have caused scholars to note cross cultural pollination between the Hellenic and Indus River Valley sculptural terracotta traditions.

The Coade ladies[dubious – discuss] popularized the grey mix of terracotta as an alternative to stone with the help of architects like Horace Walpole and Sir John Soane.

The earliest manufacturer of architectural terracotta in the United States was started by Henry Tolman Jr. in Worcester, Massachusetts, around 1849.

This usage diminished as time went on, especially after the success of Cass Gilbert's Woolworth Building increased demand for monochromatic terracotta.

Trends in the 1920s favored setbacks in skyscraper towers, leading to increasing demand for sculpted forms in low relief.

This was largely attributed to architect's increasing preference for building with cheaper metal, glass, and cement.

For much of the 20th century the American terracotta industry was a fraction of its earlier scale and the few surviving companies largely subsisted on jobs producing less complex products like machine-produced ceramic veneers.

Architects during this time period did not embrace terracotta's natural properties and instead tended to use it to imitate other materials.

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.

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.

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.

Wire mesh or other stiffeners are added to create the web, or clay body that surrounds the hollow cell.

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.

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.

The interior chamber radiated heat around the terracotta by pulling in hot air from behind an exterior cavity wall.

The development of cast and later wrought iron as a structural material was closely linked to the rise of terracotta.

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.

Kaolinite, halloysite, montmorillonite, illite and mica are all good types of clays for ceramic production.

Fluxes add oxygen when they burn to create more uniform melting of the silica particles throughout the body of the ceramic.

[19] The most common reasons for terracotta to fail are: poor manufacturing, improper installation, weathering, freeze/thaw cycling, and salt formation from atmospheric pollution.

The ability or inability for water and pollutants to enter into the material is directly correlated to its structural capacity.

Corroding interior metal anchors expand at a faster rate than the surrounding ceramic body causing it to fail from the inside out.

The pollution creates a mildly acidic solution that eats at the clay body or a salt crust forms, causing similar issues as ice.

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.