Terra Cotta

Summary

Terra cotta is a clay-based, non-vitreous ceramic fired at relatively low temperatures, typically around 1,000°C. Architectural terra cotta is fired near vitrification for cladding and ornament, so the body must resist freeze-thaw and salt, not only the glaze.

Cols

Image

Terra Cotta.jpg

Terra cotta: Italian for baked earth
Architectural terra cotta: body fired to or near vitrification for exterior use
Burnishing: polishing the surface before firing to reduce porosity

History and demand

First uses in construction

Mid-19th century: architectural revival
1871: Chicago Fire increased demand for fireproof urban materials

Sourcing

United States architectural terra cotta makers include Gladding, McBean (California, founded 1875) and Boston Valley Terra Cotta (New York, origins in 1889, terra cotta since 1981); NBK Terracotta is also a leading supplier

Demand trend

Demand fell from the 1930s, then recovered

Composition and production

Raw materials

Clay with iron oxides giving a reddish tone; also yellow, orange, buff, pink, grey, and brown

Manufacturing process

Fired at about 1,000°C (1,830°F); as low as 600°C (1,112°F) in historic examples
Throwing and slip casting; fired in a kiln

Properties

Physical

Unglazed terra cotta is not watertight; glazes reduce permeability
Much lighter than marble; outdoor works need greater thickness

Durability

Architectural body fired to or near vitrification to withstand freeze-thaw cycles and salt intrusion
Glaze alone does not seal the body; a non-porous clay body prevents water-related failure

Uses in buildings

Building elements

Wall Cladding: exterior cladding, ornament, and friezes
Glazed terra cotta serves building decoration in freezing climates

Pros and cons

Pros

Molded ornament in many shapes
Lighter than marble
Lower material cost than bronze

Cons

Unglazed terra cotta absorbs water
Needs a vitrified body for freeze-thaw exposure

Manufacturing and cost

Main manufacturers and suppliers

Sources

Wikipedia: Terracotta, https://en.wikipedia.org/wiki/Terracotta
Wikimedia Commons: Terracotta wall tiles.jpg, photo by Salil Kumar Mukherjee (CC BY-SA 4.0), https://commons.wikimedia.org/wiki/File:Terracotta_wall_tiles.jpg
Gladding, McBean: historical terra cotta, https://www.gladdingmcbean.com/historical-terra-cotta-working
Facade Tectonics: Boston Valley Terra Cotta, https://facadetectonics.org/organizations/boston-valley-terra-cotta