Terra-cotta isn't stone, but looks like it

August 11, 2010

Terra-cotta was a popular exterior cladding material in the United States between 1880 and 1930. Terra-cotta clay was formed in plaster molds at 10 percent to 12 percent larger than the required finish dimensions, and kiln-fired like brick to achieve the final product. The terra-cotta elements were then coated with a variety of glazes, usually to replicate different types of stone. The baked clay product was much cheaper than true masonry, but offered the appearance and solidity of real stone cladding.

As costs and the demand for terra-cotta increased, manufacturers began to glaze the clay elements prior to the first kiln firing, and in many cases only glazed the visible side of the product to increase production and profits. This "shortcutting" is the cause of most terra-cotta failures today.

Unlike real masonry, the oven-dried terra-cotta rehydrates some when placed in the natural environment. "Most terra-cotta-clad skyscrapers went through an initial, one-time rehydration sometime in the first two to three years," says Theodore Prudon, professor of historic preservation at Columbia University. The rehydration was often never calculated into the joint spacing of the tile skin.

The result is many buildings with highly stressed skins, compromised mortar joints and-in time-structural failure of the panel or the iron fasteners, or both.

         
 

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