The Basics of Terra Cotta
By: Orion Doscher
Restoration work often focuses on the functional capabilities of a façade, namely water infiltration; however, restoration designers are also challenged to maintain visual aesthetics in addition to façade function. Terra cotta performs well in both areas and is one of the most durable building materials there is. If terra cotta is properly manufactured and installed, it can remain on a building almost indefinitely. Terra cotta was used in ancient architecture all over the world. It made its debut in New York City in the 1850s and remained very popular until the mid 1900s. It is often seen as accent pieces such as lintels and cornices, but it also has been used to fully clad buildings like the Woolworth Building in the Financial District or the Flat Iron Building in Midtown Manhattan.
The manufacturing of terra cotta will determine everything about the stone, from its color to its longevity. Terra cotta stones are manufactured similarly to ceramics. First, a special blend of fine clays is mixed to the required composition, then the clay is hydrated and aged. Only after moisture has blended with the clay on a molecular level, can the mixture be saturated and mixed like cement. It is then molded or extruded, fully dried, and fired in a kiln at a minimum of 2000°F for several days. Other factors like shrinkage and custom glazing must also be accounted for. This manufacturing process requires an artisan level of skill.
A common cause of stone deterioration is moisture infiltration, followed by freeze/thaw cycles, expanding and contracting the infiltrating water and causing stone to crack. When properly composed and formed, terra cotta can be mostly impervious to moisture. Terra cotta is also resistant to biological growth since its surface is smooth and doesn’t allow for attachment. Since it is a porous material that is formed in shell-like configurations or with webbed interiors, terra cotta is more lightweight than other architectural stone. Lastly, as a ceramic, its heat resistance makes it a good choice for ornamental work on fireproof buildings.
Despite all of the advantageous characteristics of terra cotta, it can still deteriorate. Terra cotta is usually produced in pieces no more that 2’ wide; therefore, installation requires anchoring and mortar pointing at each joint. Exposure to moisture and the thermal expansion and contraction of the building will cause mortar to deteriorate. This can lead to water infiltrating the stones and rusting the anchoring systems. Unfortunately, it is difficult to visually determine the condition of terra cotta stone without performing a physical examination, also known as sounding. All terra cotta on a building should be sounded annually to preventively discover problems before they lead to failure or unsafe conditions.
Cracks in terra cotta can sometimes be repaired with restoration mortar. But when the extent of degradation is beyond repair, terra cotta replacement options must be considered. Replacing terra cotta in kind is expensive and can have extensive lead times, but it will last a very long time if properly maintained. Another option is to replace the terra cotta with cast stone; this more economical option can last for up to 20 years but will weather differently than terra cotta. Since terra cotta is technically a ceramic, it often has a glazed finish. This finish resists discoloration, whereas cast stones will sooner show the effects of dirt and acid rain. If you look at the façade of Woolworth Building today, you can easily spot the replacement cast stone units. Applying an exterior coating to the cast stone and regularly power washing a building façade can help preserve the likeness between the terra cotta and the cast stone.
Terra cotta has grown less popular over the years, as newer and less expensive materials have emerged and general architectural style has become more minimalistic. While it is not seen on many new buildings, much of the terra cotta used in the past is still prevalent all over the United States. Whether it is the Utah Capitol building or the Brooklyn Dreams Charter School, it is important that terra-cotta be regularly maintained and inspected, not only to preserve the aesthetic of the building, but especially for safety reasons.