Terra cotta, or “burnt clay,” is a hard baked, high grade of weathered or aged clay. It is similar to brick but the clay is of higher quality and fired at higher temperatures. This article focuses on exterior architectural terra cotta as distinguished from statuary, pottery , and terra cotta blocks used as inner wythes of wall or fill material.

The 1893 Columbian Exposition in Chicago demonstrated the versatility and ornamental qualities of terra cotta. It highlighted the great variance in color and shapes possible with terra cotta and began the demand in the United States for terra cotta that lasted through the late 1930s. Terra cotta is prized for its light weight, longevity, aesthetic qualities, and unit construction. At the peak of production, almost every urban area in America was producing architectural terra cotta in some variation. Today, most replacement units are produced by either Gladding McBean or Boston Valley Terra Cotta.

Specific forming techniques including hand press, machine press, slip casting, and extrusion are used depending on the shape and style of unit required. In the analysis of terra cotta failure the forming techniques are less critical than the strength characteristics of the fired clay, the integrity of the exterior surfaces, and structural support systems.

Exterior ornamental terra cotta was marketed as a light weight water proof cladding. And if proper construction techniques were employed, and the system was maintained, and the local climates were mild, terra cotta performed as sold. However, terra cotta adorns buildings in severe weather climates, and is installed with structural materials affected by environmental conditions, and located on façade elements inaccessible for routine maintenance.

The mortar joints are the material most susceptible to failure. Joints often exist on all three axis with some units of terra cotta designed for flat horizontal surfaces. Over time and exposure, the mortar fails providing a means for water intrusion. Sever cycling of weather in simultaneous freeze/thaw conditions can cause the terra cotta clay to expand and contract, accelerating the crazing or cracking of the protective glaze. Extensive crazing can lead to glaze spalling and allow for further water intrusion.

Once water enters the system there is no weep path allowing for water egress. Construction means and methods, as well as the cellular unit design, trap water and contribute to the potential corrosion of steel lintels, wire ties, steel structural support members, and other miscellaneous metals. Rapid freezing and thawing cycles, in addition to steel corrosion, can crack terra cotta units. If the units remain unrepaired, further water intrusion and/or absorption will occur.

The repair of terra cotta will depend both on the cause and manifestation of the defect. Typical defects include crazing of the glazed finish, shallow surface spalls, deep spalls affecting the bisque, cracked units, inadequate support and / or anchorage, corrosion induced stress fractures, impact damage, mortar degradation, lack of maintenance, and inadequate repairs.

Proper terra cotta repair methods are linked to the cause of defect. Repair techniques are often performed on-site by skilled tradesmen. When damage to the terra cotta unit is severe, full replacement may be required. Defects due to inadequate support or a result of corrosion to supporting steel members is likely to require more invasive repair strategies including removal and replacement of several courses of interlocked terra cotta units.

When replacement units are not required and the scope is limited to on-site repair, labor costs exceed material costs. Since many historic terra cotta units were specialty designed and installed for the structure, a premium price is paid for replacement. New exterior decorative terra cotta is available only from the sources referenced and with small quantity orders, the first unit is approximately $5,000 with much of the costs attributed to making the form and determining the finish color and texture. Subsequent costs per unit will decrease with the range of decrease dependent upon quantities required.

The most important component of terra cotta repair is an understanding the cause of deterioration and the proper repair specifications. Both are derived after a full condition assessment and evaluation of the existing conditions.


Written by Peter Meijer AIA, NCARB, Principal


——————–
Sources
• Last of the Handmade Buildings, Virginia Guest Ferriday, Mark Publishing Co., Portland, OR 1984
• National Park Service, Preservation Brief No7, Preservation of Glazed Terra Cotta
• APT Pacific NW Chapter 2005 workshop
• Terra Cotta, Standard Construction, Revised Ed., National Terra Cotta Society, 1927